Update libjpeg_turbo to 1.4.90 from https://github.com/libjpeg-turbo/

usage.txt

+NOTE:  This file was modified by The libjpeg-turbo Project to include only
+information relevant to libjpeg-turbo and to wordsmith certain sections.
+
+USAGE instructions for the Independent JPEG Group's JPEG software
+=================================================================
+
+This file describes usage of the JPEG conversion programs cjpeg and djpeg,
+as well as the utility programs jpegtran, rdjpgcom and wrjpgcom.  (See
+the other documentation files if you wish to use the JPEG library within
+your own programs.)
+
+If you are on a Unix machine you may prefer to read the Unix-style manual
+pages in files cjpeg.1, djpeg.1, jpegtran.1, rdjpgcom.1, wrjpgcom.1.
+
+
+INTRODUCTION
+
+These programs implement JPEG image encoding, decoding, and transcoding.
+JPEG (pronounced "jay-peg") is a standardized compression method for
+full-color and grayscale images.
+
+
+GENERAL USAGE
+
+We provide two programs, cjpeg to compress an image file into JPEG format,
+and djpeg to decompress a JPEG file back into a conventional image format.
+
+On Unix-like systems, you say:
+        cjpeg [switches] [imagefile] >jpegfile
+or
+        djpeg [switches] [jpegfile]  >imagefile
+The programs read the specified input file, or standard input if none is
+named.  They always write to standard output (with trace/error messages to
+standard error).  These conventions are handy for piping images between
+programs.
+
+On most non-Unix systems, you say:
+        cjpeg [switches] imagefile jpegfile
+or
+        djpeg [switches] jpegfile  imagefile
+i.e., both the input and output files are named on the command line.  This
+style is a little more foolproof, and it loses no functionality if you don't
+have pipes.  (You can get this style on Unix too, if you prefer, by defining
+TWO_FILE_COMMANDLINE when you compile the programs; see install.txt.)
+
+You can also say:
+        cjpeg [switches] -outfile jpegfile  imagefile
+or
+        djpeg [switches] -outfile imagefile  jpegfile
+This syntax works on all systems, so it is useful for scripts.
+
+The currently supported image file formats are: PPM (PBMPLUS color format),
+PGM (PBMPLUS grayscale format), BMP, Targa, and RLE (Utah Raster Toolkit
+format).  (RLE is supported only if the URT library is available, which it
+isn't on most non-Unix systems.)  cjpeg recognizes the input image format
+automatically, with the exception of some Targa files.  You have to tell djpeg
+which format to generate.
+
+JPEG files are in the defacto standard JFIF file format.  There are other,
+less widely used JPEG-based file formats, but we don't support them.
+
+All switch names may be abbreviated; for example, -grayscale may be written
+-gray or -gr.  Most of the "basic" switches can be abbreviated to as little as
+one letter.  Upper and lower case are equivalent (-BMP is the same as -bmp).
+British spellings are also accepted (e.g., -greyscale), though for brevity
+these are not mentioned below.
+
+
+CJPEG DETAILS
+
+The basic command line switches for cjpeg are:
+
+        -quality N[,...]  Scale quantization tables to adjust image quality.
+                          Quality is 0 (worst) to 100 (best); default is 75.
+                          (See below for more info.)
+
+        -grayscale      Create monochrome JPEG file from color input.
+                        Be sure to use this switch when compressing a grayscale
+                        BMP file, because cjpeg isn't bright enough to notice
+                        whether a BMP file uses only shades of gray.  By
+                        saying -grayscale, you'll get a smaller JPEG file that
+                        takes less time to process.
+
+        -rgb            Create RGB JPEG file.
+                        Using this switch suppresses the conversion from RGB
+                        colorspace input to the default YCbCr JPEG colorspace.
+
+        -optimize       Perform optimization of entropy encoding parameters.
+                        Without this, default encoding parameters are used.
