Delete tools/telemetry.

tools/telemetry/third_party/png/png.py

Index: tools/telemetry/third_party/png/png.py
diff --git a/tools/telemetry/third_party/png/png.py b/tools/telemetry/third_party/png/png.py
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-#!/usr/bin/env python
-
-# $URL$
-# $Rev$
-
-# png.py - PNG encoder/decoder in pure Python
-#
-# Copyright (C) 2006 Johann C. Rocholl <johann@browsershots.org>
-# Portions Copyright (C) 2009 David Jones <drj@pobox.com>
-# And probably portions Copyright (C) 2006 Nicko van Someren <nicko@nicko.org>
-#
-# Original concept by Johann C. Rocholl.
-#
-# LICENSE (The MIT License)
-#
-# Permission is hereby granted, free of charge, to any person
-# obtaining a copy of this software and associated documentation files
-# (the "Software"), to deal in the Software without restriction,
-# including without limitation the rights to use, copy, modify, merge,
-# publish, distribute, sublicense, and/or sell copies of the Software,
-# and to permit persons to whom the Software is furnished to do so,
-# subject to the following conditions:
-#
-# The above copyright notice and this permission notice shall be
-# included in all copies or substantial portions of the Software.
-#
-# THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
-# EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
-# MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
-# NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS
-# BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN
-# ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
-# CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
-# SOFTWARE.
-#
-# Changelog (recent first):
-# 2009-03-11 David: interlaced bit depth < 8 (writing).
-# 2009-03-10 David: interlaced bit depth < 8 (reading).
-# 2009-03-04 David: Flat and Boxed pixel formats.
-# 2009-02-26 David: Palette support (writing).
-# 2009-02-23 David: Bit-depths < 8; better PNM support.
-# 2006-06-17 Nicko: Reworked into a class, faster interlacing.
-# 2006-06-17 Johann: Very simple prototype PNG decoder.
-# 2006-06-17 Nicko: Test suite with various image generators.
-# 2006-06-17 Nicko: Alpha-channel, grey-scale, 16-bit/plane support.
-# 2006-06-15 Johann: Scanline iterator interface for large input files.
-# 2006-06-09 Johann: Very simple prototype PNG encoder.
-
-# Incorporated into Bangai-O Development Tools by drj on 2009-02-11 from
-# http://trac.browsershots.org/browser/trunk/pypng/lib/png.py?rev=2885
-
-# Incorporated into pypng by drj on 2009-03-12 from
-# //depot/prj/bangaio/master/code/png.py#67
-
-
-"""
-Pure Python PNG Reader/Writer
-
-This Python module implements support for PNG images (see PNG
-specification at http://www.w3.org/TR/2003/REC-PNG-20031110/ ). It reads
-and writes PNG files with all allowable bit depths (1/2/4/8/16/24/32/48/64
-bits per pixel) and colour combinations: greyscale (1/2/4/8/16 bit); RGB,
-RGBA, LA (greyscale with alpha) with 8/16 bits per channel; colour mapped
-images (1/2/4/8 bit).  Adam7 interlacing is supported for reading and
-writing.  A number of optional chunks can be specified (when writing)
-and understood (when reading): ``tRNS``, ``bKGD``, ``gAMA``.
-
-For help, type ``import png; help(png)`` in your python interpreter.
-
-A good place to start is the :class:`Reader` and :class:`Writer` classes.
-
-Requires Python 2.3.  Limited support is available for Python 2.2, but
-not everything works.  Best with Python 2.4 and higher.  Installation is
-trivial, but see the ``README.txt`` file (with the source distribution)
-for details.
-
-This file can also be used as a command-line utility to convert
-`Netpbm <http://netpbm.sourceforge.net/>`_ PNM files to PNG, and the reverse conversion from PNG to
-PNM. The interface is similar to that of the ``pnmtopng`` program from
-Netpbm.  Type ``python png.py --help`` at the shell prompt
-for usage and a list of options.
-
-A note on spelling and terminology
-----------------------------------
-
-Generally British English spelling is used in the documentation.  So
-that's "greyscale" and "colour".  This not only matches the author's
-native language, it's also used by the PNG specification.
-
-The major colour models supported by PNG (and hence by PyPNG) are:
-greyscale, RGB, greyscale--alpha, RGB--alpha.  These are sometimes
-referred to using the abbreviations: L, RGB, LA, RGBA.  In this case
-each letter abbreviates a single channel: *L* is for Luminance or Luma or
-Lightness which is the channel used in greyscale images; *R*, *G*, *B* stand
-for Red, Green, Blue, the components of a colour image; *A* stands for
-Alpha, the opacity channel (used for transparency effects, but higher
-values are more opaque, so it makes sense to call it opacity).
-
-A note on formats
------------------
-
-When getting pixel data out of this module (reading) and presenting
-data to this module (writing) there are a number of ways the data could
-be represented as a Python value.  Generally this module uses one of
-three formats called "flat row flat pixel", "boxed row flat pixel", and
-"boxed row boxed pixel".  Basically the concern is whether each pixel
-and each row comes in its own little tuple (box), or not.
-
-Consider an image that is 3 pixels wide by 2 pixels high, and each pixel
-has RGB components:
-
-Boxed row flat pixel::
-
-  list([R,G,B, R,G,B, R,G,B],
-       [R,G,B, R,G,B, R,G,B])
-
-Each row appears as its own list, but the pixels are flattened so that
-three values for one pixel simply follow the three values for the previous
-pixel.  This is the most common format used, because it provides a good
-compromise between space and convenience.  PyPNG regards itself as
-at liberty to replace any sequence type with any sufficiently compatible
-other sequence type; in practice each row is an array (from the array
-module), and the outer list is sometimes an iterator rather than an
-explicit list (so that streaming is possible).
-
-Flat row flat pixel::
-
-  [R,G,B, R,G,B, R,G,B,
-   R,G,B, R,G,B, R,G,B]
-
-The entire image is one single giant sequence of colour values.
-Generally an array will be used (to save space), not a list.
-
-Boxed row boxed pixel::
-
-  list([ (R,G,B), (R,G,B), (R,G,B) ],
-       [ (R,G,B), (R,G,B), (R,G,B) ])
-
-Each row appears in its own list, but each pixel also appears in its own
-tuple.  A serious memory burn in Python.
-
-In all cases the top row comes first, and for each row the pixels are
-ordered from left-to-right.  Within a pixel the values appear in the
-order, R-G-B-A (or L-A for greyscale--alpha).
-
-There is a fourth format, mentioned because it is used internally,
-is close to what lies inside a PNG file itself, and has some support
-from the public API.  This format is called packed.  When packed,
-each row is a sequence of bytes (integers from 0 to 255), just as
-it is before PNG scanline filtering is applied.  When the bit depth
-is 8 this is essentially the same as boxed row flat pixel; when the
-bit depth is less than 8, several pixels are packed into each byte;
-when the bit depth is 16 (the only value more than 8 that is supported
-by the PNG image format) each pixel value is decomposed into 2 bytes
-(and `packed` is a misnomer).  This format is used by the
-:meth:`Writer.write_packed` method.  It isn't usually a convenient
-format, but may be just right if the source data for the PNG image
-comes from something that uses a similar format (for example, 1-bit
-BMPs, or another PNG file).
-
-And now, my famous members
---------------------------
-"""
-
-# http://www.python.org/doc/2.2.3/whatsnew/node5.html
-from __future__ import generators
-
-__version__ = "$URL$ $Rev$"
-
-from array import array
-try: # See :pyver:old
-    import itertools
-except:
-    pass
-import math
-# http://www.python.org/doc/2.4.4/lib/module-operator.html
-import operator
-import struct
-import sys
-import zlib
-# http://www.python.org/doc/2.4.4/lib/module-warnings.html
-import warnings
-try:
-    import pyximport
-    pyximport.install()
-    import cpngfilters as pngfilters
-except ImportError:
-    pass
-
-
-__all__ = ['Image', 'Reader', 'Writer', 'write_chunks', 'from_array']
-
-
-# The PNG signature.
-# http://www.w3.org/TR/PNG/#5PNG-file-signature
-_signature = struct.pack('8B', 137, 80, 78, 71, 13, 10, 26, 10)
-
-_adam7 = ((0, 0, 8, 8),
-          (4, 0, 8, 8),
-          (0, 4, 4, 8),
-          (2, 0, 4, 4),
-          (0, 2, 2, 4),
-          (1, 0, 2, 2),
-          (0, 1, 1, 2))
-
-def group(s, n):
-    # See
-    # http://www.python.org/doc/2.6/library/functions.html#zip
-    return zip(*[iter(s)]*n)
-
-def isarray(x):
-    """Same as ``isinstance(x, array)`` except on Python 2.2, where it
-    always returns ``False``.  This helps PyPNG work on Python 2.2.
-    """
-
-    try:
-        return isinstance(x, array)
-    except:
-        return False
-
-try:  # see :pyver:old
-    array.tostring
-except:
-    def tostring(row):
-        l = len(row)
-        return struct.pack('%dB' % l, *row)
-else:
-    def tostring(row):
-        """Convert row of bytes to string.  Expects `row` to be an
-        ``array``.
-        """
-        return row.tostring()
-
-# Conditionally convert to bytes.  Works on Python 2 and Python 3.
-try:
-    bytes('', 'ascii')
-    def strtobytes(x): return bytes(x, 'iso8859-1')
-    def bytestostr(x): return str(x, 'iso8859-1')
-except:
-    strtobytes = str
-    bytestostr = str
-
-def interleave_planes(ipixels, apixels, ipsize, apsize):
-    """
-    Interleave (colour) planes, e.g. RGB + A = RGBA.
-
-    Return an array of pixels consisting of the `ipsize` elements of data
-    from each pixel in `ipixels` followed by the `apsize` elements of data
-    from each pixel in `apixels`.  Conventionally `ipixels` and
-    `apixels` are byte arrays so the sizes are bytes, but it actually
-    works with any arrays of the same type.  The returned array is the
-    same type as the input arrays which should be the same type as each other.
-    """
-
-    itotal = len(ipixels)
-    atotal = len(apixels)
-    newtotal = itotal + atotal
-    newpsize = ipsize + apsize
-    # Set up the output buffer
-    # See http://www.python.org/doc/2.4.4/lib/module-array.html#l2h-1356
-    out = array(ipixels.typecode)
-    # It's annoying that there is no cheap way to set the array size :-(
-    out.extend(ipixels)
-    out.extend(apixels)
-    # Interleave in the pixel data
-    for i in range(ipsize):
-        out[i:newtotal:newpsize] = ipixels[i:itotal:ipsize]
-    for i in range(apsize):
-        out[i+ipsize:newtotal:newpsize] = apixels[i:atotal:apsize]
-    return out
-
-def check_palette(palette):
-    """Check a palette argument (to the :class:`Writer` class) for validity.
-    Returns the palette as a list if okay; raises an exception otherwise.
-    """
-
-    # None is the default and is allowed.
-    if palette is None:
-        return None
-
-    p = list(palette)
-    if not (0 < len(p) <= 256):
-        raise ValueError("a palette must have between 1 and 256 entries")
-    seen_triple = False
-    for i,t in enumerate(p):
-        if len(t) not in (3,4):
-            raise ValueError(
-              "palette entry %d: entries must be 3- or 4-tuples." % i)
-        if len(t) == 3:
-            seen_triple = True
-        if seen_triple and len(t) == 4:
-            raise ValueError(
-              "palette entry %d: all 4-tuples must precede all 3-tuples" % i)
-        for x in t:
-            if int(x) != x or not(0 <= x <= 255):
-                raise ValueError(
-                  "palette entry %d: values must be integer: 0 <= x <= 255" % i)
-    return p
-
-class Error(Exception):
-    prefix = 'Error'
-    def __str__(self):
-        return self.prefix + ': ' + ' '.join(self.args)
-
-class FormatError(Error):
-    """Problem with input file format.  In other words, PNG file does
-    not conform to the specification in some way and is invalid.
-    """
-
-    prefix = 'FormatError'
-
-class ChunkError(FormatError):
-    prefix = 'ChunkError'
-
-
-class Writer:
-    """
-    PNG encoder in pure Python.
-    """
-
-    def __init__(self, width=None, height=None,
-                 size=None,
-                 greyscale=False,
-                 alpha=False,
-                 bitdepth=8,
-                 palette=None,
-                 transparent=None,
-                 background=None,
-                 gamma=None,
-                 compression=None,
-                 interlace=False,
-                 bytes_per_sample=None, # deprecated
-                 planes=None,
-                 colormap=None,
-                 maxval=None,
-                 chunk_limit=2**20):
-        """
-        Create a PNG encoder object.
-
-        Arguments:
-
-        width, height
-          Image size in pixels, as two separate arguments.
-        size
-          Image size (w,h) in pixels, as single argument.
-        greyscale
-          Input data is greyscale, not RGB.
-        alpha
-          Input data has alpha channel (RGBA or LA).
-        bitdepth
-          Bit depth: from 1 to 16.
-        palette
-          Create a palette for a colour mapped image (colour type 3).
-        transparent
-          Specify a transparent colour (create a ``tRNS`` chunk).
-        background
-          Specify a default background colour (create a ``bKGD`` chunk).
-        gamma
-          Specify a gamma value (create a ``gAMA`` chunk).
-        compression
-          zlib compression level: 0 (none) to 9 (more compressed); default: -1 or None.
-        interlace
-          Create an interlaced image.
-        chunk_limit
-          Write multiple ``IDAT`` chunks to save memory.
-
-        The image size (in pixels) can be specified either by using the
-        `width` and `height` arguments, or with the single `size`
-        argument.  If `size` is used it should be a pair (*width*,
-        *height*).
-
-        `greyscale` and `alpha` are booleans that specify whether
-        an image is greyscale (or colour), and whether it has an
-        alpha channel (or not).
-
-        `bitdepth` specifies the bit depth of the source pixel values.
-        Each source pixel value must be an integer between 0 and
-        ``2**bitdepth-1``.  For example, 8-bit images have values
-        between 0 and 255.  PNG only stores images with bit depths of
-        1,2,4,8, or 16.  When `bitdepth` is not one of these values,
-        the next highest valid bit depth is selected, and an ``sBIT``
-        (significant bits) chunk is generated that specifies the original
-        precision of the source image.  In this case the supplied pixel
-        values will be rescaled to fit the range of the selected bit depth.
-
-        The details of which bit depth / colour model combinations the
-        PNG file format supports directly, are somewhat arcane
-        (refer to the PNG specification for full details).  Briefly:
-        "small" bit depths (1,2,4) are only allowed with greyscale and
-        colour mapped images; colour mapped images cannot have bit depth
-        16.
-
-        For colour mapped images (in other words, when the `palette`
-        argument is specified) the `bitdepth` argument must match one of
-        the valid PNG bit depths: 1, 2, 4, or 8.  (It is valid to have a
-        PNG image with a palette and an ``sBIT`` chunk, but the meaning
-        is slightly different; it would be awkward to press the
-        `bitdepth` argument into service for this.)
-
-        The `palette` option, when specified, causes a colour mapped image
-        to be created: the PNG colour type is set to 3; greyscale
-        must not be set; alpha must not be set; transparent must
-        not be set; the bit depth must be 1,2,4, or 8.  When a colour
-        mapped image is created, the pixel values are palette indexes
-        and the `bitdepth` argument specifies the size of these indexes
-        (not the size of the colour values in the palette).
-
-        The palette argument value should be a sequence of 3- or
-        4-tuples.  3-tuples specify RGB palette entries; 4-tuples
-        specify RGBA palette entries.  If both 4-tuples and 3-tuples
-        appear in the sequence then all the 4-tuples must come
-        before all the 3-tuples.  A ``PLTE`` chunk is created; if there
-        are 4-tuples then a ``tRNS`` chunk is created as well.  The
-        ``PLTE`` chunk will contain all the RGB triples in the same
-        sequence; the ``tRNS`` chunk will contain the alpha channel for
-        all the 4-tuples, in the same sequence.  Palette entries
-        are always 8-bit.
-
-        If specified, the `transparent` and `background` parameters must
-        be a tuple with three integer values for red, green, blue, or
-        a simple integer (or singleton tuple) for a greyscale image.
