matplotlib.testing.compare

Utilities for comparing image results.

Functions

  • calculate_rms(): Calculate the per-pixel errors, then compute the root mean square error.

  • comparable_formats(): Return the list of file formats that .compare_images can compare

  • compare_images(): Compare two “image” files checking differences within a tolerance.

matplotlib.testing.compare.calculate_rms(expected_image, actual_image)

Calculate the per-pixel errors, then compute the root mean square error.

matplotlib.testing.compare.comparable_formats()

Return the list of file formats that .compare_images can compare on this system.

Returns:

E.g. ['png', 'pdf', 'svg', 'eps'].

Return type:

list of str

matplotlib.testing.compare.compare_images(expected, actual, tol, in_decorator=False)

Compare two “image” files checking differences within a tolerance.

The two given filenames may point to files which are convertible to PNG via the .converter dictionary. The underlying RMS is calculated with the .calculate_rms function.

Parameters:

  • expected (str) – The filename of the expected image.

  • actual (str) – The filename of the actual image.

  • tol (float) – The tolerance (a color value difference, where 255 is the maximal difference). The test fails if the average pixel difference is greater than this value.

  • in_decorator (bool) – Determines the output format. If called from image_comparison decorator, this should be True. (default=False)

Returns:

Return None if the images are equal within the given tolerance.

If the images differ, the return value depends on in_decorator. If in_decorator is true, a dict with the following entries is returned:

  • rms: The RMS of the image difference.

  • expected: The filename of the expected image.

  • actual: The filename of the actual image.

  • diff_image: The filename of the difference image.

  • tol: The comparison tolerance.

Otherwise, a human-readable multi-line string representation of this information is returned.

Return type:

None or dict or str

Examples

img1 = "./baseline/plot.png"
img2 = "./output/plot.png"
compare_images(img1, img2, 0.001)