+                        -optimize usually makes the JPEG file a little smaller,
+                        but cjpeg runs somewhat slower and needs much more
+                        memory.  Image quality and speed of decompression are
+                        unaffected by -optimize.
+
+        -progressive    Create progressive JPEG file (see below).
+
+        -targa          Input file is Targa format.  Targa files that contain
+                        an "identification" field will not be automatically
+                        recognized by cjpeg; for such files you must specify
+                        -targa to make cjpeg treat the input as Targa format.
+                        For most Targa files, you won't need this switch.
+
+The -quality switch lets you trade off compressed file size against quality of
+the reconstructed image: the higher the quality setting, the larger the JPEG
+file, and the closer the output image will be to the original input.  Normally
+you want to use the lowest quality setting (smallest file) that decompresses
+into something visually indistinguishable from the original image.  For this
+purpose the quality setting should generally be between 50 and 95 (the default
+is 75) for photographic images.  If you see defects at -quality 75, then go up
+5 or 10 counts at a time until you are happy with the output image.  (The
+optimal setting will vary from one image to another.)
+
+-quality 100 will generate a quantization table of all 1's, minimizing loss
+in the quantization step (but there is still information loss in subsampling,
+as well as roundoff error.)  For most images, specifying a quality value above
+about 95 will increase the size of the compressed file dramatically, and while
+the quality gain from these higher quality values is measurable (using metrics
+such as PSNR or SSIM), it is rarely perceivable by human vision.
+
+In the other direction, quality values below 50 will produce very small files
+of low image quality.  Settings around 5 to 10 might be useful in preparing an
+index of a large image library, for example.  Try -quality 2 (or so) for some
+amusing Cubist effects.  (Note: quality values below about 25 generate 2-byte
+quantization tables, which are considered optional in the JPEG standard.
+cjpeg emits a warning message when you give such a quality value, because some
+other JPEG programs may be unable to decode the resulting file.  Use -baseline
+if you need to ensure compatibility at low quality values.)
+
+The -quality option has been extended in this version of cjpeg to support
+separate quality settings for luminance and chrominance (or, in general,
+separate settings for every quantization table slot.)  The principle is the
+same as chrominance subsampling:  since the human eye is more sensitive to
+spatial changes in brightness than spatial changes in color, the chrominance
+components can be quantized more than the luminance components without
+incurring any visible image quality loss.  However, unlike subsampling, this
+feature reduces data in the frequency domain instead of the spatial domain,
+which allows for more fine-grained control.  This option is useful in
+quality-sensitive applications, for which the artifacts generated by
+subsampling may be unacceptable.
+
+The -quality option accepts a comma-separated list of parameters, which
+respectively refer to the quality levels that should be assigned to the
+quantization table slots.  If there are more q-table slots than parameters,
+then the last parameter is replicated.  Thus, if only one quality parameter is
+given, this is used for both luminance and chrominance (slots 0 and 1,
+respectively), preserving the legacy behavior of cjpeg v6b and prior.  More (or
+customized) quantization tables can be set with the -qtables option and
+assigned to components with the -qslots option (see the "wizard" switches
+below.)
+
+JPEG  files  generated  with separate luminance and chrominance quality are
+fully compliant with standard JPEG decoders.
+
+CAUTION: For this setting to be useful, be sure to pass an argument of
+-sample 1x1 to cjpeg to disable chrominance subsampling.  Otherwise, the
+default subsampling level (2x2, AKA "4:2:0") will be used.
+
+The -progressive switch creates a "progressive JPEG" file.  In this type of
+JPEG file, the data is stored in multiple scans of increasing quality.  If the
+file is being transmitted over a slow communications link, the decoder can use
+the first scan to display a low-quality image very quickly, and can then
+improve the display with each subsequent scan.  The final image is exactly
+equivalent to a standard JPEG file of the same quality setting, and the total
+file size is about the same --- often a little smaller.