-
-        If specified, the `gamma` parameter must be a positive number
-        (generally, a float).  A ``gAMA`` chunk will be created.  Note that
-        this will not change the values of the pixels as they appear in
-        the PNG file, they are assumed to have already been converted
-        appropriately for the gamma specified.
-
-        The `compression` argument specifies the compression level to
-        be used by the ``zlib`` module.  Values from 1 to 9 specify
-        compression, with 9 being "more compressed" (usually smaller
-        and slower, but it doesn't always work out that way).  0 means
-        no compression.  -1 and ``None`` both mean that the default
-        level of compession will be picked by the ``zlib`` module
-        (which is generally acceptable).
-
-        If `interlace` is true then an interlaced image is created
-        (using PNG's so far only interace method, *Adam7*).  This does not
-        affect how the pixels should be presented to the encoder, rather
-        it changes how they are arranged into the PNG file.  On slow
-        connexions interlaced images can be partially decoded by the
-        browser to give a rough view of the image that is successively
-        refined as more image data appears.
-        
-        .. note ::
-        
-          Enabling the `interlace` option requires the entire image
-          to be processed in working memory.
-
-        `chunk_limit` is used to limit the amount of memory used whilst
-        compressing the image.  In order to avoid using large amounts of
-        memory, multiple ``IDAT`` chunks may be created.
-        """
-
-        # At the moment the `planes` argument is ignored;
-        # its purpose is to act as a dummy so that
-        # ``Writer(x, y, **info)`` works, where `info` is a dictionary
-        # returned by Reader.read and friends.
-        # Ditto for `colormap`.
-
-        # A couple of helper functions come first.  Best skipped if you
-        # are reading through.
-
-        def isinteger(x):
-            try:
-                return int(x) == x
-            except:
-                return False
-
-        def check_color(c, which):
-            """Checks that a colour argument for transparent or
-            background options is the right form.  Also "corrects" bare
-            integers to 1-tuples.
-            """
-
-            if c is None:
-                return c
-            if greyscale:
-                try:
-                    l = len(c)
-                except TypeError:
-                    c = (c,)
-                if len(c) != 1:
-                    raise ValueError("%s for greyscale must be 1-tuple" %
-                        which)
-                if not isinteger(c[0]):
-                    raise ValueError(
-                        "%s colour for greyscale must be integer" %
-                        which)
-            else:
-                if not (len(c) == 3 and
-                        isinteger(c[0]) and
-                        isinteger(c[1]) and
-                        isinteger(c[2])):
-                    raise ValueError(
-                        "%s colour must be a triple of integers" %
-                        which)
-            return c
-
-        if size:
-            if len(size) != 2:
-                raise ValueError(
-                  "size argument should be a pair (width, height)")
-            if width is not None and width != size[0]:
-                raise ValueError(
-                  "size[0] (%r) and width (%r) should match when both are used."
-                    % (size[0], width))
-            if height is not None and height != size[1]:
-                raise ValueError(
-                  "size[1] (%r) and height (%r) should match when both are used."
-                    % (size[1], height))
-            width,height = size
-        del size
-
-        if width <= 0 or height <= 0:
-            raise ValueError("width and height must be greater than zero")
-        if not isinteger(width) or not isinteger(height):
-            raise ValueError("width and height must be integers")
-        # http://www.w3.org/TR/PNG/#7Integers-and-byte-order
-        if width > 2**32-1 or height > 2**32-1:
-            raise ValueError("width and height cannot exceed 2**32-1")
-
-        if alpha and transparent is not None:
-            raise ValueError(
-                "transparent colour not allowed with alpha channel")
-
-        if bytes_per_sample is not None:
-            warnings.warn('please use bitdepth instead of bytes_per_sample',
-                          DeprecationWarning)
-            if bytes_per_sample not in (0.125, 0.25, 0.5, 1, 2):
-                raise ValueError(
-                    "bytes per sample must be .125, .25, .5, 1, or 2")
-            bitdepth = int(8*bytes_per_sample)
-        del bytes_per_sample
-        if not isinteger(bitdepth) or bitdepth < 1 or 16 < bitdepth:
-            raise ValueError("bitdepth (%r) must be a postive integer <= 16" %
-              bitdepth)
-
-        self.rescale = None
-        if palette:
-            if bitdepth not in (1,2,4,8):
-                raise ValueError("with palette, bitdepth must be 1, 2, 4, or 8")
-            if transparent is not None:
-                raise ValueError("transparent and palette not compatible")
-            if alpha:
-                raise ValueError("alpha and palette not compatible")
-            if greyscale:
-                raise ValueError("greyscale and palette not compatible")
-        else:
-            # No palette, check for sBIT chunk generation.
-            if alpha or not greyscale:
-                if bitdepth not in (8,16):
-                    targetbitdepth = (8,16)[bitdepth > 8]
-                    self.rescale = (bitdepth, targetbitdepth)
-                    bitdepth = targetbitdepth
-                    del targetbitdepth
-            else:
-                assert greyscale
-                assert not alpha
-                if bitdepth not in (1,2,4,8,16):
-                    if bitdepth > 8:
-                        targetbitdepth = 16
-                    elif bitdepth == 3:
-                        targetbitdepth = 4
-                    else:
-                        assert bitdepth in (5,6,7)
-                        targetbitdepth = 8
-                    self.rescale = (bitdepth, targetbitdepth)
-                    bitdepth = targetbitdepth
-                    del targetbitdepth
-
-        if bitdepth < 8 and (alpha or not greyscale and not palette):
-            raise ValueError(
-              "bitdepth < 8 only permitted with greyscale or palette")
-        if bitdepth > 8 and palette:
-            raise ValueError(
-                "bit depth must be 8 or less for images with palette")
-
-        transparent = check_color(transparent, 'transparent')
-        background = check_color(background, 'background')
-
-        # It's important that the true boolean values (greyscale, alpha,
-        # colormap, interlace) are converted to bool because Iverson's
-        # convention is relied upon later on.
-        self.width = width
-        self.height = height
-        self.transparent = transparent
-        self.background = background
-        self.gamma = gamma
-        self.greyscale = bool(greyscale)
-        self.alpha = bool(alpha)
-        self.colormap = bool(palette)
-        self.bitdepth = int(bitdepth)
-        self.compression = compression
-        self.chunk_limit = chunk_limit
-        self.interlace = bool(interlace)
-        self.palette = check_palette(palette)
-
-        self.color_type = 4*self.alpha + 2*(not greyscale) + 1*self.colormap
-        assert self.color_type in (0,2,3,4,6)
-
-        self.color_planes = (3,1)[self.greyscale or self.colormap]
-        self.planes = self.color_planes + self.alpha
-        # :todo: fix for bitdepth < 8
-        self.psize = (self.bitdepth/8) * self.planes
-
-    def make_palette(self):
-        """Create the byte sequences for a ``PLTE`` and if necessary a
-        ``tRNS`` chunk.  Returned as a pair (*p*, *t*).  *t* will be
-        ``None`` if no ``tRNS`` chunk is necessary.
-        """
-
-        p = array('B')
-        t = array('B')
-
-        for x in self.palette:
-            p.extend(x[0:3])
-            if len(x) > 3:
-                t.append(x[3])
-        p = tostring(p)
-        t = tostring(t)
-        if t:
-            return p,t
-        return p,None
-
-    def write(self, outfile, rows):
-        """Write a PNG image to the output file.  `rows` should be
-        an iterable that yields each row in boxed row flat pixel format.
-        The rows should be the rows of the original image, so there
-        should be ``self.height`` rows of ``self.width * self.planes`` values.
-        If `interlace` is specified (when creating the instance), then
-        an interlaced PNG file will be written.  Supply the rows in the
-        normal image order; the interlacing is carried out internally.
-        
-        .. note ::
-
-          Interlacing will require the entire image to be in working memory.
-        """
-
-        if self.interlace:
-            fmt = 'BH'[self.bitdepth > 8]
-            a = array(fmt, itertools.chain(*rows))
-            return self.write_array(outfile, a)
-        else:
-            nrows = self.write_passes(outfile, rows)
-            if nrows != self.height:
-                raise ValueError(
-                  "rows supplied (%d) does not match height (%d)" %
-                  (nrows, self.height))
-
-    def write_passes(self, outfile, rows, packed=False):
-        """
-        Write a PNG image to the output file.
-
-        Most users are expected to find the :meth:`write` or
-        :meth:`write_array` method more convenient.
-        
-        The rows should be given to this method in the order that
-        they appear in the output file.  For straightlaced images,
-        this is the usual top to bottom ordering, but for interlaced
-        images the rows should have already been interlaced before
-        passing them to this function.
-
-        `rows` should be an iterable that yields each row.  When
-        `packed` is ``False`` the rows should be in boxed row flat pixel
-        format; when `packed` is ``True`` each row should be a packed
-        sequence of bytes.
-
-        """
-
-        # http://www.w3.org/TR/PNG/#5PNG-file-signature
-        outfile.write(_signature)
-
-        # http://www.w3.org/TR/PNG/#11IHDR
-        write_chunk(outfile, 'IHDR',
-                    struct.pack("!2I5B", self.width, self.height,
-                                self.bitdepth, self.color_type,
-                                0, 0, self.interlace))
-
-        # See :chunk:order
-        # http://www.w3.org/TR/PNG/#11gAMA
-        if self.gamma is not None:
-            write_chunk(outfile, 'gAMA',
-                        struct.pack("!L", int(round(self.gamma*1e5))))
-
-        # See :chunk:order
-        # http://www.w3.org/TR/PNG/#11sBIT
-        if self.rescale:
-            write_chunk(outfile, 'sBIT',
-                struct.pack('%dB' % self.planes,
-                            *[self.rescale[0]]*self.planes))
-        
-        # :chunk:order: Without a palette (PLTE chunk), ordering is
-        # relatively relaxed.  With one, gAMA chunk must precede PLTE
-        # chunk which must precede tRNS and bKGD.
-        # See http://www.w3.org/TR/PNG/#5ChunkOrdering
-        if self.palette:
-            p,t = self.make_palette()
-            write_chunk(outfile, 'PLTE', p)
-            if t:
-                # tRNS chunk is optional.  Only needed if palette entries
-                # have alpha.
-                write_chunk(outfile, 'tRNS', t)
-
-        # http://www.w3.org/TR/PNG/#11tRNS
-        if self.transparent is not None:
-            if self.greyscale:
-                write_chunk(outfile, 'tRNS',
-                            struct.pack("!1H", *self.transparent))
-            else:
-                write_chunk(outfile, 'tRNS',
-                            struct.pack("!3H", *self.transparent))
-
-        # http://www.w3.org/TR/PNG/#11bKGD
-        if self.background is not None:
-            if self.greyscale:
-                write_chunk(outfile, 'bKGD',
-                            struct.pack("!1H", *self.background))
-            else:
-                write_chunk(outfile, 'bKGD',
-                            struct.pack("!3H", *self.background))
-
-        # http://www.w3.org/TR/PNG/#11IDAT
-        if self.compression is not None:
-            compressor = zlib.compressobj(self.compression)
-        else:
-            compressor = zlib.compressobj()
-
-        # Choose an extend function based on the bitdepth.  The extend
-        # function packs/decomposes the pixel values into bytes and
-        # stuffs them onto the data array.
-        data = array('B')
-        if self.bitdepth == 8 or packed:
-            extend = data.extend
-        elif self.bitdepth == 16:
-            # Decompose into bytes
-            def extend(sl):
-                fmt = '!%dH' % len(sl)
-                data.extend(array('B', struct.pack(fmt, *sl)))
-        else:
-            # Pack into bytes
-            assert self.bitdepth < 8
-            # samples per byte
-            spb = int(8/self.bitdepth)
-            def extend(sl):
-                a = array('B', sl)
-                # Adding padding bytes so we can group into a whole
-                # number of spb-tuples.
-                l = float(len(a))
-                extra = math.ceil(l / float(spb))*spb - l
-                a.extend([0]*int(extra))
-                # Pack into bytes
-                l = group(a, spb)
-                l = map(lambda e: reduce(lambda x,y:
-                                           (x << self.bitdepth) + y, e), l)
-                data.extend(l)
-        if self.rescale:
-            oldextend = extend
-            factor = \
-              float(2**self.rescale[1]-1) / float(2**self.rescale[0]-1)
-            def extend(sl):
-                oldextend(map(lambda x: int(round(factor*x)), sl))
-
-        # Build the first row, testing mostly to see if we need to
-        # changed the extend function to cope with NumPy integer types
-        # (they cause our ordinary definition of extend to fail, so we
-        # wrap it).  See
-        # http://code.google.com/p/pypng/issues/detail?id=44
-        enumrows = enumerate(rows)
-        del rows
-
-        # First row's filter type.
-        data.append(0)
-        # :todo: Certain exceptions in the call to ``.next()`` or the
-        # following try would indicate no row data supplied.
-        # Should catch.
-        i,row = enumrows.next()
-        try:
-            # If this fails...
-            extend(row)
-        except:
-            # ... try a version that converts the values to int first.
-            # Not only does this work for the (slightly broken) NumPy
-            # types, there are probably lots of other, unknown, "nearly"
-            # int types it works for.
-            def wrapmapint(f):
-                return lambda sl: f(map(int, sl))
-            extend = wrapmapint(extend)
-            del wrapmapint
-            extend(row)
-
-        for i,row in enumrows:
-            # Add "None" filter type.  Currently, it's essential that
-            # this filter type be used for every scanline as we do not
-            # mark the first row of a reduced pass image; that means we
-            # could accidentally compute the wrong filtered scanline if
-            # we used "up", "average", or "paeth" on such a line.
-            data.append(0)
-            extend(row)
-            if len(data) > self.chunk_limit:
-                compressed = compressor.compress(tostring(data))
-                if len(compressed):
-                    # print >> sys.stderr, len(data), len(compressed)
-                    write_chunk(outfile, 'IDAT', compressed)
-                # Because of our very witty definition of ``extend``,
-                # above, we must re-use the same ``data`` object.  Hence
-                # we use ``del`` to empty this one, rather than create a
-                # fresh one (which would be my natural FP instinct).
-                del data[:]
-        if len(data):
-            compressed = compressor.compress(tostring(data))
-        else:
-            compressed = ''
-        flushed = compressor.flush()
-        if len(compressed) or len(flushed):
-            # print >> sys.stderr, len(data), len(compressed), len(flushed)
-            write_chunk(outfile, 'IDAT', compressed + flushed)
-        # http://www.w3.org/TR/PNG/#11IEND
-        write_chunk(outfile, 'IEND')
-        return i+1
-
-    def write_array(self, outfile, pixels):
-        """
-        Write an array in flat row flat pixel format as a PNG file on
-        the output file.  See also :meth:`write` method.
-        """
-
-        if self.interlace:
-            self.write_passes(outfile, self.array_scanlines_interlace(pixels))
-        else:
-            self.write_passes(outfile, self.array_scanlines(pixels))
-
-    def write_packed(self, outfile, rows):
-        """
-        Write PNG file to `outfile`.  The pixel data comes from `rows`
-        which should be in boxed row packed format.  Each row should be
-        a sequence of packed bytes.
-
-        Technically, this method does work for interlaced images but it
-        is best avoided.  For interlaced images, the rows should be
-        presented in the order that they appear in the file.
-
-        This method should not be used when the source image bit depth
-        is not one naturally supported by PNG; the bit depth should be
-        1, 2, 4, 8, or 16.
-        """
-
-        if self.rescale:
-            raise Error("write_packed method not suitable for bit depth %d" %
-              self.rescale[0])
-        return self.write_passes(outfile, rows, packed=True)
-
-    def convert_pnm(self, infile, outfile):
-        """
-        Convert a PNM file containing raw pixel data into a PNG file
-        with the parameters set in the writer object.  Works for
-        (binary) PGM, PPM, and PAM formats.
-        """
-
-        if self.interlace:
-            pixels = array('B')
-            pixels.fromfile(infile,
-                            (self.bitdepth/8) * self.color_planes *
-                            self.width * self.height)
-            self.write_passes(outfile, self.array_scanlines_interlace(pixels))
-        else:
-            self.write_passes(outfile, self.file_scanlines(infile))
-
-    def convert_ppm_and_pgm(self, ppmfile, pgmfile, outfile):
-        """
-        Convert a PPM and PGM file containing raw pixel data into a
-        PNG outfile with the parameters set in the writer object.
