Added compare_images function to ImageChops

[johnnygitgud]

This pull request implements the feature requested in issue #6478 by adding a new helper function compare_images to ImageChops.

The function compares two images pixel-by-pixel using ImageChops.difference and returns a dictionary containing:

different_pixels — the number of pixels that differ

percent_difference — the percentage of differing pixels

It raises a ValueError if the images have different modes or sizes, matching existing behavior elsewhere in Pillow.

A new test file (Tests/test_compare_images.py) is included to validate that:

identical images return zero difference

different images return nonzero values

This feature builds directly on existing ImageChops functionality and follows the project's testing and docstring conventions.

Closes #6478

[radarhere]

Suggested change

	raise ValueError("Images must have the same mode")
	if image1.size != image2.size:
	raise ValueError("Images must have the same size")
	msg = "Images must have the same mode"
	raise ValueError(msg)
	if image1.size != image2.size:
	msg = "Images must have the same size"
	raise ValueError(msg)

This should fix the Lint error - https://github.com/python-pillow/Pillow/actions/runs/19978770041/job/57301143480?pr=9328

[radarhere]

Suggested change

	def test_compare_identical_images():
	def test_compare_identical_images() -> None:

Type hint.

[radarhere]

Suggested change

	def test_compare_different_images():
	def test_compare_different_images() -> None:

[radarhere]

Suggested change

	nonzero = sum(1 for px in diff.getdata() if px != 0)
	nonzero = sum(1 for px in tuple(diff.getdata()) if px != 0)

Another type hint.

[radarhere]

Suggested change

	def compare_images(image1, image2):
	def compare_images(image1: Image.Image, image2: Image.Image) -> dict[str, float]:

[radarhere]

Hi. I don't think this does actually resolve #6478, as I think that issue wants to compare all frames of an image, and to have images automatically convert before comparison. That issue also only expected a boolean to be returned, rather than any information about how different the two images are.

So let me ask - ignoring #6478, do you personally think what you're put together would be a helpful addition to Pillow?