+
+Switches for advanced users:
+
+        -arithmetic     Use arithmetic coding.  CAUTION: arithmetic coded JPEG
+                        is not yet widely implemented, so many decoders will
+                        be unable to view an arithmetic coded JPEG file at
+                        all.
+
+        -dct int        Use integer DCT method (default).
+        -dct fast       Use fast integer DCT (less accurate).
+                        In libjpeg-turbo, the fast method is generally about
+                        5-15% faster than the int method when using the
+                        x86/x86-64 SIMD extensions (results may vary with other
+                        SIMD implementations, or when using libjpeg-turbo
+                        without SIMD extensions.)  For quality levels of 90 and
+                        below, there should be little or no perceptible
+                        difference between the two algorithms.  For quality
+                        levels above 90, however, the difference between
+                        the fast and the int methods becomes more pronounced.
+                        With quality=97, for instance, the fast method incurs
+                        generally about a 1-3 dB loss (in PSNR) relative to
+                        the int method, but this can be larger for some images.
+                        Do not use the fast method with quality levels above
+                        97.  The algorithm often degenerates at quality=98 and
+                        above and can actually produce a more lossy image than
+                        if lower quality levels had been used.  Also, in
+                        libjpeg-turbo, the fast method is not fully accerated
+                        for quality levels above 97, so it will be slower than
+                        the int method.
+        -dct float      Use floating-point DCT method.
+                        The float method is mainly a legacy feature.  It does
+                        not produce significantly more accurate results than
+                        the int method, and it is much slower.  The float
+                        method may also give different results on different
+                        machines due to varying roundoff behavior, whereas the
+                        integer methods should give the same results on all
+                        machines.
+
+        -restart N      Emit a JPEG restart marker every N MCU rows, or every
+                        N MCU blocks if "B" is attached to the number.
+                        -restart 0 (the default) means no restart markers.
+
+        -smooth N       Smooth the input image to eliminate dithering noise.
+                        N, ranging from 1 to 100, indicates the strength of
+                        smoothing.  0 (the default) means no smoothing.
+
+        -maxmemory N    Set limit for amount of memory to use in processing
+                        large images.  Value is in thousands of bytes, or
+                        millions of bytes if "M" is attached to the number.
+                        For example, -max 4m selects 4000000 bytes.  If more
+                        space is needed, temporary files will be used.
+
+        -verbose        Enable debug printout.  More -v's give more printout.
+        or  -debug      Also, version information is printed at startup.
+
+The -restart option inserts extra markers that allow a JPEG decoder to
+resynchronize after a transmission error.  Without restart markers, any damage
+to a compressed file will usually ruin the image from the point of the error
+to the end of the image; with restart markers, the damage is usually confined
+to the portion of the image up to the next restart marker.  Of course, the
+restart markers occupy extra space.  We recommend -restart 1 for images that
+will be transmitted across unreliable networks such as Usenet.
+
+The -smooth option filters the input to eliminate fine-scale noise.  This is
+often useful when converting dithered images to JPEG: a moderate smoothing
+factor of 10 to 50 gets rid of dithering patterns in the input file, resulting
+in a smaller JPEG file and a better-looking image.  Too large a smoothing
+factor will visibly blur the image, however.
+
+Switches for wizards:
+
+        -baseline       Force baseline-compatible quantization tables to be
+                        generated.  This clamps quantization values to 8 bits
+                        even at low quality settings.  (This switch is poorly
+                        named, since it does not ensure that the output is
+                        actually baseline JPEG.  For example, you can use
+                        -baseline and -progressive together.)
+
+        -qtables file   Use the quantization tables given in the specified
+                        text file.
+
+        -qslots N[,...] Select which quantization table to use for each color
+                        component.
+
+        -sample HxV[,...]  Set JPEG sampling factors for each color component.
+
+        -scans file     Use the scan script given in the specified text file.