-        """
-        pixels = array('B')
-        pixels.fromfile(ppmfile,
-                        (self.bitdepth/8) * self.color_planes *
-                        self.width * self.height)
-        apixels = array('B')
-        apixels.fromfile(pgmfile,
-                         (self.bitdepth/8) *
-                         self.width * self.height)
-        pixels = interleave_planes(pixels, apixels,
-                                   (self.bitdepth/8) * self.color_planes,
-                                   (self.bitdepth/8))
-        if self.interlace:
-            self.write_passes(outfile, self.array_scanlines_interlace(pixels))
-        else:
-            self.write_passes(outfile, self.array_scanlines(pixels))
-
-    def file_scanlines(self, infile):
-        """
-        Generates boxed rows in flat pixel format, from the input file
-        `infile`.  It assumes that the input file is in a "Netpbm-like"
-        binary format, and is positioned at the beginning of the first
-        pixel.  The number of pixels to read is taken from the image
-        dimensions (`width`, `height`, `planes`) and the number of bytes
-        per value is implied by the image `bitdepth`.
-        """
-
-        # Values per row
-        vpr = self.width * self.planes
-        row_bytes = vpr
-        if self.bitdepth > 8:
-            assert self.bitdepth == 16
-            row_bytes *= 2
-            fmt = '>%dH' % vpr
-            def line():
-                return array('H', struct.unpack(fmt, infile.read(row_bytes)))
-        else:
-            def line():
-                scanline = array('B', infile.read(row_bytes))
-                return scanline
-        for y in range(self.height):
-            yield line()
-
-    def array_scanlines(self, pixels):
-        """
-        Generates boxed rows (flat pixels) from flat rows (flat pixels)
-        in an array.
-        """
-
-        # Values per row
-        vpr = self.width * self.planes
-        stop = 0
-        for y in range(self.height):
-            start = stop
-            stop = start + vpr
-            yield pixels[start:stop]
-
-    def array_scanlines_interlace(self, pixels):
-        """
-        Generator for interlaced scanlines from an array.  `pixels` is
-        the full source image in flat row flat pixel format.  The
-        generator yields each scanline of the reduced passes in turn, in
-        boxed row flat pixel format.
-        """
-
-        # http://www.w3.org/TR/PNG/#8InterlaceMethods
-        # Array type.
-        fmt = 'BH'[self.bitdepth > 8]
-        # Value per row
-        vpr = self.width * self.planes
-        for xstart, ystart, xstep, ystep in _adam7:
-            if xstart >= self.width:
-                continue
-            # Pixels per row (of reduced image)
-            ppr = int(math.ceil((self.width-xstart)/float(xstep)))
-            # number of values in reduced image row.
-            row_len = ppr*self.planes
-            for y in range(ystart, self.height, ystep):
-                if xstep == 1:
-                    offset = y * vpr
-                    yield pixels[offset:offset+vpr]
-                else:
-                    row = array(fmt)
-                    # There's no easier way to set the length of an array
-                    row.extend(pixels[0:row_len])
-                    offset = y * vpr + xstart * self.planes
-                    end_offset = (y+1) * vpr
-                    skip = self.planes * xstep
-                    for i in range(self.planes):
-                        row[i::self.planes] = \
-                            pixels[offset+i:end_offset:skip]
-                    yield row
-
-def write_chunk(outfile, tag, data=strtobytes('')):
-    """
-    Write a PNG chunk to the output file, including length and
-    checksum.
-    """
-
-    # http://www.w3.org/TR/PNG/#5Chunk-layout
-    outfile.write(struct.pack("!I", len(data)))
-    tag = strtobytes(tag)
-    outfile.write(tag)
-    outfile.write(data)
-    checksum = zlib.crc32(tag)
-    checksum = zlib.crc32(data, checksum)
-    checksum &= 2**32-1
-    outfile.write(struct.pack("!I", checksum))
-
-def write_chunks(out, chunks):
-    """Create a PNG file by writing out the chunks."""
-
-    out.write(_signature)
-    for chunk in chunks:
-        write_chunk(out, *chunk)
-
-def filter_scanline(type, line, fo, prev=None):
-    """Apply a scanline filter to a scanline.  `type` specifies the
-    filter type (0 to 4); `line` specifies the current (unfiltered)
-    scanline as a sequence of bytes; `prev` specifies the previous
-    (unfiltered) scanline as a sequence of bytes. `fo` specifies the
-    filter offset; normally this is size of a pixel in bytes (the number
-    of bytes per sample times the number of channels), but when this is
-    < 1 (for bit depths < 8) then the filter offset is 1.
-    """
-
-    assert 0 <= type < 5
-
-    # The output array.  Which, pathetically, we extend one-byte at a
-    # time (fortunately this is linear).
-    out = array('B', [type])
-
-    def sub():
-        ai = -fo
-        for x in line:
-            if ai >= 0:
-                x = (x - line[ai]) & 0xff
-            out.append(x)
-            ai += 1
-    def up():
-        for i,x in enumerate(line):
-            x = (x - prev[i]) & 0xff
-            out.append(x)
-    def average():
-        ai = -fo
-        for i,x in enumerate(line):
-            if ai >= 0:
-                x = (x - ((line[ai] + prev[i]) >> 1)) & 0xff
-            else:
-                x = (x - (prev[i] >> 1)) & 0xff
-            out.append(x)
-            ai += 1
-    def paeth():
-        # http://www.w3.org/TR/PNG/#9Filter-type-4-Paeth
-        ai = -fo # also used for ci
-        for i,x in enumerate(line):
-            a = 0
-            b = prev[i]
-            c = 0
-
-            if ai >= 0:
-                a = line[ai]
-                c = prev[ai]
-            p = a + b - c
-            pa = abs(p - a)
-            pb = abs(p - b)
-            pc = abs(p - c)
-            if pa <= pb and pa <= pc: Pr = a
-            elif pb <= pc: Pr = b
-            else: Pr = c
-
-            x = (x - Pr) & 0xff
-            out.append(x)
-            ai += 1
-
-    if not prev:
-        # We're on the first line.  Some of the filters can be reduced
-        # to simpler cases which makes handling the line "off the top"
-        # of the image simpler.  "up" becomes "none"; "paeth" becomes
-        # "left" (non-trivial, but true). "average" needs to be handled
-        # specially.
-        if type == 2: # "up"
-            return line # type = 0
-        elif type == 3:
-            prev = [0]*len(line)
-        elif type == 4: # "paeth"
-            type = 1
-    if type == 0:
-        out.extend(line)
-    elif type == 1:
-        sub()
-    elif type == 2:
-        up()
-    elif type == 3:
-        average()
-    else: # type == 4
-        paeth()
-    return out
-
-
-def from_array(a, mode=None, info={}):
-    """Create a PNG :class:`Image` object from a 2- or 3-dimensional array.
-    One application of this function is easy PIL-style saving:
-    ``png.from_array(pixels, 'L').save('foo.png')``.
-
-    .. note :
-
-      The use of the term *3-dimensional* is for marketing purposes
-      only.  It doesn't actually work.  Please bear with us.  Meanwhile
-      enjoy the complimentary snacks (on request) and please use a
-      2-dimensional array.
-    
-    Unless they are specified using the *info* parameter, the PNG's
-    height and width are taken from the array size.  For a 3 dimensional
-    array the first axis is the height; the second axis is the width;
-    and the third axis is the channel number.  Thus an RGB image that is
-    16 pixels high and 8 wide will use an array that is 16x8x3.  For 2
-    dimensional arrays the first axis is the height, but the second axis
-    is ``width*channels``, so an RGB image that is 16 pixels high and 8
-    wide will use a 2-dimensional array that is 16x24 (each row will be
-    8*3==24 sample values).
-
-    *mode* is a string that specifies the image colour format in a
-    PIL-style mode.  It can be:
-
-    ``'L'``
-      greyscale (1 channel)
-    ``'LA'``
-      greyscale with alpha (2 channel)
-    ``'RGB'``
-      colour image (3 channel)
-    ``'RGBA'``
-      colour image with alpha (4 channel)
-
-    The mode string can also specify the bit depth (overriding how this
-    function normally derives the bit depth, see below).  Appending
-    ``';16'`` to the mode will cause the PNG to be 16 bits per channel;
-    any decimal from 1 to 16 can be used to specify the bit depth.
-
-    When a 2-dimensional array is used *mode* determines how many
-    channels the image has, and so allows the width to be derived from
-    the second array dimension.
-
-    The array is expected to be a ``numpy`` array, but it can be any
-    suitable Python sequence.  For example, a list of lists can be used:
-    ``png.from_array([[0, 255, 0], [255, 0, 255]], 'L')``.  The exact
-    rules are: ``len(a)`` gives the first dimension, height;
-    ``len(a[0])`` gives the second dimension; ``len(a[0][0])`` gives the
-    third dimension, unless an exception is raised in which case a
-    2-dimensional array is assumed.  It's slightly more complicated than
-    that because an iterator of rows can be used, and it all still
-    works.  Using an iterator allows data to be streamed efficiently.
-
-    The bit depth of the PNG is normally taken from the array element's
-    datatype (but if *mode* specifies a bitdepth then that is used
-    instead).  The array element's datatype is determined in a way which
-    is supposed to work both for ``numpy`` arrays and for Python
-    ``array.array`` objects.  A 1 byte datatype will give a bit depth of
-    8, a 2 byte datatype will give a bit depth of 16.  If the datatype
-    does not have an implicit size, for example it is a plain Python
-    list of lists, as above, then a default of 8 is used.
-
-    The *info* parameter is a dictionary that can be used to specify
-    metadata (in the same style as the arguments to the
-    :class:``png.Writer`` class).  For this function the keys that are
-    useful are:
-    
-    height
-      overrides the height derived from the array dimensions and allows
-      *a* to be an iterable.
-    width
-      overrides the width derived from the array dimensions.
-    bitdepth
-      overrides the bit depth derived from the element datatype (but
-      must match *mode* if that also specifies a bit depth).
-
-    Generally anything specified in the
-    *info* dictionary will override any implicit choices that this
-    function would otherwise make, but must match any explicit ones.
-    For example, if the *info* dictionary has a ``greyscale`` key then
-    this must be true when mode is ``'L'`` or ``'LA'`` and false when
-    mode is ``'RGB'`` or ``'RGBA'``.
-    """
-
-    # We abuse the *info* parameter by modifying it.  Take a copy here.
-    # (Also typechecks *info* to some extent).
-    info = dict(info)
-
-    # Syntax check mode string.
-    bitdepth = None
-    try:
-        mode = mode.split(';')
-        if len(mode) not in (1,2):
-            raise Error()
-        if mode[0] not in ('L', 'LA', 'RGB', 'RGBA'):
-            raise Error()
-        if len(mode) == 2:
-            try:
-                bitdepth = int(mode[1])
-            except:
-                raise Error()
-    except Error:
-        raise Error("mode string should be 'RGB' or 'L;16' or similar.")
-    mode = mode[0]
-
-    # Get bitdepth from *mode* if possible.
-    if bitdepth:
-        if info.get('bitdepth') and bitdepth != info['bitdepth']:
-            raise Error("mode bitdepth (%d) should match info bitdepth (%d)." %
-              (bitdepth, info['bitdepth']))
-        info['bitdepth'] = bitdepth
-
-    # Fill in and/or check entries in *info*.
-    # Dimensions.
-    if 'size' in info:
-        # Check width, height, size all match where used.
-        for dimension,axis in [('width', 0), ('height', 1)]:
-            if dimension in info:
-                if info[dimension] != info['size'][axis]:
-                    raise Error(
-                      "info[%r] shhould match info['size'][%r]." %
-                      (dimension, axis))
-        info['width'],info['height'] = info['size']
-    if 'height' not in info:
-        try:
-            l = len(a)
-        except:
-            raise Error(
-              "len(a) does not work, supply info['height'] instead.")
-        info['height'] = l
-    # Colour format.
-    if 'greyscale' in info:
-        if bool(info['greyscale']) != ('L' in mode):
-            raise Error("info['greyscale'] should match mode.")
-    info['greyscale'] = 'L' in mode
-    if 'alpha' in info:
-        if bool(info['alpha']) != ('A' in mode):
-            raise Error("info['alpha'] should match mode.")
-    info['alpha'] = 'A' in mode
-
-    planes = len(mode)
-    if 'planes' in info:
-        if info['planes'] != planes:
-            raise Error("info['planes'] should match mode.")
-
-    # In order to work out whether we the array is 2D or 3D we need its
-    # first row, which requires that we take a copy of its iterator.
-    # We may also need the first row to derive width and bitdepth.
-    a,t = itertools.tee(a)
-    row = t.next()
-    del t
-    try:
-        row[0][0]
-        threed = True
-        testelement = row[0]
-    except:
-        threed = False
-        testelement = row
-    if 'width' not in info:
-        if threed:
-            width = len(row)
-        else:
-            width = len(row) // planes
-        info['width'] = width
-
-    # Not implemented yet
-    assert not threed
-
-    if 'bitdepth' not in info:
-        try:
-            dtype = testelement.dtype
-            # goto the "else:" clause.  Sorry.
-        except:
-            try:
-                # Try a Python array.array.
-                bitdepth = 8 * testelement.itemsize
-            except:
-                # We can't determine it from the array element's
-                # datatype, use a default of 8.
-                bitdepth = 8
-        else:
-            # If we got here without exception, we now assume that
-            # the array is a numpy array.
-            if dtype.kind == 'b':
-                bitdepth = 1
-            else:
-                bitdepth = 8 * dtype.itemsize
-        info['bitdepth'] = bitdepth
-
-    for thing in 'width height bitdepth greyscale alpha'.split():
-        assert thing in info
-    return Image(a, info)
-
-# So that refugee's from PIL feel more at home.  Not documented.
-fromarray = from_array
-
-class Image:
-    """A PNG image.
-    You can create an :class:`Image` object from an array of pixels by calling
-    :meth:`png.from_array`.  It can be saved to disk with the
-    :meth:`save` method."""
-    def __init__(self, rows, info):
-        """
-        .. note ::
-        
-          The constructor is not public.  Please do not call it.
-        """
-        
-        self.rows = rows
-        self.info = info
-
-    def save(self, file):
-        """Save the image to *file*.  If *file* looks like an open file
-        descriptor then it is used, otherwise it is treated as a
-        filename and a fresh file is opened.
-
-        In general, you can only call this method once; after it has
-        been called the first time and the PNG image has been saved, the
-        source data will have been streamed, and cannot be streamed
-        again.
-        """
-
-        w = Writer(**self.info)
-
-        try:
-            file.write
-            def close(): pass
-        except:
-            file = open(file, 'wb')
-            def close(): file.close()
-
-        try:
-            w.write(file, self.rows)
-        finally:
-            close()
-
-class _readable:
-    """
-    A simple file-like interface for strings and arrays.
-    """
-
-    def __init__(self, buf):
-        self.buf = buf
-        self.offset = 0
-
-    def read(self, n):
-        r = self.buf[self.offset:self.offset+n]
-        if isarray(r):
-            r = r.tostring()
-        self.offset += n
-        return r
-
-
-class Reader:
-    """
-    PNG decoder in pure Python.
-    """
-
-    def __init__(self, _guess=None, **kw):
-        """
-        Create a PNG decoder object.
-
-        The constructor expects exactly one keyword argument. If you
-        supply a positional argument instead, it will guess the input
-        type. You can choose among the following keyword arguments:
-
-        filename
-          Name of input file (a PNG file).
-        file
-          A file-like object (object with a read() method).
-        bytes
-          ``array`` or ``string`` with PNG data.
-
-        """
-        if ((_guess is not None and len(kw) != 0) or
-            (_guess is None and len(kw) != 1)):
-            raise TypeError("Reader() takes exactly 1 argument")
-
-        # Will be the first 8 bytes, later on.  See validate_signature.
