Python Tools for Pixel-by-Pixel Image Comparison

chrome/test/functional/ispy/image_tools.py

Index: chrome/test/functional/ispy/image_tools.py
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+# Copyright (c) 2013 The Chromium Authos. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+
+"""Utilities for performing pixel-by-pixel image comparision."""
+
+import itertools
+import PIL
+from PIL import Image
+
+
+def _AreTheSameSize(images):
+  """Returns whether a set of images are the size size.
+
+  Args:
+    images: a list of images to compare.
+
+  Returns:
+    boolean.
+
+  Raises:
+    Exception: One image or fewer is passed in.
+  """
+  if len(images) > 1:
+    return all(images[0].size == img.size for img in images[1:])
+  else:
+    raise Exception('No images passed in.')
+
+
+def _GetDifferenceWithMask(image1, image2, mask=None,
+                           masked_color=(0, 0, 0),
+                           same_color=(0, 0, 0),
+                           different_color=(255, 255, 255)):
+  """Returns an image representing the difference between the two images.
+
+  This function computes the difference between two images taking into
+  account a mask if it is provided. The final three arguments represent
+  the coloration of the generated image.
+
+  Args:
+    image1: the first image to compare.
+    image2: the second image to compare.
+    mask: an optional mask to take into consideration.
+    masked_color: the color of a masked section in the resulting image.
+    same_color: the color of an unmasked section that is the same.
+      between images 1 and 2 in the resulting image.
+    different_color: the color of an unmasked section that is different
+      between images 1 and 2 in the resulting image.
+
+  Returns:
+    an image repesenting the difference between the two images.
+
+  Raises:
+    Exception: if image1, image2, and mask are not the same size.
+  """
+  if mask:
+    if not _AreTheSameSize([image1, image2, mask]):
+      raise Exception('images and mask must be the same size.')
+    image_diff = PIL.Image.new('RGB', image1.size, (0, 0, 0))
+    data = list(

[craigdh, 2013/06/18 19:16:11]

use [] instead of list(). That will result in this being a list comprehension
instead of a list that gets constructed from a generator expression.

[cgrimm, 2013/06/18 20:21:01]

Done.

+        masked_color if m == (255, 255, 255)
+        else same_color if px1 == px2
+        else different_color
+        for m, px1, px2 in itertools.izip(mask.getdata(),
+                                          image1.getdata(),
+                                          image2.getdata())
+        )
+    image_diff.putdata(data)
+    return image_diff
+  else:
+    if not _AreTheSameSize([image1, image2]):
+      raise Exception('images must be the same size.')
+    image_diff = PIL.Image.new('RGB', image1.size, (0, 0, 0))
+    data = list(

[craigdh, 2013/06/18 19:16:11]

ditto

[cgrimm, 2013/06/18 20:21:01]

Done.

+        same_color if px1 == px2
+        else different_color
+        for px1, px2 in itertools.izip(image1.getdata(),
+                                       image2.getdata())
+        )
+    image_diff.putdata(data)
+    return image_diff
+
+
+def CreateMask(images):
+  """Computes a mask for a set of images.
+
+  Returns an image that is computed from the images
+  passed in. The mask is generated by continually

[craigdh, 2013/06/18 19:16:11]

Just say the mask will have a white pixel anywhere the input images differ and
black everywhere else. Don't need to go through how it's implemented.

[cgrimm, 2013/06/18 20:21:01]

Done.