+
+The "wizard" switches are intended for experimentation with JPEG.  If you
+don't know what you are doing, DON'T USE THEM.  These switches are documented
+further in the file wizard.txt.
+
+
+DJPEG DETAILS
+
+The basic command line switches for djpeg are:
+
+        -colors N       Reduce image to at most N colors.  This reduces the
+        or -quantize N  number of colors used in the output image, so that it
+                        can be displayed on a colormapped display or stored in
+                        a colormapped file format.  For example, if you have
+                        an 8-bit display, you'd need to reduce to 256 or fewer
+                        colors.  (-colors is the recommended name, -quantize
+                        is provided only for backwards compatibility.)
+
+        -fast           Select recommended processing options for fast, low
+                        quality output.  (The default options are chosen for
+                        highest quality output.)  Currently, this is equivalent
+                        to "-dct fast -nosmooth -onepass -dither ordered".
+
+        -grayscale      Force grayscale output even if JPEG file is color.
+                        Useful for viewing on monochrome displays; also,
+                        djpeg runs noticeably faster in this mode.
+
+        -rgb            Force RGB output even if JPEG file is grayscale.
+
+        -scale M/N      Scale the output image by a factor M/N.  Currently
+                        the scale factor must be M/8, where M is an integer
+                        between 1 and 16 inclusive, or any reduced fraction
+                        thereof (such as 1/2, 3/4, etc.  Scaling is handy if
+                        the image is larger than your screen; also, djpeg runs
+                        much faster when scaling down the output.
+
+        -bmp            Select BMP output format (Windows flavor).  8-bit
+                        colormapped format is emitted if -colors or -grayscale
+                        is specified, or if the JPEG file is grayscale;
+                        otherwise, 24-bit full-color format is emitted.
+
+        -gif            Select GIF output format.  Since GIF does not support
+                        more than 256 colors, -colors 256 is assumed (unless
+                        you specify a smaller number of colors).  If you
+                        specify -fast, the default number of colors is 216.
+
+        -os2            Select BMP output format (OS/2 1.x flavor).  8-bit
+                        colormapped format is emitted if -colors or -grayscale
+                        is specified, or if the JPEG file is grayscale;
+                        otherwise, 24-bit full-color format is emitted.
+
+        -pnm            Select PBMPLUS (PPM/PGM) output format (this is the
+                        default format).  PGM is emitted if the JPEG file is
+                        grayscale or if -grayscale is specified; otherwise
+                        PPM is emitted.
+
+        -rle            Select RLE output format.  (Requires URT library.)
+
+        -targa          Select Targa output format.  Grayscale format is
+                        emitted if the JPEG file is grayscale or if
+                        -grayscale is specified; otherwise, colormapped format
+                        is emitted if -colors is specified; otherwise, 24-bit
+                        full-color format is emitted.
+
+Switches for advanced users:
+
+        -dct int        Use integer DCT method (default).
+        -dct fast       Use fast integer DCT (less accurate).
+                        In libjpeg-turbo, the fast method is generally about
+                        5-15% faster than the int method when using the
+                        x86/x86-64 SIMD extensions (results may vary with other
+                        SIMD implementations, or when using libjpeg-turbo
+                        without SIMD extensions.)  If the JPEG image was
+                        compressed using a quality level of 85 or below, then
+                        there should be little or no perceptible difference
+                        between the two algorithms.  When decompressing images
+                        that were compressed using quality levels above 85,
+                        however, the difference between the fast and int
+                        methods becomes more pronounced.  With images
+                        compressed using quality=97, for instance, the fast
+                        method incurs generally about a 4-6 dB loss (in PSNR)
+                        relative to the int method, but this can be larger for
+                        some images.  If you can avoid it, do not use the fast
+                        method when decompressing images that were compressed
+                        using quality levels above 97.  The algorithm often
+                        degenerates for such images and can actually produce
+                        a more lossy output image than if the JPEG image had
+                        been compressed using lower quality levels.