-        self.signature = None
-        self.transparent = None
-        # A pair of (len,type) if a chunk has been read but its data and
-        # checksum have not (in other words the file position is just
-        # past the 4 bytes that specify the chunk type).  See preamble
-        # method for how this is used.
-        self.atchunk = None
-
-        if _guess is not None:
-            if isarray(_guess):
-                kw["bytes"] = _guess
-            elif isinstance(_guess, str):
-                kw["filename"] = _guess
-            elif isinstance(_guess, file):
-                kw["file"] = _guess
-
-        if "filename" in kw:
-            self.file = open(kw["filename"], "rb")
-        elif "file" in kw:
-            self.file = kw["file"]
-        elif "bytes" in kw:
-            self.file = _readable(kw["bytes"])
-        else:
-            raise TypeError("expecting filename, file or bytes array")
-
-
-    def chunk(self, seek=None, lenient=False):
-        """
-        Read the next PNG chunk from the input file; returns a
-        (*type*,*data*) tuple.  *type* is the chunk's type as a string
-        (all PNG chunk types are 4 characters long).  *data* is the
-        chunk's data content, as a string.
-
-        If the optional `seek` argument is
-        specified then it will keep reading chunks until it either runs
-        out of file or finds the type specified by the argument.  Note
-        that in general the order of chunks in PNGs is unspecified, so
-        using `seek` can cause you to miss chunks.
-
-        If the optional `lenient` argument evaluates to True,
-        checksum failures will raise warnings rather than exceptions.
-        """
-
-        self.validate_signature()
-
-        while True:
-            # http://www.w3.org/TR/PNG/#5Chunk-layout
-            if not self.atchunk:
-                self.atchunk = self.chunklentype()
-            length,type = self.atchunk
-            self.atchunk = None
-            data = self.file.read(length)
-            if len(data) != length:
-                raise ChunkError('Chunk %s too short for required %i octets.'
-                  % (type, length))
-            checksum = self.file.read(4)
-            if len(checksum) != 4:
-                raise ValueError('Chunk %s too short for checksum.', tag)
-            if seek and type != seek:
-                continue
-            verify = zlib.crc32(strtobytes(type))
-            verify = zlib.crc32(data, verify)
-            # Whether the output from zlib.crc32 is signed or not varies
-            # according to hideous implementation details, see
-            # http://bugs.python.org/issue1202 .
-            # We coerce it to be positive here (in a way which works on
-            # Python 2.3 and older).
-            verify &= 2**32 - 1
-            verify = struct.pack('!I', verify)
-            if checksum != verify:
-                # print repr(checksum)
-                (a, ) = struct.unpack('!I', checksum)
-                (b, ) = struct.unpack('!I', verify)
-                message = "Checksum error in %s chunk: 0x%08X != 0x%08X." % (type, a, b)
-                if lenient:
-                    warnings.warn(message, RuntimeWarning)
-                else:
-                    raise ChunkError(message)
-            return type, data
-
-    def chunks(self):
-        """Return an iterator that will yield each chunk as a
-        (*chunktype*, *content*) pair.
-        """
-
-        while True:
-            t,v = self.chunk()
-            yield t,v
-            if t == 'IEND':
-                break
-
-    def undo_filter(self, filter_type, scanline, previous):
-        """Undo the filter for a scanline.  `scanline` is a sequence of
-        bytes that does not include the initial filter type byte.
-        `previous` is decoded previous scanline (for straightlaced
-        images this is the previous pixel row, but for interlaced
-        images, it is the previous scanline in the reduced image, which
-        in general is not the previous pixel row in the final image).
-        When there is no previous scanline (the first row of a
-        straightlaced image, or the first row in one of the passes in an
-        interlaced image), then this argument should be ``None``.
-
-        The scanline will have the effects of filtering removed, and the
-        result will be returned as a fresh sequence of bytes.
-        """
-
-        # :todo: Would it be better to update scanline in place?
-        # Yes, with the Cython extension making the undo_filter fast,
-        # updating scanline inplace makes the code 3 times faster
-        # (reading 50 images of 800x800 went from 40s to 16s)
-        result = scanline
-
-        if filter_type == 0:
-            return result
-
-        if filter_type not in (1,2,3,4):
-            raise FormatError('Invalid PNG Filter Type.'
-              '  See http://www.w3.org/TR/2003/REC-PNG-20031110/#9Filters .')
-
-        # Filter unit.  The stride from one pixel to the corresponding
-        # byte from the previous previous.  Normally this is the pixel
-        # size in bytes, but when this is smaller than 1, the previous
-        # byte is used instead.
-        fu = max(1, self.psize)
-
-        # For the first line of a pass, synthesize a dummy previous
-        # line.  An alternative approach would be to observe that on the
-        # first line 'up' is the same as 'null', 'paeth' is the same
-        # as 'sub', with only 'average' requiring any special case.
-        if not previous:
-            previous = array('B', [0]*len(scanline))
-
-        def sub():
-            """Undo sub filter."""
-
-            ai = 0
-            # Loops starts at index fu.  Observe that the initial part
-            # of the result is already filled in correctly with
-            # scanline.
-            for i in range(fu, len(result)):
-                x = scanline[i]
-                a = result[ai]
-                result[i] = (x + a) & 0xff
-                ai += 1
-
-        def up():
-            """Undo up filter."""
-
-            for i in range(len(result)):
-                x = scanline[i]
-                b = previous[i]
-                result[i] = (x + b) & 0xff
-
-        def average():
-            """Undo average filter."""
-
-            ai = -fu
-            for i in range(len(result)):
-                x = scanline[i]
-                if ai < 0:
-                    a = 0
-                else:
-                    a = result[ai]
-                b = previous[i]
-                result[i] = (x + ((a + b) >> 1)) & 0xff
-                ai += 1
-
-        def paeth():
-            """Undo Paeth filter."""
-
-            # Also used for ci.
-            ai = -fu
-            for i in range(len(result)):
-                x = scanline[i]
-                if ai < 0:
-                    a = c = 0
-                else:
-                    a = result[ai]
-                    c = previous[ai]
-                b = previous[i]
-                p = a + b - c
-                pa = abs(p - a)
-                pb = abs(p - b)
-                pc = abs(p - c)
-                if pa <= pb and pa <= pc:
-                    pr = a
-                elif pb <= pc:
-                    pr = b
-                else:
-                    pr = c
-                result[i] = (x + pr) & 0xff
-                ai += 1
-
-        # Call appropriate filter algorithm.  Note that 0 has already
-        # been dealt with.
-        (None,
-         pngfilters.undo_filter_sub,
-         pngfilters.undo_filter_up,
-         pngfilters.undo_filter_average,
-         pngfilters.undo_filter_paeth)[filter_type](fu, scanline, previous, result)
-        return result
-
-    def deinterlace(self, raw):
-        """
-        Read raw pixel data, undo filters, deinterlace, and flatten.
-        Return in flat row flat pixel format.
-        """
-
-        # print >> sys.stderr, ("Reading interlaced, w=%s, r=%s, planes=%s," +
-        #     " bpp=%s") % (self.width, self.height, self.planes, self.bps)
-        # Values per row (of the target image)
-        vpr = self.width * self.planes
-
-        # Make a result array, and make it big enough.  Interleaving
-        # writes to the output array randomly (well, not quite), so the
-        # entire output array must be in memory.
-        fmt = 'BH'[self.bitdepth > 8]
-        a = array(fmt, [0]*vpr*self.height)
-        source_offset = 0
-
-        for xstart, ystart, xstep, ystep in _adam7:
-            # print >> sys.stderr, "Adam7: start=%s,%s step=%s,%s" % (
-            #     xstart, ystart, xstep, ystep)
-            if xstart >= self.width:
-                continue
-            # The previous (reconstructed) scanline.  None at the
-            # beginning of a pass to indicate that there is no previous
-            # line.
-            recon = None
-            # Pixels per row (reduced pass image)
-            ppr = int(math.ceil((self.width-xstart)/float(xstep)))
-            # Row size in bytes for this pass.
-            row_size = int(math.ceil(self.psize * ppr))
-            for y in range(ystart, self.height, ystep):
-                filter_type = raw[source_offset]
-                source_offset += 1
-                scanline = raw[source_offset:source_offset+row_size]
-                source_offset += row_size
-                recon = self.undo_filter(filter_type, scanline, recon)
-                # Convert so that there is one element per pixel value
-                flat = self.serialtoflat(recon, ppr)
-                if xstep == 1:
-                    assert xstart == 0
-                    offset = y * vpr
-                    a[offset:offset+vpr] = flat
-                else:
-                    offset = y * vpr + xstart * self.planes
-                    end_offset = (y+1) * vpr
-                    skip = self.planes * xstep
-                    for i in range(self.planes):
-                        a[offset+i:end_offset:skip] = \
-                            flat[i::self.planes]
-        return a
-
-    def iterboxed(self, rows):
-        """Iterator that yields each scanline in boxed row flat pixel
-        format.  `rows` should be an iterator that yields the bytes of
-        each row in turn.
-        """
-
-        def asvalues(raw):
-            """Convert a row of raw bytes into a flat row.  Result may
-            or may not share with argument"""
-
-            if self.bitdepth == 8:
-                return raw
-            if self.bitdepth == 16:
-                raw = tostring(raw)
-                return array('H', struct.unpack('!%dH' % (len(raw)//2), raw))
-            assert self.bitdepth < 8
-            width = self.width
-            # Samples per byte
-            spb = 8//self.bitdepth
-            out = array('B')
-            mask = 2**self.bitdepth - 1
-            shifts = map(self.bitdepth.__mul__, reversed(range(spb)))
-            for o in raw:
-                out.extend(map(lambda i: mask&(o>>i), shifts))
-            return out[:width]
-
-        return itertools.imap(asvalues, rows)
-
-    def serialtoflat(self, bytes, width=None):
-        """Convert serial format (byte stream) pixel data to flat row
-        flat pixel.
-        """
-
-        if self.bitdepth == 8:
-            return bytes
-        if self.bitdepth == 16:
-            bytes = tostring(bytes)
-            return array('H',
-              struct.unpack('!%dH' % (len(bytes)//2), bytes))
-        assert self.bitdepth < 8
-        if width is None:
-            width = self.width
-        # Samples per byte
-        spb = 8//self.bitdepth
-        out = array('B')
-        mask = 2**self.bitdepth - 1
-        shifts = map(self.bitdepth.__mul__, reversed(range(spb)))
-        l = width
-        for o in bytes:
-            out.extend([(mask&(o>>s)) for s in shifts][:l])
-            l -= spb
-            if l <= 0:
-                l = width
-        return out
-
-    def iterstraight(self, raw):
-        """Iterator that undoes the effect of filtering, and yields each
-        row in serialised format (as a sequence of bytes).  Assumes input
-        is straightlaced.  `raw` should be an iterable that yields the
-        raw bytes in chunks of arbitrary size."""
-
-        # length of row, in bytes
-        rb = self.row_bytes
-        a = array('B')
-        # The previous (reconstructed) scanline.  None indicates first
-        # line of image.
-        recon = None
-        for some in raw:
-            a.extend(some)
-            while len(a) >= rb + 1:
-                filter_type = a[0]
-                scanline = a[1:rb+1]
-                del a[:rb+1]
-                recon = self.undo_filter(filter_type, scanline, recon)
-                yield recon
-        if len(a) != 0:
-            # :file:format We get here with a file format error: when the
-            # available bytes (after decompressing) do not pack into exact
-            # rows.
-            raise FormatError(
-              'Wrong size for decompressed IDAT chunk.')
-        assert len(a) == 0
-
-    def validate_signature(self):
-        """If signature (header) has not been read then read and
-        validate it; otherwise do nothing.
-        """
-
-        if self.signature:
-            return
-        self.signature = self.file.read(8)
-        if self.signature != _signature:
-            raise FormatError("PNG file has invalid signature.")
-
-    def preamble(self, lenient=False):
-        """
-        Extract the image metadata by reading the initial part of the PNG
-        file up to the start of the ``IDAT`` chunk.  All the chunks that
-        precede the ``IDAT`` chunk are read and either processed for
-        metadata or discarded.
-
-        If the optional `lenient` argument evaluates to True,
-        checksum failures will raise warnings rather than exceptions.
-        """
-
-        self.validate_signature()
-
-        while True:
-            if not self.atchunk:
-                self.atchunk = self.chunklentype()
-                if self.atchunk is None:
-                    raise FormatError(
-                      'This PNG file has no IDAT chunks.')
-            if self.atchunk[1] == 'IDAT':
-                return
-            self.process_chunk(lenient=lenient)
-
-    def chunklentype(self):
-        """Reads just enough of the input to determine the next
-        chunk's length and type, returned as a (*length*, *type*) pair
-        where *type* is a string.  If there are no more chunks, ``None``
-        is returned.
-        """
-
-        x = self.file.read(8)
-        if not x:
-            return None
-        if len(x) != 8:
-            raise FormatError(
-              'End of file whilst reading chunk length and type.')
-        length,type = struct.unpack('!I4s', x)
-        type = bytestostr(type)
-        if length > 2**31-1:
-            raise FormatError('Chunk %s is too large: %d.' % (type,length))
-        return length,type
-
-    def process_chunk(self, lenient=False):
-        """Process the next chunk and its data.  This only processes the
-        following chunk types, all others are ignored: ``IHDR``,
-        ``PLTE``, ``bKGD``, ``tRNS``, ``gAMA``, ``sBIT``.
-
-        If the optional `lenient` argument evaluates to True,
-        checksum failures will raise warnings rather than exceptions.
-        """
-
-        type, data = self.chunk(lenient=lenient)
-        if type == 'IHDR':
-            # http://www.w3.org/TR/PNG/#11IHDR
-            if len(data) != 13:
-                raise FormatError('IHDR chunk has incorrect length.')
-            (self.width, self.height, self.bitdepth, self.color_type,
-             self.compression, self.filter,
-             self.interlace) = struct.unpack("!2I5B", data)
-
-            # Check that the header specifies only valid combinations.
-            if self.bitdepth not in (1,2,4,8,16):
-                raise Error("invalid bit depth %d" % self.bitdepth)
-            if self.color_type not in (0,2,3,4,6):
-                raise Error("invalid colour type %d" % self.color_type)
-            # Check indexed (palettized) images have 8 or fewer bits
-            # per pixel; check only indexed or greyscale images have
-            # fewer than 8 bits per pixel.
-            if ((self.color_type & 1 and self.bitdepth > 8) or
-                (self.bitdepth < 8 and self.color_type not in (0,3))):
-                raise FormatError("Illegal combination of bit depth (%d)"
-                  " and colour type (%d)."
-                  " See http://www.w3.org/TR/2003/REC-PNG-20031110/#table111 ."
-                  % (self.bitdepth, self.color_type))
-            if self.compression != 0:
-                raise Error("unknown compression method %d" % self.compression)
-            if self.filter != 0:
-                raise FormatError("Unknown filter method %d,"
-                  " see http://www.w3.org/TR/2003/REC-PNG-20031110/#9Filters ."
-                  % self.filter)
-            if self.interlace not in (0,1):
-                raise FormatError("Unknown interlace method %d,"
-                  " see http://www.w3.org/TR/2003/REC-PNG-20031110/#8InterlaceMethods ."
-                  % self.interlace)
-
-            # Derived values
-            # http://www.w3.org/TR/PNG/#6Colour-values
-            colormap =  bool(self.color_type & 1)
-            greyscale = not (self.color_type & 2)
-            alpha = bool(self.color_type & 4)
-            color_planes = (3,1)[greyscale or colormap]
-            planes = color_planes + alpha
-
-            self.colormap = colormap
-            self.greyscale = greyscale
-            self.alpha = alpha
-            self.color_planes = color_planes
-            self.planes = planes
-            self.psize = float(self.bitdepth)/float(8) * planes
-            if int(self.psize) == self.psize:
-                self.psize = int(self.psize)
-            self.row_bytes = int(math.ceil(self.width * self.psize))
-            # Stores PLTE chunk if present, and is used to check
-            # chunk ordering constraints.
-            self.plte = None
-            # Stores tRNS chunk if present, and is used to check chunk
-            # ordering constraints.
-            self.trns = None
-            # Stores sbit chunk if present.