+  finding the difference between two images in the list
+  with respect to a mask which is initialized as a black
+  image, then setting the value of the mask to the result.
+
+  Args:
+    images: the images to compute the mask from.
+
+  Returns:
+    an image that is a mask of the passed in images.
+
+  Raises:
+    Exception: if the images passed in are not of the same size.
+    Exception: if fewer than two images are passed in.
+  """
+  if len(images) < 2:
+    raise Exception('mask must be created from two or more images.')
+  mask = Image.new('RGB', images[0].size, (0, 0, 0))
+  image = images[0]
+  for other_image in images[1:]:
+    mask = _GetDifferenceWithMask(
+        image,
+        other_image,
+        mask,
+        masked_color=(255, 255, 255))
+  return mask
+
+
+def VisualizeImageDifferences(image1, image2, mask=None):
+  """Returns an image repesenting the unmasked differences between two images.
+
+  Iterates through the pixel values of two images and an optional
+  mask. If the pixel values are the same, or the pixel is masked,
+  (0,0,0) is stored for that pixel. Otherwise, (255,255,255) is stored.
+  This ultimately produces an image where unmasked differences between
+  the two images are white pixels, and everything else is black.
+
+  Args:
+    image1: an Image
+    image2: another Image of the same size as image1.
+    mask: an optional mask that represents parts of the two
+      images to ignore when generating the difference image.
+
+  Returns:
+    a black and white image representing the unmasked difference between
+    the two input images.
+
+  Raises:
+    Exception: if the two images and optional mask are different sizes.
+  """
+  return _GetDifferenceWithMask(image1, image2, mask)
+
+
+def TotalDifferentPixels(image1, image2, mask=None):
+  """Computes the number of different pixels between two images.
+
+  Args:
+    image1: the first RGB PIL.Image to be compared.
+    image2: the second RGB PIL.Image to be compared.
+    mask: an optional mask to occlude parts of the images from calculation.

[craigdh, 2013/06/18 19:16:11]

Mention the format of the mask. Is it an image or a list of booleans? If it is
an image is black masked out or white?

[cgrimm, 2013/06/18 20:21:01]

Done.

+
+  Returns:
+    the number of differing pixels between the images.
+
+  Raises:
+    Exception: if the images to be compared and the mask are not the same size.
+  """
+  if mask:

[craigdh, 2013/06/18 19:16:11]

Entirely different codepaths are being followed depending whether there is a
mask. Just have two separate functions, one that takes a mask and one that
doesn't. SameImage() can have the logic to decide which to call.

[cgrimm, 2013/06/18 20:21:01]

Similar problem in _GetDifferenceWithMask, fixed both. 

+    if _AreTheSameSize([image1, image2, mask]):
+      return sum([

[craigdh, 2013/06/18 19:16:11]

remove the []. No need to create a list in memory, just have it as a generator
expression.

[cgrimm, 2013/06/18 20:21:01]

Done.

+          0 if m == (255, 255, 255)
+          else 1 if px1 != px2
+          else 0
+          for px1, px2, m in itertools.izip(
+              image1.getdata(),
+              image2.getdata(),
+              mask.getdata()
+              )
+          ])
+    else:
+      raise Exception('images and mask must be the same size')
+  else:
+    if _AreTheSameSize([image1, image2]):
+      return sum([

[craigdh, 2013/06/18 19:16:11]

ditto

[cgrimm, 2013/06/18 20:21:01]

Done.

+          1 if px1 != px2
+          else 0
+          for px1, px2 in itertools.izip(
+              image1.getdata(),
+              image2.getdata()
+              )
+          ])
+    else:
+      raise Exception('images must be the same size')
+
+
+def SameImage(image1, image2, max_different_pixels=0, mask=None):
+  """Returns a boolean representing whether the images are the same.
+
+  Returns a boolean indicating whether two images are similar
+  enough to be considered the same. Essentially wraps the
+  TotalDifferentPixels function and adds a max_different_pixels
+  cutoff for whether or not the images are the same.
+
+  Args:
+    image1: an RGB PIL.Image to compare.
+    image2: an RGB PIL.Image to compare.
+    max_different_pixels: a number that is the cutoff for whether or not
+      the two images are the same.
+    mask: an optional Image that occludes parts of the images from
+      the calculation.
+
+  Returns:
+    True if the images are similar, False otherwise.
+
+  Raises:
+    Exception: if the images (and mask) are different sizes.
+  """
+  different_pixels = TotalDifferentPixels(image1, image2, mask)
+  return different_pixels <= max_different_pixels