+        -dct float      Use floating-point DCT method.
+                        The float method is mainly a legacy feature.  It does
+                        not produce significantly more accurate results than
+                        the int method, and it is much slower.  The float
+                        method may also give different results on different
+                        machines due to varying roundoff behavior, whereas the
+                        integer methods should give the same results on all
+                        machines.
+
+        -dither fs      Use Floyd-Steinberg dithering in color quantization.
+        -dither ordered Use ordered dithering in color quantization.
+        -dither none    Do not use dithering in color quantization.
+                        By default, Floyd-Steinberg dithering is applied when
+                        quantizing colors; this is slow but usually produces
+                        the best results.  Ordered dither is a compromise
+                        between speed and quality; no dithering is fast but
+                        usually looks awful.  Note that these switches have
+                        no effect unless color quantization is being done.
+                        Ordered dither is only available in -onepass mode.
+
+        -map FILE       Quantize to the colors used in the specified image
+                        file.  This is useful for producing multiple files
+                        with identical color maps, or for forcing a predefined
+                        set of colors to be used.  The FILE must be a GIF
+                        or PPM file.  This option overrides -colors and
+                        -onepass.
+
+        -nosmooth       Use a faster, lower-quality upsampling routine.
+
+        -onepass        Use one-pass instead of two-pass color quantization.
+                        The one-pass method is faster and needs less memory,
+                        but it produces a lower-quality image.  -onepass is
+                        ignored unless you also say -colors N.  Also,
+                        the one-pass method is always used for grayscale
+                        output (the two-pass method is no improvement then).
+
+        -maxmemory N    Set limit for amount of memory to use in processing
+                        large images.  Value is in thousands of bytes, or
+                        millions of bytes if "M" is attached to the number.
+                        For example, -max 4m selects 4000000 bytes.  If more
+                        space is needed, temporary files will be used.
+
+        -verbose        Enable debug printout.  More -v's give more printout.
+        or  -debug      Also, version information is printed at startup.
+
+
+HINTS FOR CJPEG
+
+Color GIF files are not the ideal input for JPEG; JPEG is really intended for
+compressing full-color (24-bit) images.  In particular, don't try to convert
+cartoons, line drawings, and other images that have only a few distinct
+colors.  GIF works great on these, JPEG does not.  If you want to convert a
+GIF to JPEG, you should experiment with cjpeg's -quality and -smooth options
+to get a satisfactory conversion.  -smooth 10 or so is often helpful.
+
+Avoid running an image through a series of JPEG compression/decompression
+cycles.  Image quality loss will accumulate; after ten or so cycles the image
+may be noticeably worse than it was after one cycle.  It's best to use a
+lossless format while manipulating an image, then convert to JPEG format when
+you are ready to file the image away.
+
+The -optimize option to cjpeg is worth using when you are making a "final"
+version for posting or archiving.  It's also a win when you are using low
+quality settings to make very small JPEG files; the percentage improvement
+is often a lot more than it is on larger files.  (At present, -optimize
+mode is always selected when generating progressive JPEG files.)
+
+Support for GIF input files was removed in cjpeg v6b due to concerns over
+the Unisys LZW patent.  Although this patent expired in 2006, cjpeg still
+lacks GIF support, for these historical reasons.  (Conversion of GIF files to
+JPEG is usually a bad idea anyway.)
+
+
+HINTS FOR DJPEG
+
+To get a quick preview of an image, use the -grayscale and/or -scale switches.
+"-grayscale -scale 1/8" is the fastest case.
+
+Several options are available that trade off image quality to gain speed.
+"-fast" turns on the recommended settings.
+
+"-dct fast" and/or "-nosmooth" gain speed at a small sacrifice in quality.
+When producing a color-quantized image, "-onepass -dither ordered" is fast but
+much lower quality than the default behavior.  "-dither none" may give
+acceptable results in two-pass mode, but is seldom tolerable in one-pass mode.