-            self.sbit = None
-        elif type == 'PLTE':
-            # http://www.w3.org/TR/PNG/#11PLTE
-            if self.plte:
-                warnings.warn("Multiple PLTE chunks present.")
-            self.plte = data
-            if len(data) % 3 != 0:
-                raise FormatError(
-                  "PLTE chunk's length should be a multiple of 3.")
-            if len(data) > (2**self.bitdepth)*3:
-                raise FormatError("PLTE chunk is too long.")
-            if len(data) == 0:
-                raise FormatError("Empty PLTE is not allowed.")
-        elif type == 'bKGD':
-            try:
-                if self.colormap:
-                    if not self.plte:
-                        warnings.warn(
-                          "PLTE chunk is required before bKGD chunk.")
-                    self.background = struct.unpack('B', data)
-                else:
-                    self.background = struct.unpack("!%dH" % self.color_planes,
-                      data)
-            except struct.error:
-                raise FormatError("bKGD chunk has incorrect length.")
-        elif type == 'tRNS':
-            # http://www.w3.org/TR/PNG/#11tRNS
-            self.trns = data
-            if self.colormap:
-                if not self.plte:
-                    warnings.warn("PLTE chunk is required before tRNS chunk.")
-                else:
-                    if len(data) > len(self.plte)/3:
-                        # Was warning, but promoted to Error as it
-                        # would otherwise cause pain later on.
-                        raise FormatError("tRNS chunk is too long.")
-            else:
-                if self.alpha:
-                    raise FormatError(
-                      "tRNS chunk is not valid with colour type %d." %
-                      self.color_type)
-                try:
-                    self.transparent = \
-                        struct.unpack("!%dH" % self.color_planes, data)
-                except struct.error:
-                    raise FormatError("tRNS chunk has incorrect length.")
-        elif type == 'gAMA':
-            try:
-                self.gamma = struct.unpack("!L", data)[0] / 100000.0
-            except struct.error:
-                raise FormatError("gAMA chunk has incorrect length.")
-        elif type == 'sBIT':
-            self.sbit = data
-            if (self.colormap and len(data) != 3 or
-                not self.colormap and len(data) != self.planes):
-                raise FormatError("sBIT chunk has incorrect length.")
-
-    def read(self, lenient=False):
-        """
-        Read the PNG file and decode it.  Returns (`width`, `height`,
-        `pixels`, `metadata`).
-
-        May use excessive memory.
-
-        `pixels` are returned in boxed row flat pixel format.
-
-        If the optional `lenient` argument evaluates to True,
-        checksum failures will raise warnings rather than exceptions.
-        """
-
-        def iteridat():
-            """Iterator that yields all the ``IDAT`` chunks as strings."""
-            while True:
-                try:
-                    type, data = self.chunk(lenient=lenient)
-                except ValueError, e:
-                    raise ChunkError(e.args[0])
-                if type == 'IEND':
-                    # http://www.w3.org/TR/PNG/#11IEND
-                    break
-                if type != 'IDAT':
-                    continue
-                # type == 'IDAT'
-                # http://www.w3.org/TR/PNG/#11IDAT
-                if self.colormap and not self.plte:
-                    warnings.warn("PLTE chunk is required before IDAT chunk")
-                yield data
-
-        def iterdecomp(idat):
-            """Iterator that yields decompressed strings.  `idat` should
-            be an iterator that yields the ``IDAT`` chunk data.
-            """
-
-            # Currently, with no max_length paramter to decompress, this
-            # routine will do one yield per IDAT chunk.  So not very
-            # incremental.
-            d = zlib.decompressobj()
-            # Each IDAT chunk is passed to the decompressor, then any
-            # remaining state is decompressed out.
-            for data in idat:
-                # :todo: add a max_length argument here to limit output
-                # size.
-                yield array('B', d.decompress(data))
-            yield array('B', d.flush())
-
-        self.preamble(lenient=lenient)
-        raw = iterdecomp(iteridat())
-
-        if self.interlace:
-            raw = array('B', itertools.chain(*raw))
-            arraycode = 'BH'[self.bitdepth>8]
-            # Like :meth:`group` but producing an array.array object for
-            # each row.
-            pixels = itertools.imap(lambda *row: array(arraycode, row),
-                       *[iter(self.deinterlace(raw))]*self.width*self.planes)
-        else:
-            pixels = self.iterboxed(self.iterstraight(raw))
-        meta = dict()
-        for attr in 'greyscale alpha planes bitdepth interlace'.split():
-            meta[attr] = getattr(self, attr)
-        meta['size'] = (self.width, self.height)
-        for attr in 'gamma transparent background'.split():
-            a = getattr(self, attr, None)
-            if a is not None:
-                meta[attr] = a
-        if self.plte:
-            meta['palette'] = self.palette()
-        return self.width, self.height, pixels, meta
-
-
-    def read_flat(self):
-        """
-        Read a PNG file and decode it into flat row flat pixel format.
-        Returns (*width*, *height*, *pixels*, *metadata*).
-
-        May use excessive memory.
-
-        `pixels` are returned in flat row flat pixel format.
-
-        See also the :meth:`read` method which returns pixels in the
-        more stream-friendly boxed row flat pixel format.
-        """
-
-        x, y, pixel, meta = self.read()
-        arraycode = 'BH'[meta['bitdepth']>8]
-        pixel = array(arraycode, itertools.chain(*pixel))
-        return x, y, pixel, meta
-
-    def palette(self, alpha='natural'):
-        """Returns a palette that is a sequence of 3-tuples or 4-tuples,
-        synthesizing it from the ``PLTE`` and ``tRNS`` chunks.  These
-        chunks should have already been processed (for example, by
-        calling the :meth:`preamble` method).  All the tuples are the
-        same size: 3-tuples if there is no ``tRNS`` chunk, 4-tuples when
-        there is a ``tRNS`` chunk.  Assumes that the image is colour type
-        3 and therefore a ``PLTE`` chunk is required.
-
-        If the `alpha` argument is ``'force'`` then an alpha channel is
-        always added, forcing the result to be a sequence of 4-tuples.
-        """
-
-        if not self.plte:
-            raise FormatError(
-                "Required PLTE chunk is missing in colour type 3 image.")
-        plte = group(array('B', self.plte), 3)
-        if self.trns or alpha == 'force':
-            trns = array('B', self.trns or '')
-            trns.extend([255]*(len(plte)-len(trns)))
-            plte = map(operator.add, plte, group(trns, 1))
-        return plte
-
-    def asDirect(self):
-        """Returns the image data as a direct representation of an
-        ``x * y * planes`` array.  This method is intended to remove the
-        need for callers to deal with palettes and transparency
-        themselves.  Images with a palette (colour type 3)
-        are converted to RGB or RGBA; images with transparency (a
-        ``tRNS`` chunk) are converted to LA or RGBA as appropriate.
-        When returned in this format the pixel values represent the
-        colour value directly without needing to refer to palettes or
-        transparency information.
-
-        Like the :meth:`read` method this method returns a 4-tuple:
-
-        (*width*, *height*, *pixels*, *meta*)
-
-        This method normally returns pixel values with the bit depth
-        they have in the source image, but when the source PNG has an
-        ``sBIT`` chunk it is inspected and can reduce the bit depth of
-        the result pixels; pixel values will be reduced according to
-        the bit depth specified in the ``sBIT`` chunk (PNG nerds should
-        note a single result bit depth is used for all channels; the
-        maximum of the ones specified in the ``sBIT`` chunk.  An RGB565
-        image will be rescaled to 6-bit RGB666).
-
-        The *meta* dictionary that is returned reflects the `direct`
-        format and not the original source image.  For example, an RGB
-        source image with a ``tRNS`` chunk to represent a transparent
-        colour, will have ``planes=3`` and ``alpha=False`` for the
-        source image, but the *meta* dictionary returned by this method
-        will have ``planes=4`` and ``alpha=True`` because an alpha
-        channel is synthesized and added.
-
-        *pixels* is the pixel data in boxed row flat pixel format (just
-        like the :meth:`read` method).
-
-        All the other aspects of the image data are not changed.
-        """
-
-        self.preamble()
-
-        # Simple case, no conversion necessary.
-        if not self.colormap and not self.trns and not self.sbit:
-            return self.read()
-
-        x,y,pixels,meta = self.read()
-
-        if self.colormap:
-            meta['colormap'] = False
-            meta['alpha'] = bool(self.trns)
-            meta['bitdepth'] = 8
-            meta['planes'] = 3 + bool(self.trns)
-            plte = self.palette()
-            def iterpal(pixels):
-                for row in pixels:
-                    row = map(plte.__getitem__, row)
-                    yield array('B', itertools.chain(*row))
-            pixels = iterpal(pixels)
-        elif self.trns:
-            # It would be nice if there was some reasonable way of doing
-            # this without generating a whole load of intermediate tuples.
-            # But tuples does seem like the easiest way, with no other way
-            # clearly much simpler or much faster.  (Actually, the L to LA
-            # conversion could perhaps go faster (all those 1-tuples!), but
-            # I still wonder whether the code proliferation is worth it)
-            it = self.transparent
-            maxval = 2**meta['bitdepth']-1
-            planes = meta['planes']
-            meta['alpha'] = True
-            meta['planes'] += 1
-            typecode = 'BH'[meta['bitdepth']>8]
-            def itertrns(pixels):
-                for row in pixels:
-                    # For each row we group it into pixels, then form a
-                    # characterisation vector that says whether each pixel
-                    # is opaque or not.  Then we convert True/False to
-                    # 0/maxval (by multiplication), and add it as the extra
-                    # channel.
-                    row = group(row, planes)
-                    opa = map(it.__ne__, row)
-                    opa = map(maxval.__mul__, opa)
-                    opa = zip(opa) # convert to 1-tuples
-                    yield array(typecode,
-                      itertools.chain(*map(operator.add, row, opa)))
-            pixels = itertrns(pixels)
-        targetbitdepth = None
-        if self.sbit:
-            sbit = struct.unpack('%dB' % len(self.sbit), self.sbit)
-            targetbitdepth = max(sbit)
-            if targetbitdepth > meta['bitdepth']:
-                raise Error('sBIT chunk %r exceeds bitdepth %d' %
-                    (sbit,self.bitdepth))
-            if min(sbit) <= 0:
-                raise Error('sBIT chunk %r has a 0-entry' % sbit)
-            if targetbitdepth == meta['bitdepth']:
-                targetbitdepth = None
-        if targetbitdepth:
-            shift = meta['bitdepth'] - targetbitdepth
-            meta['bitdepth'] = targetbitdepth
-            def itershift(pixels):
-                for row in pixels:
-                    yield map(shift.__rrshift__, row)
-            pixels = itershift(pixels)
-        return x,y,pixels,meta
-
-    def asFloat(self, maxval=1.0):
-        """Return image pixels as per :meth:`asDirect` method, but scale
-        all pixel values to be floating point values between 0.0 and
-        *maxval*.
-        """
-
-        x,y,pixels,info = self.asDirect()
-        sourcemaxval = 2**info['bitdepth']-1
-        del info['bitdepth']
-        info['maxval'] = float(maxval)
-        factor = float(maxval)/float(sourcemaxval)
-        def iterfloat():
-            for row in pixels:
-                yield map(factor.__mul__, row)
-        return x,y,iterfloat(),info
-
-    def _as_rescale(self, get, targetbitdepth):
-        """Helper used by :meth:`asRGB8` and :meth:`asRGBA8`."""
-
-        width,height,pixels,meta = get()
-        maxval = 2**meta['bitdepth'] - 1
-        targetmaxval = 2**targetbitdepth - 1
-        factor = float(targetmaxval) / float(maxval)
-        meta['bitdepth'] = targetbitdepth
-        def iterscale():
-            for row in pixels:
-                yield map(lambda x: int(round(x*factor)), row)
-        if maxval == targetmaxval:
-            return width, height, pixels, meta
-        else:
-            return width, height, iterscale(), meta
-
-    def asRGB8(self):
-        """Return the image data as an RGB pixels with 8-bits per
-        sample.  This is like the :meth:`asRGB` method except that
-        this method additionally rescales the values so that they
-        are all between 0 and 255 (8-bit).  In the case where the
-        source image has a bit depth < 8 the transformation preserves
-        all the information; where the source image has bit depth
-        > 8, then rescaling to 8-bit values loses precision.  No
-        dithering is performed.  Like :meth:`asRGB`, an alpha channel
-        in the source image will raise an exception.
-
-        This function returns a 4-tuple:
-        (*width*, *height*, *pixels*, *metadata*).
-        *width*, *height*, *metadata* are as per the :meth:`read` method.
-        
-        *pixels* is the pixel data in boxed row flat pixel format.
-        """
-
-        return self._as_rescale(self.asRGB, 8)
-
-    def asRGBA8(self):
-        """Return the image data as RGBA pixels with 8-bits per
-        sample.  This method is similar to :meth:`asRGB8` and
-        :meth:`asRGBA`:  The result pixels have an alpha channel, *and*
-        values are rescaled to the range 0 to 255.  The alpha channel is
-        synthesized if necessary (with a small speed penalty).
-        """
-
-        return self._as_rescale(self.asRGBA, 8)
-
-    def asRGB(self):
-        """Return image as RGB pixels.  RGB colour images are passed
-        through unchanged; greyscales are expanded into RGB
-        triplets (there is a small speed overhead for doing this).
-
-        An alpha channel in the source image will raise an
-        exception.
-
-        The return values are as for the :meth:`read` method
-        except that the *metadata* reflect the returned pixels, not the
-        source image.  In particular, for this method
-        ``metadata['greyscale']`` will be ``False``.
-        """
-
-        width,height,pixels,meta = self.asDirect()
-        if meta['alpha']:
-            raise Error("will not convert image with alpha channel to RGB")
-        if not meta['greyscale']:
-            return width,height,pixels,meta
-        meta['greyscale'] = False
-        typecode = 'BH'[meta['bitdepth'] > 8]
-        def iterrgb():
-            for row in pixels:
-                a = array(typecode, [0]) * 3 * width
-                for i in range(3):
-                    a[i::3] = row
-                yield a
-        return width,height,iterrgb(),meta
-
-    def asRGBA(self):
-        """Return image as RGBA pixels.  Greyscales are expanded into
-        RGB triplets; an alpha channel is synthesized if necessary.
-        The return values are as for the :meth:`read` method
-        except that the *metadata* reflect the returned pixels, not the
-        source image.  In particular, for this method
-        ``metadata['greyscale']`` will be ``False``, and
-        ``metadata['alpha']`` will be ``True``.
-        """
-
-        width,height,pixels,meta = self.asDirect()
-        if meta['alpha'] and not meta['greyscale']:
-            return width,height,pixels,meta
-        typecode = 'BH'[meta['bitdepth'] > 8]
-        maxval = 2**meta['bitdepth'] - 1
-        maxbuffer = struct.pack('=' + typecode, maxval) * 4 * width
-        def newarray():
-            return array(typecode, maxbuffer)
-
-        if meta['alpha'] and meta['greyscale']:
-            # LA to RGBA
-            def convert():
-                for row in pixels:
-                    # Create a fresh target row, then copy L channel
-                    # into first three target channels, and A channel
-                    # into fourth channel.
-                    a = newarray()
-                    pngfilters.convert_la_to_rgba(row, a)
-                    yield a
-        elif meta['greyscale']:
-            # L to RGBA
-            def convert():
-                for row in pixels:
-                    a = newarray()
-                    pngfilters.convert_l_to_rgba(row, a)
-                    yield a
-        else:
-            assert not meta['alpha'] and not meta['greyscale']
-            # RGB to RGBA
-            def convert():
-                for row in pixels:
-                    a = newarray()
-                    pngfilters.convert_rgb_to_rgba(row, a)
-                    yield a
-        meta['alpha'] = True
-        meta['greyscale'] = False
-        return width,height,convert(),meta
-
-
-# === Legacy Version Support ===
-
-# :pyver:old:  PyPNG works on Python versions 2.3 and 2.2, but not
-# without some awkward problems.  Really PyPNG works on Python 2.4 (and
-# above); it works on Pythons 2.3 and 2.2 by virtue of fixing up
-# problems here.  It's a bit ugly (which is why it's hidden down here).