+
+Two-pass color quantization requires a good deal of memory; on MS-DOS machines
+it may run out of memory even with -maxmemory 0.  In that case you can still
+decompress, with some loss of image quality, by specifying -onepass for
+one-pass quantization.
+
+To avoid the Unisys LZW patent (now expired), djpeg produces uncompressed GIF
+files.  These are larger than they should be, but are readable by standard GIF
+decoders.
+
+
+HINTS FOR BOTH PROGRAMS
+
+If more space is needed than will fit in the available main memory (as
+determined by -maxmemory), temporary files will be used.  (MS-DOS versions
+will try to get extended or expanded memory first.)  The temporary files are
+often rather large: in typical cases they occupy three bytes per pixel, for
+example 3*800*600 = 1.44Mb for an 800x600 image.  If you don't have enough
+free disk space, leave out -progressive and -optimize (for cjpeg) or specify
+-onepass (for djpeg).
+
+On MS-DOS, the temporary files are created in the directory named by the TMP
+or TEMP environment variable, or in the current directory if neither of those
+exist.  Amiga implementations put the temp files in the directory named by
+JPEGTMP:, so be sure to assign JPEGTMP: to a disk partition with adequate free
+space.
+
+The default memory usage limit (-maxmemory) is set when the software is
+compiled.  If you get an "insufficient memory" error, try specifying a smaller
+-maxmemory value, even -maxmemory 0 to use the absolute minimum space.  You
+may want to recompile with a smaller default value if this happens often.
+
+On machines that have "environment" variables, you can define the environment
+variable JPEGMEM to set the default memory limit.  The value is specified as
+described for the -maxmemory switch.  JPEGMEM overrides the default value
+specified when the program was compiled, and itself is overridden by an
+explicit -maxmemory switch.
+
+On MS-DOS machines, -maxmemory is the amount of main (conventional) memory to
+use.  (Extended or expanded memory is also used if available.)  Most
+DOS-specific versions of this software do their own memory space estimation
+and do not need you to specify -maxmemory.
+
+
+JPEGTRAN
+
+jpegtran performs various useful transformations of JPEG files.
+It can translate the coded representation from one variant of JPEG to another,
+for example from baseline JPEG to progressive JPEG or vice versa.  It can also
+perform some rearrangements of the image data, for example turning an image
+from landscape to portrait format by rotation.  For EXIF files and JPEG files
+containing Exif data, you may prefer to use exiftran instead.
+
+jpegtran works by rearranging the compressed data (DCT coefficients), without
+ever fully decoding the image.  Therefore, its transformations are lossless:
+there is no image degradation at all, which would not be true if you used
+djpeg followed by cjpeg to accomplish the same conversion.  But by the same
+token, jpegtran cannot perform lossy operations such as changing the image
+quality.  However, while the image data is losslessly transformed, metadata
+can be removed.  See the -copy option for specifics.
+
+jpegtran uses a command line syntax similar to cjpeg or djpeg.
+On Unix-like systems, you say:
+        jpegtran [switches] [inputfile] >outputfile
+On most non-Unix systems, you say:
+        jpegtran [switches] inputfile outputfile
+where both the input and output files are JPEG files.
+
+To specify the coded JPEG representation used in the output file,
+jpegtran accepts a subset of the switches recognized by cjpeg:
+        -optimize       Perform optimization of entropy encoding parameters.
+        -progressive    Create progressive JPEG file.
+        -arithmetic     Use arithmetic coding.
+        -restart N      Emit a JPEG restart marker every N MCU rows, or every
+                        N MCU blocks if "B" is attached to the number.
+        -scans file     Use the scan script given in the specified text file.
+See the previous discussion of cjpeg for more details about these switches.
+If you specify none of these switches, you get a plain baseline-JPEG output
+file.  The quality setting and so forth are determined by the input file.
+
+The image can be losslessly transformed by giving one of these switches:
+        -flip horizontal        Mirror image horizontally (left-right).
+        -flip vertical          Mirror image vertically (top-bottom).
+        -rotate 90              Rotate image 90 degrees clockwise.
+        -rotate 180             Rotate image 180 degrees.
+        -rotate 270             Rotate image 270 degrees clockwise (or 90 ccw).
+        -transpose              Transpose image (across UL-to-LR axis).
+        -transverse             Transverse transpose (across UR-to-LL axis).
+
+The transpose transformation has no restrictions regarding image dimensions.
+The other transformations operate rather oddly if the image dimensions are not
+a multiple of the iMCU size (usually 8 or 16 pixels), because they can only
+transform complete blocks of DCT coefficient data in the desired way.
+
+jpegtran's default behavior when transforming an odd-size image is designed
+to preserve exact reversibility and mathematical consistency of the
+transformation set.  As stated, transpose is able to flip the entire image
+area.  Horizontal mirroring leaves any partial iMCU column at the right edge
+untouched, but is able to flip all rows of the image.  Similarly, vertical
+mirroring leaves any partial iMCU row at the bottom edge untouched, but is
+able to flip all columns.  The other transforms can be built up as sequences
+of transpose and flip operations; for consistency, their actions on edge
+pixels are defined to be the same as the end result of the corresponding
+transpose-and-flip sequence.
+
+For practical use, you may prefer to discard any untransformable edge pixels
+rather than having a strange-looking strip along the right and/or bottom edges
+of a transformed image.  To do this, add the -trim switch:
+        -trim           Drop non-transformable edge blocks.
+Obviously, a transformation with -trim is not reversible, so strictly speaking
+jpegtran with this switch is not lossless.  Also, the expected mathematical
+equivalences between the transformations no longer hold.  For example,
+"-rot 270 -trim" trims only the bottom edge, but "-rot 90 -trim" followed by
+"-rot 180 -trim" trims both edges.
+
+If you are only interested in perfect transformations, add the -perfect switch:
+        -perfect        Fail with an error if the transformation is not
+                        perfect.
+For example, you may want to do
+  jpegtran -rot 90 -perfect foo.jpg || djpeg foo.jpg | pnmflip -r90 | cjpeg
+to do a perfect rotation, if available, or an approximated one if not.
+
+This version of jpegtran also offers a lossless crop option, which discards
+data outside of a given image region but losslessly preserves what is inside.
+Like the rotate and flip transforms, lossless crop is restricted by the current
+JPEG format; the upper left corner of the selected region must fall on an iMCU
+boundary.  If it doesn't, then it is silently moved up and/or left to the
+nearest iMCU boundary (the lower right corner is unchanged.)  Thus, the output
+image covers at least the requested region, but it may cover more.  The
+adjustment of the region dimensions may be optionally disabled by attaching an
+'f' character ("force") to the width or height number.
+
+The image can be losslessly cropped by giving the switch:
+        -crop WxH+X+Y   Crop to a rectangular region of width W and height H,
+                        starting at point X,Y.
+
+Other not-strictly-lossless transformation switches are:
+
+        -grayscale      Force grayscale output.
+This option discards the chrominance channels if the input image is YCbCr
+(ie, a standard color JPEG), resulting in a grayscale JPEG file.  The
+luminance channel is preserved exactly, so this is a better method of reducing
+to grayscale than decompression, conversion, and recompression.  This switch
+is particularly handy for fixing a monochrome picture that was mistakenly
+encoded as a color JPEG.  (In such a case, the space savings from getting rid
+of the near-empty chroma channels won't be large; but the decoding time for
+a grayscale JPEG is substantially less than that for a color JPEG.)
+
+jpegtran also recognizes these switches that control what to do with "extra"
+markers, such as comment blocks:
+        -copy none      Copy no extra markers from source file.  This setting
+                        suppresses all comments and other metadata in the
+                        source file.