-#
-# Generally the strategy is one of pretending that we're running on
-# Python 2.4 (or above), and patching up the library support on earlier
-# versions so that it looks enough like Python 2.4.  When it comes to
-# Python 2.2 there is one thing we cannot patch: extended slices
-# http://www.python.org/doc/2.3/whatsnew/section-slices.html.
-# Instead we simply declare that features that are implemented using
-# extended slices will not work on Python 2.2.
-#
-# In order to work on Python 2.3 we fix up a recurring annoyance involving
-# the array type.  In Python 2.3 an array cannot be initialised with an
-# array, and it cannot be extended with a list (or other sequence).
-# Both of those are repeated issues in the code.  Whilst I would not
-# normally tolerate this sort of behaviour, here we "shim" a replacement
-# for array into place (and hope no-ones notices).  You never read this.
-#
-# In an amusing case of warty hacks on top of warty hacks... the array
-# shimming we try and do only works on Python 2.3 and above (you can't
-# subclass array.array in Python 2.2).  So to get it working on Python
-# 2.2 we go for something much simpler and (probably) way slower.
-try:
-    array('B').extend([])
-    array('B', array('B'))
-except:
-    # Expect to get here on Python 2.3
-    try:
-        class _array_shim(array):
-            true_array = array
-            def __new__(cls, typecode, init=None):
-                super_new = super(_array_shim, cls).__new__
-                it = super_new(cls, typecode)
-                if init is None:
-                    return it
-                it.extend(init)
-                return it
-            def extend(self, extension):
-                super_extend = super(_array_shim, self).extend
-                if isinstance(extension, self.true_array):
-                    return super_extend(extension)
-                if not isinstance(extension, (list, str)):
-                    # Convert to list.  Allows iterators to work.
-                    extension = list(extension)
-                return super_extend(self.true_array(self.typecode, extension))
-        array = _array_shim
-    except:
-        # Expect to get here on Python 2.2
-        def array(typecode, init=()):
-            if type(init) == str:
-                return map(ord, init)
-            return list(init)
-
-# Further hacks to get it limping along on Python 2.2
-try:
-    enumerate
-except:
-    def enumerate(seq):
-        i=0
-        for x in seq:
-            yield i,x
-            i += 1
-
-try:
-    reversed
-except:
-    def reversed(l):
-        l = list(l)
-        l.reverse()
-        for x in l:
-            yield x
-
-try:
-    itertools
-except:
-    class _dummy_itertools:
-        pass
-    itertools = _dummy_itertools()
-    def _itertools_imap(f, seq):
-        for x in seq:
-            yield f(x)
-    itertools.imap = _itertools_imap
-    def _itertools_chain(*iterables):
-        for it in iterables:
-            for element in it:
-                yield element
-    itertools.chain = _itertools_chain
-
-
-# === Support for users without Cython ===
-
-try:
-    pngfilters
-except:
-    class pngfilters(object):
-        def undo_filter_sub(filter_unit, scanline, previous, result):
-            """Undo sub filter."""
-
-            ai = 0
-            # Loops starts at index fu.  Observe that the initial part
-            # of the result is already filled in correctly with
-            # scanline.
-            for i in range(filter_unit, len(result)):
-                x = scanline[i]
-                a = result[ai]
-                result[i] = (x + a) & 0xff
-                ai += 1
-        undo_filter_sub = staticmethod(undo_filter_sub)
-
-        def undo_filter_up(filter_unit, scanline, previous, result):
-            """Undo up filter."""
-
-            for i in range(len(result)):
-                x = scanline[i]
-                b = previous[i]
-                result[i] = (x + b) & 0xff
-        undo_filter_up = staticmethod(undo_filter_up)
-
-        def undo_filter_average(filter_unit, scanline, previous, result):
-            """Undo up filter."""
-
-            ai = -filter_unit
-            for i in range(len(result)):
-                x = scanline[i]
-                if ai < 0:
-                    a = 0
-                else:
-                    a = result[ai]
-                b = previous[i]
-                result[i] = (x + ((a + b) >> 1)) & 0xff
-                ai += 1
-        undo_filter_average = staticmethod(undo_filter_average)
-
-        def undo_filter_paeth(filter_unit, scanline, previous, result):
-            """Undo Paeth filter."""
-
-            # Also used for ci.
-            ai = -filter_unit
-            for i in range(len(result)):
-                x = scanline[i]
-                if ai < 0:
-                    a = c = 0
-                else:
-                    a = result[ai]
-                    c = previous[ai]
-                b = previous[i]
-                p = a + b - c
-                pa = abs(p - a)
-                pb = abs(p - b)
-                pc = abs(p - c)
-                if pa <= pb and pa <= pc:
-                    pr = a
-                elif pb <= pc:
-                    pr = b
-                else:
-                    pr = c
-                result[i] = (x + pr) & 0xff
-                ai += 1
-        undo_filter_paeth = staticmethod(undo_filter_paeth)
-
-        def convert_la_to_rgba(row, result):
-            for i in range(3):
-                result[i::4] = row[0::2]
-            result[3::4] = row[1::2]
-        convert_la_to_rgba = staticmethod(convert_la_to_rgba)
-
-        def convert_l_to_rgba(row, result):
-            """Convert a grayscale image to RGBA. This method assumes the alpha
-            channel in result is already correctly initialized."""
-            for i in range(3):
-                result[i::4] = row
-        convert_l_to_rgba = staticmethod(convert_l_to_rgba)
-
-        def convert_rgb_to_rgba(row, result):
-            """Convert an RGB image to RGBA. This method assumes the alpha
-            channel in result is already correctly initialized."""
-            for i in range(3):
-                result[i::4] = row[i::3]
-        convert_rgb_to_rgba = staticmethod(convert_rgb_to_rgba)
-
-
-# === Internal Test Support ===
-
-# This section comprises the tests that are internally validated (as
-# opposed to tests which produce output files that are externally
-# validated).  Primarily they are unittests.
-
-# Note that it is difficult to internally validate the results of
-# writing a PNG file.  The only thing we can do is read it back in
-# again, which merely checks consistency, not that the PNG file we
-# produce is valid.
-
-# Run the tests from the command line:
-# python -c 'import png;png.test()'
-
-# (For an in-memory binary file IO object) We use BytesIO where
-# available, otherwise we use StringIO, but name it BytesIO.
-try:
-    from io import BytesIO
-except:
-    from StringIO import StringIO as BytesIO
-import tempfile
-# http://www.python.org/doc/2.4.4/lib/module-unittest.html
-import unittest
-
-
-def test():
-    unittest.main(__name__)
-
-def topngbytes(name, rows, x, y, **k):
-    """Convenience function for creating a PNG file "in memory" as a
-    string.  Creates a :class:`Writer` instance using the keyword arguments,
-    then passes `rows` to its :meth:`Writer.write` method.  The resulting
-    PNG file is returned as a string.  `name` is used to identify the file for
-    debugging.
-    """
-
-    import os
-
-    print name
-    f = BytesIO()
-    w = Writer(x, y, **k)
-    w.write(f, rows)
-    if os.environ.get('PYPNG_TEST_TMP'):
-        w = open(name, 'wb')
-        w.write(f.getvalue())
-        w.close()
-    return f.getvalue()
-
-def testWithIO(inp, out, f):
-    """Calls the function `f` with ``sys.stdin`` changed to `inp`
-    and ``sys.stdout`` changed to `out`.  They are restored when `f`
-    returns.  This function returns whatever `f` returns.
-    """
-
-    import os
-
-    try:
-        oldin,sys.stdin = sys.stdin,inp
-        oldout,sys.stdout = sys.stdout,out
-        x = f()
-    finally:
-        sys.stdin = oldin
-        sys.stdout = oldout
-    if os.environ.get('PYPNG_TEST_TMP') and hasattr(out,'getvalue'):
-        name = mycallersname()
-        if name:
-            w = open(name+'.png', 'wb')
-            w.write(out.getvalue())
-            w.close()
-    return x
-
-def mycallersname():
-    """Returns the name of the caller of the caller of this function
-    (hence the name of the caller of the function in which
-    "mycallersname()" textually appears).  Returns None if this cannot
-    be determined."""
-
-    # http://docs.python.org/library/inspect.html#the-interpreter-stack
-    import inspect
-
-    frame = inspect.currentframe()
-    if not frame:
-        return None
-    frame_,filename_,lineno_,funname,linelist_,listi_ = (
-      inspect.getouterframes(frame)[2])
-    return funname
-
-def seqtobytes(s):
-    """Convert a sequence of integers to a *bytes* instance.  Good for
-    plastering over Python 2 / Python 3 cracks.
-    """
-
-    return strtobytes(''.join(chr(x) for x in s))
-
-class Test(unittest.TestCase):
-    # This member is used by the superclass.  If we don't define a new
-    # class here then when we use self.assertRaises() and the PyPNG code
-    # raises an assertion then we get no proper traceback.  I can't work
-    # out why, but defining a new class here means we get a proper
-    # traceback.
-    class failureException(Exception):
-        pass
-
-    def helperLN(self, n):
-        mask = (1 << n) - 1
-        # Use small chunk_limit so that multiple chunk writing is
-        # tested.  Making it a test for Issue 20.
-        w = Writer(15, 17, greyscale=True, bitdepth=n, chunk_limit=99)
-        f = BytesIO()
-        w.write_array(f, array('B', map(mask.__and__, range(1, 256))))
-        r = Reader(bytes=f.getvalue())
-        x,y,pixels,meta = r.read()
-        self.assertEqual(x, 15)
-        self.assertEqual(y, 17)
-        self.assertEqual(list(itertools.chain(*pixels)),
-                         map(mask.__and__, range(1,256)))
-    def testL8(self):
-        return self.helperLN(8)
-    def testL4(self):
-        return self.helperLN(4)
-    def testL2(self):
-        "Also tests asRGB8."
-        w = Writer(1, 4, greyscale=True, bitdepth=2)
-        f = BytesIO()
-        w.write_array(f, array('B', range(4)))
-        r = Reader(bytes=f.getvalue())
-        x,y,pixels,meta = r.asRGB8()
-        self.assertEqual(x, 1)
-        self.assertEqual(y, 4)
-        for i,row in enumerate(pixels):
-            self.assertEqual(len(row), 3)
-            self.assertEqual(list(row), [0x55*i]*3)
-    def testP2(self):
-        "2-bit palette."
-        a = (255,255,255)
-        b = (200,120,120)
-        c = (50,99,50)
-        w = Writer(1, 4, bitdepth=2, palette=[a,b,c])
-        f = BytesIO()
-        w.write_array(f, array('B', (0,1,1,2)))
-        r = Reader(bytes=f.getvalue())
-        x,y,pixels,meta = r.asRGB8()
-        self.assertEqual(x, 1)
-        self.assertEqual(y, 4)
-        self.assertEqual(map(list, pixels), map(list, [a, b, b, c]))
-    def testPtrns(self):
-        "Test colour type 3 and tRNS chunk (and 4-bit palette)."
-        a = (50,99,50,50)
-        b = (200,120,120,80)
-        c = (255,255,255)
-        d = (200,120,120)
-        e = (50,99,50)
-        w = Writer(3, 3, bitdepth=4, palette=[a,b,c,d,e])
-        f = BytesIO()
-        w.write_array(f, array('B', (4, 3, 2, 3, 2, 0, 2, 0, 1)))
-        r = Reader(bytes=f.getvalue())
-        x,y,pixels,meta = r.asRGBA8()
-        self.assertEqual(x, 3)
-        self.assertEqual(y, 3)
-        c = c+(255,)
-        d = d+(255,)
-        e = e+(255,)
-        boxed = [(e,d,c),(d,c,a),(c,a,b)]
-        flat = map(lambda row: itertools.chain(*row), boxed)
-        self.assertEqual(map(list, pixels), map(list, flat))
-    def testRGBtoRGBA(self):
-        "asRGBA8() on colour type 2 source."""
-        # Test for Issue 26
-        r = Reader(bytes=_pngsuite['basn2c08'])
-        x,y,pixels,meta = r.asRGBA8()
-        # Test the pixels at row 9 columns 0 and 1.
-        row9 = list(pixels)[9]
-        self.assertEqual(list(row9[0:8]),
-                         [0xff, 0xdf, 0xff, 0xff, 0xff, 0xde, 0xff, 0xff])
-    def testLtoRGBA(self):
-        "asRGBA() on grey source."""
-        # Test for Issue 60
-        r = Reader(bytes=_pngsuite['basi0g08'])
-        x,y,pixels,meta = r.asRGBA()
-        row9 = list(list(pixels)[9])
-        self.assertEqual(row9[0:8],
-          [222, 222, 222, 255, 221, 221, 221, 255])
-    def testCtrns(self):
-        "Test colour type 2 and tRNS chunk."
-        # Test for Issue 25
-        r = Reader(bytes=_pngsuite['tbrn2c08'])
-        x,y,pixels,meta = r.asRGBA8()
-        # I just happen to know that the first pixel is transparent.
-        # In particular it should be #7f7f7f00
-        row0 = list(pixels)[0]
-        self.assertEqual(tuple(row0[0:4]), (0x7f, 0x7f, 0x7f, 0x00))
-    def testAdam7read(self):
-        """Adam7 interlace reading.
-        Specifically, test that for images in the PngSuite that
-        have both an interlaced and straightlaced pair that both
-        images from the pair produce the same array of pixels."""
-        for candidate in _pngsuite:
-            if not candidate.startswith('basn'):
-                continue
-            candi = candidate.replace('n', 'i')
-            if candi not in _pngsuite:
-                continue
-            print 'adam7 read', candidate
-            straight = Reader(bytes=_pngsuite[candidate])
-            adam7 = Reader(bytes=_pngsuite[candi])
-            # Just compare the pixels.  Ignore x,y (because they're
-            # likely to be correct?); metadata is ignored because the
-            # "interlace" member differs.  Lame.
-            straight = straight.read()[2]
-            adam7 = adam7.read()[2]
-            self.assertEqual(map(list, straight), map(list, adam7))
-    def testAdam7write(self):
-        """Adam7 interlace writing.
-        For each test image in the PngSuite, write an interlaced
-        and a straightlaced version.  Decode both, and compare results.
-        """
-        # Not such a great test, because the only way we can check what
-        # we have written is to read it back again.
-
-        for name,bytes in _pngsuite.items():
-            # Only certain colour types supported for this test.
-            if name[3:5] not in ['n0', 'n2', 'n4', 'n6']:
-                continue
-            it = Reader(bytes=bytes)
-            x,y,pixels,meta = it.read()
-            pngi = topngbytes('adam7wn'+name+'.png', pixels,
-              x=x, y=y, bitdepth=it.bitdepth,
-              greyscale=it.greyscale, alpha=it.alpha,
-              transparent=it.transparent,
-              interlace=False)
-            x,y,ps,meta = Reader(bytes=pngi).read()
-            it = Reader(bytes=bytes)
-            x,y,pixels,meta = it.read()
-            pngs = topngbytes('adam7wi'+name+'.png', pixels,
-              x=x, y=y, bitdepth=it.bitdepth,
-              greyscale=it.greyscale, alpha=it.alpha,
-              transparent=it.transparent,
-              interlace=True)
-            x,y,pi,meta = Reader(bytes=pngs).read()
-            self.assertEqual(map(list, ps), map(list, pi))
-    def testPGMin(self):
-        """Test that the command line tool can read PGM files."""
-        def do():
-            return _main(['testPGMin'])
-        s = BytesIO()
-        s.write(strtobytes('P5 2 2 3\n'))
-        s.write(strtobytes('\x00\x01\x02\x03'))
-        s.flush()
-        s.seek(0)
-        o = BytesIO()
-        testWithIO(s, o, do)
-        r = Reader(bytes=o.getvalue())
-        x,y,pixels,meta = r.read()
-        self.assertTrue(r.greyscale)
-        self.assertEqual(r.bitdepth, 2)
-    def testPAMin(self):
-        """Test that the command line tool can read PAM file."""
-        def do():
-            return _main(['testPAMin'])
-        s = BytesIO()
-        s.write(strtobytes('P7\nWIDTH 3\nHEIGHT 1\nDEPTH 4\nMAXVAL 255\n'
-                'TUPLTYPE RGB_ALPHA\nENDHDR\n'))
-        # The pixels in flat row flat pixel format
-        flat =  [255,0,0,255, 0,255,0,120, 0,0,255,30]
-        asbytes = seqtobytes(flat)
-        s.write(asbytes)
-        s.flush()
-        s.seek(0)
-        o = BytesIO()
-        testWithIO(s, o, do)
-        r = Reader(bytes=o.getvalue())
-        x,y,pixels,meta = r.read()
-        self.assertTrue(r.alpha)
-        self.assertTrue(not r.greyscale)
-        self.assertEqual(list(itertools.chain(*pixels)), flat)
-    def testLA4(self):
-        """Create an LA image with bitdepth 4."""
-        bytes = topngbytes('la4.png', [[5, 12]], 1, 1,
-          greyscale=True, alpha=True, bitdepth=4)
-        sbit = Reader(bytes=bytes).chunk('sBIT')[1]
-        self.assertEqual(sbit, strtobytes('\x04\x04'))
-    def testPal(self):
-        """Test that a palette PNG returns the palette in info."""
-        r = Reader(bytes=_pngsuite['basn3p04'])
-        x,y,pixels,info = r.read()
-        self.assertEqual(x, 32)
-        self.assertEqual(y, 32)
-        self.assertTrue('palette' in info)
-    def testPalWrite(self):
-        """Test metadata for paletted PNG can be passed from one PNG
-        to another."""
-        r = Reader(bytes=_pngsuite['basn3p04'])
-        x,y,pixels,info = r.read()
-        w = Writer(**info)
-        o = BytesIO()
-        w.write(o, pixels)
-        o.flush()
-        o.seek(0)
-        r = Reader(file=o)
-        _,_,_,again_info = r.read()
-        # Same palette
-        self.assertEqual(again_info['palette'], info['palette'])
-    def testPalExpand(self):
-        """Test that bitdepth can be used to fiddle with pallete image."""
-        r = Reader(bytes=_pngsuite['basn3p04'])
-        x,y,pixels,info = r.read()
-        pixels = [list(row) for row in pixels]
-        info['bitdepth'] = 8
-        w = Writer(**info)
-        o = BytesIO()
-        w.write(o, pixels)
-        o.flush()
-        o.seek(0)
-        r = Reader(file=o)
-        _,_,again_pixels,again_info = r.read()
-        # Same pixels
-        again_pixels = [list(row) for row in again_pixels]
-        self.assertEqual(again_pixels, pixels)
-
-    def testPNMsbit(self):
-        """Test that PNM files can generates sBIT chunk."""
-        def do():
-            return _main(['testPNMsbit'])
-        s = BytesIO()
-        s.write(strtobytes('P6 8 1 1\n'))
-        for pixel in range(8):
-            s.write(struct.pack('<I', (0x4081*pixel)&0x10101)[:3])
-        s.flush()
-        s.seek(0)
-        o = BytesIO()
-        testWithIO(s, o, do)
-        r = Reader(bytes=o.getvalue())
-        sbit = r.chunk('sBIT')[1]
-        self.assertEqual(sbit, strtobytes('\x01\x01\x01'))
-    def testLtrns0(self):
-        """Create greyscale image with tRNS chunk."""
-        return self.helperLtrns(0)
-    def testLtrns1(self):
-        """Using 1-tuple for transparent arg."""
-        return self.helperLtrns((0,))
-    def helperLtrns(self, transparent):
-        """Helper used by :meth:`testLtrns*`."""
-        pixels = zip([0x00, 0x38, 0x4c, 0x54, 0x5c, 0x40, 0x38, 0x00])
-        o = BytesIO()
-        w = Writer(8, 8, greyscale=True, bitdepth=1, transparent=transparent)
-        w.write_packed(o, pixels)
-        r = Reader(bytes=o.getvalue())
-        x,y,pixels,meta = r.asDirect()
-        self.assertTrue(meta['alpha'])
-        self.assertTrue(meta['greyscale'])
-        self.assertEqual(meta['bitdepth'], 1)
-    def testWinfo(self):
-        """Test the dictionary returned by a `read` method can be used
-        as args for :meth:`Writer`.
-        """
-        r = Reader(bytes=_pngsuite['basn2c16'])
-        info = r.read()[3]
-        w = Writer(**info)
-    def testPackedIter(self):
-        """Test iterator for row when using write_packed.
-
-        Indicative for Issue 47.
-        """
-        w = Writer(16, 2, greyscale=True, alpha=False, bitdepth=1)
-        o = BytesIO()
-        w.write_packed(o, [itertools.chain([0x0a], [0xaa]),
-                           itertools.chain([0x0f], [0xff])])
-        r = Reader(bytes=o.getvalue())
-        x,y,pixels,info = r.asDirect()
-        pixels = list(pixels)
-        self.assertEqual(len(pixels), 2)
-        self.assertEqual(len(pixels[0]), 16)
-    def testInterlacedArray(self):
-        """Test that reading an interlaced PNG yields each row as an
-        array."""
-        r = Reader(bytes=_pngsuite['basi0g08'])
-        list(r.read()[2])[0].tostring
-    def testTrnsArray(self):
-        """Test that reading a type 2 PNG with tRNS chunk yields each
-        row as an array (using asDirect)."""
-        r = Reader(bytes=_pngsuite['tbrn2c08'])
-        list(r.asDirect()[2])[0].tostring
-
-    # Invalid file format tests.  These construct various badly
-    # formatted PNG files, then feed them into a Reader.  When
-    # everything is working properly, we should get FormatError
-    # exceptions raised.
-    def testEmpty(self):
-        """Test empty file."""
-
-        r = Reader(bytes='')
-        self.assertRaises(FormatError, r.asDirect)
-    def testSigOnly(self):
-        """Test file containing just signature bytes."""
-
-        r = Reader(bytes=_signature)
-        self.assertRaises(FormatError, r.asDirect)
-    def testExtraPixels(self):
-        """Test file that contains too many pixels."""
-
-        def eachchunk(chunk):
-            if chunk[0] != 'IDAT':
-                return chunk
-            data = zlib.decompress(chunk[1])
-            data += strtobytes('\x00garbage')
-            data = zlib.compress(data)
-            chunk = (chunk[0], data)
-            return chunk
-        self.assertRaises(FormatError, self.helperFormat, eachchunk)
-    def testNotEnoughPixels(self):
-        def eachchunk(chunk):
-            if chunk[0] != 'IDAT':
-                return chunk
-            # Remove last byte.
-            data = zlib.decompress(chunk[1])
-            data = data[:-1]
-            data = zlib.compress(data)
-            return (chunk[0], data)
-        self.assertRaises(FormatError, self.helperFormat, eachchunk)
-    def helperFormat(self, f):
-        r = Reader(bytes=_pngsuite['basn0g01'])
-        o = BytesIO()
-        def newchunks():
-            for chunk in r.chunks():
-                yield f(chunk)
-        write_chunks(o, newchunks())
-        r = Reader(bytes=o.getvalue())
-        return list(r.asDirect()[2])
-    def testBadFilter(self):
-        def eachchunk(chunk):
-            if chunk[0] != 'IDAT':
-                return chunk
-            data = zlib.decompress(chunk[1])
-            # Corrupt the first filter byte
-            data = strtobytes('\x99') + data[1:]
-            data = zlib.compress(data)
-            return (chunk[0], data)
-        self.assertRaises(FormatError, self.helperFormat, eachchunk)
-
-    def testFlat(self):
-        """Test read_flat."""
-        import hashlib
-
-        r = Reader(bytes=_pngsuite['basn0g02'])
-        x,y,pixel,meta = r.read_flat()
-        d = hashlib.md5(seqtobytes(pixel)).digest()
-        self.assertEqual(_enhex(d), '255cd971ab8cd9e7275ff906e5041aa0')
-    def testfromarray(self):
-        img = from_array([[0, 0x33, 0x66], [0xff, 0xcc, 0x99]], 'L')
-        img.save('testfromarray.png')
-    def testfromarrayL16(self):
-        img = from_array(group(range(2**16), 256), 'L;16')
-        img.save('testL16.png')
-    def testfromarrayRGB(self):
-        img = from_array([[0,0,0, 0,0,1, 0,1,0, 0,1,1],
-                          [1,0,0, 1,0,1, 1,1,0, 1,1,1]], 'RGB;1')
-        o = BytesIO()
-        img.save(o)
-    def testfromarrayIter(self):
-        import itertools
-
-        i = itertools.islice(itertools.count(10), 20)
-        i = itertools.imap(lambda x: [x, x, x], i)
-        img = from_array(i, 'RGB;5', dict(height=20))
-        f = open('testiter.png', 'wb')
-        img.save(f)
-        f.close()
-
-    # numpy dependent tests.  These are skipped (with a message to
-    # sys.stderr) if numpy cannot be imported.
-    def testNumpyuint16(self):
-        """numpy uint16."""
-
-        try:
-            import numpy
-        except ImportError:
-            print >>sys.stderr, "skipping numpy test"
-            return
-
-        rows = [map(numpy.uint16, range(0,0x10000,0x5555))]
-        b = topngbytes('numpyuint16.png', rows, 4, 1,
-            greyscale=True, alpha=False, bitdepth=16)
-    def testNumpyuint8(self):
-        """numpy uint8."""
-
-        try:
-            import numpy
-        except ImportError:
-            print >>sys.stderr, "skipping numpy test"
-            return
-
-        rows = [map(numpy.uint8, range(0,0x100,0x55))]
-        b = topngbytes('numpyuint8.png', rows, 4, 1,
-            greyscale=True, alpha=False, bitdepth=8)
-    def testNumpybool(self):
-        """numpy bool."""
-
-        try:
-            import numpy
-        except ImportError:
-            print >>sys.stderr, "skipping numpy test"
-            return
-
-        rows = [map(numpy.bool, [0,1])]
-        b = topngbytes('numpybool.png', rows, 2, 1,
-            greyscale=True, alpha=False, bitdepth=1)
-    def testNumpyarray(self):
-        """numpy array."""
-        try:
-            import numpy
-        except ImportError:
-            print >>sys.stderr, "skipping numpy test"
-            return
-
-        pixels = numpy.array([[0,0x5555],[0x5555,0xaaaa]], numpy.uint16)
-        img = from_array(pixels, 'L')
-        img.save('testnumpyL16.png')
-
-    def paeth(self, x, a, b, c):
-        p = a + b - c
-        pa = abs(p - a)
-        pb = abs(p - b)
-        pc = abs(p - c)
-        if pa <= pb and pa <= pc:
-            pr = a
-        elif pb <= pc:
-            pr = b
-        else:
-            pr = c
-        return x - pr
-
-    # test filters and unfilters
-    def testFilterScanlineFirstLine(self):
-        fo = 3  # bytes per pixel
-        line = [30, 31, 32, 230, 231, 232]
-        out = filter_scanline(0, line, fo, None)  # none
-        self.assertEqual(list(out), [0, 30, 31, 32, 230, 231, 232])
-        out = filter_scanline(1, line, fo, None)  # sub
-        self.assertEqual(list(out), [1, 30, 31, 32, 200, 200, 200])
-        out = filter_scanline(2, line, fo, None)  # up
-        # TODO: All filtered scanlines start with a byte indicating the filter
-        # algorithm, except "up". Is this a bug? Should the expected output
-        # start with 2 here?
-        self.assertEqual(list(out), [30, 31, 32, 230, 231, 232])
-        out = filter_scanline(3, line, fo, None)  # average
-        self.assertEqual(list(out), [3, 30, 31, 32, 215, 216, 216])
-        out = filter_scanline(4, line, fo, None)  # paeth
-        self.assertEqual(list(out), [
-            4, self.paeth(30, 0, 0, 0), self.paeth(31, 0, 0, 0),
-            self.paeth(32, 0, 0, 0), self.paeth(230, 30, 0, 0),
-            self.paeth(231, 31, 0, 0), self.paeth(232, 32, 0, 0)
-            ])
-    def testFilterScanline(self):
-        prev = [20, 21, 22, 210, 211, 212]
-        line = [30, 32, 34, 230, 233, 236]
-        fo = 3
-        out = filter_scanline(0, line, fo, prev)  # none
-        self.assertEqual(list(out), [0, 30, 32, 34, 230, 233, 236])
-        out = filter_scanline(1, line, fo, prev)  # sub
-        self.assertEqual(list(out), [1, 30, 32, 34, 200, 201, 202])
-        out = filter_scanline(2, line, fo, prev)  # up
-        self.assertEqual(list(out), [2, 10, 11, 12, 20, 22, 24])
-        out = filter_scanline(3, line, fo, prev)  # average
-        self.assertEqual(list(out), [3, 20, 22, 23, 110, 112, 113])
-        out = filter_scanline(4, line, fo, prev)  # paeth
-        self.assertEqual(list(out), [
-            4, self.paeth(30, 0, 20, 0), self.paeth(32, 0, 21, 0),
-            self.paeth(34, 0, 22, 0), self.paeth(230, 30, 210, 20),
-            self.paeth(233, 32, 211, 21), self.paeth(236, 34, 212, 22)
-            ])
-    def testUnfilterScanline(self):
-        reader = Reader(bytes='')
-        reader.psize = 3
-        scanprev = array('B', [20, 21, 22, 210, 211, 212])
-        scanline = array('B', [30, 32, 34, 230, 233, 236])
-        def cp(a):
-            return array('B', a)
-
-        out = reader.undo_filter(0, cp(scanline), cp(scanprev))
-        self.assertEqual(list(out), list(scanline))  # none
-        out = reader.undo_filter(1, cp(scanline), cp(scanprev))
-        self.assertEqual(list(out), [30, 32, 34, 4, 9, 14])  # sub
-        out = reader.undo_filter(2, cp(scanline), cp(scanprev))
-        self.assertEqual(list(out), [50, 53, 56, 184, 188, 192])  # up
-        out = reader.undo_filter(3, cp(scanline), cp(scanprev))
-        self.assertEqual(list(out), [40, 42, 45, 99, 103, 108])  # average
-        out = reader.undo_filter(4, cp(scanline), cp(scanprev))
-        self.assertEqual(list(out), [50, 53, 56, 184, 188, 192])  # paeth
-    def testUnfilterScanlinePaeth(self):
-        # This tests more edge cases in the paeth unfilter
-        reader = Reader(bytes='')
-        reader.psize = 3
-        scanprev = array('B', [2, 0, 0, 0, 9, 11])
-        scanline = array('B', [6, 10, 9, 100, 101, 102])
-
-        out = reader.undo_filter(4, scanline, scanprev)
-        self.assertEqual(list(out), [8, 10, 9, 108, 111, 113])  # paeth
-    def testIterstraight(self):
-        def arraify(list_of_str):
-            return [array('B', s) for s in list_of_str]
-        reader = Reader(bytes='')
-        reader.row_bytes = 6
-        reader.psize = 3
-        rows = reader.iterstraight(arraify(['\x00abcdef', '\x00ghijkl']))
-        self.assertEqual(list(rows), arraify(['abcdef', 'ghijkl']))
-
-        rows = reader.iterstraight(arraify(['\x00abc', 'def\x00ghijkl']))
-        self.assertEqual(list(rows), arraify(['abcdef', 'ghijkl']))
-
-        rows = reader.iterstraight(arraify(['\x00abcdef\x00ghijkl']))
-        self.assertEqual(list(rows), arraify(['abcdef', 'ghijkl']))
-
-        rows = reader.iterstraight(arraify(['\x00abcdef\x00ghi', 'jkl']))
-        self.assertEqual(list(rows), arraify(['abcdef', 'ghijkl']))
-
-# === Command Line Support ===
-
-def _dehex(s):
-    """Liberally convert from hex string to binary string."""
-    import re
-    import binascii
-
-    # Remove all non-hexadecimal digits
-    s = re.sub(r'[^a-fA-F\d]', '', s)
-    # binscii.unhexlify works in Python 2 and Python 3 (unlike
-    # thing.decode('hex')).
-    return binascii.unhexlify(strtobytes(s))
-def _enhex(s):
-    """Convert from binary string (bytes) to hex string (str)."""