+        -copy comments  Copy only comment markers.  This setting copies
+                        comments from the source file but discards any other
+                        metadata.
+        -copy all       Copy all extra markers.  This setting preserves
+                        miscellaneous markers found in the source file, such
+                        as JFIF thumbnails, Exif data, and Photoshop settings.
+                        In some files, these extra markers can be sizable.
+                        Note that this option will copy thumbnails as-is;
+                        they will not be transformed.
+The default behavior is -copy comments.  (Note: in IJG releases v6 and v6a,
+jpegtran always did the equivalent of -copy none.)
+
+Additional switches recognized by jpegtran are:
+        -outfile filename
+        -maxmemory N
+        -verbose
+        -debug
+These work the same as in cjpeg or djpeg.
+
+
+THE COMMENT UTILITIES
+
+The JPEG standard allows "comment" (COM) blocks to occur within a JPEG file.
+Although the standard doesn't actually define what COM blocks are for, they
+are widely used to hold user-supplied text strings.  This lets you add
+annotations, titles, index terms, etc to your JPEG files, and later retrieve
+them as text.  COM blocks do not interfere with the image stored in the JPEG
+file.  The maximum size of a COM block is 64K, but you can have as many of
+them as you like in one JPEG file.
+
+We provide two utility programs to display COM block contents and add COM
+blocks to a JPEG file.
+
+rdjpgcom searches a JPEG file and prints the contents of any COM blocks on
+standard output.  The command line syntax is
+        rdjpgcom [-raw] [-verbose] [inputfilename]
+The switch "-raw" (or just "-r") causes rdjpgcom to output non-printable
+characters in JPEG comments.  These characters are normally escaped for
+security reasons.
+The switch "-verbose" (or just "-v") causes rdjpgcom to also display the JPEG
+image dimensions.  If you omit the input file name from the command line,
+the JPEG file is read from standard input.  (This may not work on some
+operating systems, if binary data can't be read from stdin.)
+
+wrjpgcom adds a COM block, containing text you provide, to a JPEG file.
+Ordinarily, the COM block is added after any existing COM blocks, but you
+can delete the old COM blocks if you wish.  wrjpgcom produces a new JPEG
+file; it does not modify the input file.  DO NOT try to overwrite the input
+file by directing wrjpgcom's output back into it; on most systems this will
+just destroy your file.
+
+The command line syntax for wrjpgcom is similar to cjpeg's.  On Unix-like
+systems, it is
+        wrjpgcom [switches] [inputfilename]
+The output file is written to standard output.  The input file comes from
+the named file, or from standard input if no input file is named.
+
+On most non-Unix systems, the syntax is
+        wrjpgcom [switches] inputfilename outputfilename
+where both input and output file names must be given explicitly.
+
+wrjpgcom understands three switches:
+        -replace                 Delete any existing COM blocks from the file.
+        -comment "Comment text"  Supply new COM text on command line.
+        -cfile name              Read text for new COM block from named file.
+(Switch names can be abbreviated.)  If you have only one line of comment text
+to add, you can provide it on the command line with -comment.  The comment
+text must be surrounded with quotes so that it is treated as a single
+argument.  Longer comments can be read from a text file.
+
+If you give neither -comment nor -cfile, then wrjpgcom will read the comment
+text from standard input.  (In this case an input image file name MUST be
+supplied, so that the source JPEG file comes from somewhere else.)  You can
+enter multiple lines, up to 64KB worth.  Type an end-of-file indicator
+(usually control-D or control-Z) to terminate the comment text entry.
+
+wrjpgcom will not add a COM block if the provided comment string is empty.
+Therefore -replace -comment "" can be used to delete all COM blocks from a
+file.
+
+These utility programs do not depend on the IJG JPEG library.  In
+particular, the source code for rdjpgcom is intended as an illustration of
+the minimum amount of code required to parse a JPEG file header correctly.