-
-    import binascii
-
-    return bytestostr(binascii.hexlify(s))
-
-# Copies of PngSuite test files taken
-# from http://www.schaik.com/pngsuite/pngsuite_bas_png.html
-# on 2009-02-19 by drj and converted to hex.
-# Some of these are not actually in PngSuite (but maybe they should
-# be?), they use the same naming scheme, but start with a capital
-# letter.
-_pngsuite = {
-  'basi0g01': _dehex("""
-89504e470d0a1a0a0000000d49484452000000200000002001000000012c0677
-cf0000000467414d41000186a031e8965f0000009049444154789c2d8d310ec2
-300c45dfc682c415187a00a42e197ab81e83b127e00c5639001363a580d8582c
-65c910357c4b78b0bfbfdf4f70168c19e7acb970a3f2d1ded9695ce5bf5963df
-d92aaf4c9fd927ea449e6487df5b9c36e799b91bdf082b4d4bd4014fe4014b01
-ab7a17aee694d28d328a2d63837a70451e1648702d9a9ff4a11d2f7a51aa21e5
-a18c7ffd0094e3511d661822f20000000049454e44ae426082
-"""),
-  'basi0g02': _dehex("""
-89504e470d0a1a0a0000000d49484452000000200000002002000000016ba60d
-1f0000000467414d41000186a031e8965f0000005149444154789c635062e860
-00e17286bb609c93c370ec189494960631366e4467b3ae675dcf10f521ea0303
-90c1ca006444e11643482064114a4852c710baea3f18c31918020c30410403a6
-0ac1a09239009c52804d85b6d97d0000000049454e44ae426082
-"""),
-  'basi0g04': _dehex("""
-89504e470d0a1a0a0000000d4948445200000020000000200400000001e4e6f8
-bf0000000467414d41000186a031e8965f000000ae49444154789c658e5111c2
-301044171c141c141c041c843a287510ea20d441c041c141c141c04191102454
-03994998cecd7edcecedbb9bdbc3b2c2b6457545fbc4bac1be437347f7c66a77
-3c23d60db15e88f5c5627338a5416c2e691a9b475a89cd27eda12895ae8dfdab
-43d61e590764f5c83a226b40d669bec307f93247701687723abf31ff83a2284b
-a5b4ae6b63ac6520ad730ca4ed7b06d20e030369bd6720ed383290360406d24e
-13811f2781eba9d34d07160000000049454e44ae426082
-"""),
-  'basi0g08': _dehex("""
-89504e470d0a1a0a0000000d4948445200000020000000200800000001211615
-be0000000467414d41000186a031e8965f000000b549444154789cb5905d0ac2
-3010849dbac81c42c47bf843cf253e8878b0aa17110f214bdca6be240f5d21a5
-94ced3e49bcd322c1624115515154998aa424822a82a5624a1aa8a8b24c58f99
-999908130989a04a00d76c2c09e76cf21adcb209393a6553577da17140a2c59e
-70ecbfa388dff1f03b82fb82bd07f05f7cb13f80bb07ad2fd60c011c3c588eef
-f1f4e03bbec7ce832dca927aea005e431b625796345307b019c845e6bfc3bb98
-769d84f9efb02ea6c00f9bb9ff45e81f9f280000000049454e44ae426082
-"""),
-  'basi0g16': _dehex("""
-89504e470d0a1a0a0000000d49484452000000200000002010000000017186c9
-fd0000000467414d41000186a031e8965f000000e249444154789cb5913b0ec2
-301044c7490aa8f85d81c3e4301c8f53a4ca0da8902c8144b3920b4043111282
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-00000049454e44ae426082
-"""),
-  'basi2c08': _dehex("""
-89504e470d0a1a0a0000000d49484452000000200000002008020000018b1fdd
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-"""),
-  'basi2c16': _dehex("""
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-"""),
-  'basi3p08': _dehex("""
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-"""),
-  'basi6a08': _dehex("""
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-"""),
-  'basn0g01': _dehex("""
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-ae426082
-"""),
-  'basn0g02': _dehex("""
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-49454e44ae426082
-"""),
-  # A version of basn0g04 dithered down to 3 bits.
-  'Basn0g03': _dehex("""
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-"""),
-  'basn0g04': _dehex("""
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-"""),
-  'basn0g08': _dehex("""
-89504e470d0a1a0a0000000d4948445200000020000000200800000000561125
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-000049454e44ae426082
-"""),
-  'basn0g16': _dehex("""
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-"""),
-  'basn2c08': _dehex("""
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-"""),
-  'basn2c16': _dehex("""
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-"""),
-  'basn3p04': _dehex("""
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-"""),
-  'basn6a08': _dehex("""
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-"""),
-  'cs3n3p08': _dehex("""
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-426082
-"""),
-  's09n3p02': _dehex("""
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-"""),
-  'tbgn3p08': _dehex("""
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-9e854c38e5191878fbe471e749b0112bb18902008dc473b2b2e8e72700000000
-49454e44ae426082
-"""),
-  'Tp2n3p08': _dehex("""
-89504e470d0a1a0a0000000d494844520000002000000020080300000044a48a
-c60000000467414d41000186a031e8965f00000300504c544502ffff80ff05ff
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-}
-
-def read_pam_header(infile):
-    """
-    Read (the rest of a) PAM header.  `infile` should be positioned
-    immediately after the initial 'P7' line (at the beginning of the
-    second line).  Returns are as for `read_pnm_header`.
-    """
-    
-    # Unlike PBM, PGM, and PPM, we can read the header a line at a time.
-    header = dict()
-    while True:
-        l = infile.readline().strip()
-        if l == strtobytes('ENDHDR'):
-            break
-        if not l:
-            raise EOFError('PAM ended prematurely')
-        if l[0] == strtobytes('#'):
-            continue
-        l = l.split(None, 1)
-        if l[0] not in header:
-            header[l[0]] = l[1]
-        else:
-            header[l[0]] += strtobytes(' ') + l[1]
-
-    required = ['WIDTH', 'HEIGHT', 'DEPTH', 'MAXVAL']
-    required = [strtobytes(x) for x in required]
-    WIDTH,HEIGHT,DEPTH,MAXVAL = required
-    present = [x for x in required if x in header]
-    if len(present) != len(required):
-        raise Error('PAM file must specify WIDTH, HEIGHT, DEPTH, and MAXVAL')
-    width = int(header[WIDTH])
-    height = int(header[HEIGHT])
-    depth = int(header[DEPTH])
-    maxval = int(header[MAXVAL])
-    if (width <= 0 or
-        height <= 0 or
-        depth <= 0 or
-        maxval <= 0):
-        raise Error(
-          'WIDTH, HEIGHT, DEPTH, MAXVAL must all be positive integers')
-    return 'P7', width, height, depth, maxval
-
-def read_pnm_header(infile, supported=('P5','P6')):
-    """
-    Read a PNM header, returning (format,width,height,depth,maxval).
-    `width` and `height` are in pixels.  `depth` is the number of
-    channels in the image; for PBM and PGM it is synthesized as 1, for
-    PPM as 3; for PAM images it is read from the header.  `maxval` is
-    synthesized (as 1) for PBM images.
-    """
-
-    # Generally, see http://netpbm.sourceforge.net/doc/ppm.html
-    # and http://netpbm.sourceforge.net/doc/pam.html
-
-    supported = [strtobytes(x) for x in supported]
-
-    # Technically 'P7' must be followed by a newline, so by using
-    # rstrip() we are being liberal in what we accept.  I think this
-    # is acceptable.
-    type = infile.read(3).rstrip()
-    if type not in supported:
-        raise NotImplementedError('file format %s not supported' % type)
-    if type == strtobytes('P7'):
-        # PAM header parsing is completely different.
-        return read_pam_header(infile)
-    # Expected number of tokens in header (3 for P4, 4 for P6)
-    expected = 4
-    pbm = ('P1', 'P4')
-    if type in pbm:
-        expected = 3
-    header = [type]
-
-    # We have to read the rest of the header byte by byte because the
-    # final whitespace character (immediately following the MAXVAL in
-    # the case of P6) may not be a newline.  Of course all PNM files in
-    # the wild use a newline at this point, so it's tempting to use
-    # readline; but it would be wrong.
-    def getc():
-        c = infile.read(1)
-        if not c:
-            raise Error('premature EOF reading PNM header')
-        return c
-
-    c = getc()
-    while True:
-        # Skip whitespace that precedes a token.
-        while c.isspace():
-            c = getc()
-        # Skip comments.
-        while c == '#':
-            while c not in '\n\r':
-                c = getc()
-        if not c.isdigit():
-            raise Error('unexpected character %s found in header' % c)
-        # According to the specification it is legal to have comments
-        # that appear in the middle of a token.
-        # This is bonkers; I've never seen it; and it's a bit awkward to
-        # code good lexers in Python (no goto).  So we break on such
-        # cases.
-        token = strtobytes('')
-        while c.isdigit():
-            token += c
-            c = getc()
-        # Slight hack.  All "tokens" are decimal integers, so convert
-        # them here.
-        header.append(int(token))
-        if len(header) == expected:
-            break
-    # Skip comments (again)
-    while c == '#':
-        while c not in '\n\r':
-            c = getc()
-    if not c.isspace():
-        raise Error('expected header to end with whitespace, not %s' % c)
-
-    if type in pbm:
-        # synthesize a MAXVAL
-        header.append(1)
-    depth = (1,3)[type == strtobytes('P6')]
-    return header[0], header[1], header[2], depth, header[3]
-
-def write_pnm(file, width, height, pixels, meta):
-    """Write a Netpbm PNM/PAM file."""
-
-    bitdepth = meta['bitdepth']
-    maxval = 2**bitdepth - 1
-    # Rudely, the number of image planes can be used to determine
-    # whether we are L (PGM), LA (PAM), RGB (PPM), or RGBA (PAM).
-    planes = meta['planes']
-    # Can be an assert as long as we assume that pixels and meta came
-    # from a PNG file.
-    assert planes in (1,2,3,4)
-    if planes in (1,3):
-        if 1 == planes:
-            # PGM
-            # Could generate PBM if maxval is 1, but we don't (for one
-            # thing, we'd have to convert the data, not just blat it
-            # out).
-            fmt = 'P5'
-        else:
-            # PPM
-            fmt = 'P6'
-        file.write('%s %d %d %d\n' % (fmt, width, height, maxval))
-    if planes in (2,4):
-        # PAM
-        # See http://netpbm.sourceforge.net/doc/pam.html
-        if 2 == planes:
-            tupltype = 'GRAYSCALE_ALPHA'
-        else:
-            tupltype = 'RGB_ALPHA'
-        file.write('P7\nWIDTH %d\nHEIGHT %d\nDEPTH %d\nMAXVAL %d\n'
-                   'TUPLTYPE %s\nENDHDR\n' %
-                   (width, height, planes, maxval, tupltype))
-    # Values per row
-    vpr = planes * width
-    # struct format
-    fmt = '>%d' % vpr
-    if maxval > 0xff:
-        fmt = fmt + 'H'
-    else:
-        fmt = fmt + 'B'
-    for row in pixels:
-        file.write(struct.pack(fmt, *row))
-    file.flush()
-
-def color_triple(color):
-    """
-    Convert a command line colour value to a RGB triple of integers.
-    FIXME: Somewhere we need support for greyscale backgrounds etc.
-    """
-    if color.startswith('#') and len(color) == 4:
-        return (int(color[1], 16),
-                int(color[2], 16),
-                int(color[3], 16))
-    if color.startswith('#') and len(color) == 7:
-        return (int(color[1:3], 16),
-                int(color[3:5], 16),
-                int(color[5:7], 16))
-    elif color.startswith('#') and len(color) == 13:
-        return (int(color[1:5], 16),
-                int(color[5:9], 16),
-                int(color[9:13], 16))
-
-def _add_common_options(parser):
-    """Call *parser.add_option* for each of the options that are
-    common between this PNG--PNM conversion tool and the gen
-    tool.
-    """
-    parser.add_option("-i", "--interlace",
-                      default=False, action="store_true",
-                      help="create an interlaced PNG file (Adam7)")
-    parser.add_option("-t", "--transparent",
-                      action="store", type="string", metavar="#RRGGBB",
-                      help="mark the specified colour as transparent")
-    parser.add_option("-b", "--background",
-                      action="store", type="string", metavar="#RRGGBB",
-                      help="save the specified background colour")
-    parser.add_option("-g", "--gamma",
-                      action="store", type="float", metavar="value",
-                      help="save the specified gamma value")
-    parser.add_option("-c", "--compression",
-                      action="store", type="int", metavar="level",
-                      help="zlib compression level (0-9)")
-    return parser
-
-def _main(argv):
-    """
-    Run the PNG encoder with options from the command line.
-    """
-
-    # Parse command line arguments
-    from optparse import OptionParser
-    import re
-    version = '%prog ' + re.sub(r'( ?\$|URL: |Rev:)', '', __version__)
-    parser = OptionParser(version=version)
-    parser.set_usage("%prog [options] [imagefile]")
-    parser.add_option('-r', '--read-png', default=False,
-                      action='store_true',
-                      help='Read PNG, write PNM')
-    parser.add_option("-a", "--alpha",
-                      action="store", type="string", metavar="pgmfile",
-                      help="alpha channel transparency (RGBA)")
-    _add_common_options(parser)
-
-    (options, args) = parser.parse_args(args=argv[1:])
-
-    # Convert options
-    if options.transparent is not None:
-        options.transparent = color_triple(options.transparent)
-    if options.background is not None:
-        options.background = color_triple(options.background)
-
-    # Prepare input and output files
-    if len(args) == 0:
-        infilename = '-'
-        infile = sys.stdin
-    elif len(args) == 1:
-        infilename = args[0]
-        infile = open(infilename, 'rb')
-    else:
-        parser.error("more than one input file")
-    outfile = sys.stdout
-    if sys.platform == "win32":
-        import msvcrt, os
-        msvcrt.setmode(sys.stdout.fileno(), os.O_BINARY)
-
-    if options.read_png:
-        # Encode PNG to PPM
-        png = Reader(file=infile)
-        width,height,pixels,meta = png.asDirect()
-        write_pnm(outfile, width, height, pixels, meta) 
-    else:
-        # Encode PNM to PNG
-        format, width, height, depth, maxval = \
-          read_pnm_header(infile, ('P5','P6','P7'))
-        # When it comes to the variety of input formats, we do something
-        # rather rude.  Observe that L, LA, RGB, RGBA are the 4 colour
-        # types supported by PNG and that they correspond to 1, 2, 3, 4
-        # channels respectively.  So we use the number of channels in
-        # the source image to determine which one we have.  We do not
-        # care about TUPLTYPE.
-        greyscale = depth <= 2
-        pamalpha = depth in (2,4)
-        supported = map(lambda x: 2**x-1, range(1,17))
-        try:
-            mi = supported.index(maxval)
-        except ValueError:
-            raise NotImplementedError(
-              'your maxval (%s) not in supported list %s' %
-              (maxval, str(supported)))
-        bitdepth = mi+1
-        writer = Writer(width, height,
-                        greyscale=greyscale,
-                        bitdepth=bitdepth,
-                        interlace=options.interlace,
-                        transparent=options.transparent,
-                        background=options.background,
-                        alpha=bool(pamalpha or options.alpha),
-                        gamma=options.gamma,
-                        compression=options.compression)
-        if options.alpha:
-            pgmfile = open(options.alpha, 'rb')
-            format, awidth, aheight, adepth, amaxval = \
-              read_pnm_header(pgmfile, 'P5')
-            if amaxval != '255':
-                raise NotImplementedError(
-                  'maxval %s not supported for alpha channel' % amaxval)
-            if (awidth, aheight) != (width, height):
-                raise ValueError("alpha channel image size mismatch"
-                                 " (%s has %sx%s but %s has %sx%s)"
-                                 % (infilename, width, height,
-                                    options.alpha, awidth, aheight))
-            writer.convert_ppm_and_pgm(infile, pgmfile, outfile)
-        else:
-            writer.convert_pnm(infile, outfile)
-
-
-if __name__ == '__main__':
-    try:
-        _main(sys.argv)
-    except Error, e:
-        print >>sys.stderr, e