Fix rounding error on RGB to L conversion

[homm]

Fixes #3800

[homm]

This code was altered by me but I believe the error was there from the beginning. I think this PR is relatively safe for merge in 7.0.0.

A bit more work needs for fixing other L macro calls but I'm not ready for such feats before the release.

[radarhere]

Looks like you're solving the second problem in the thread, but the original poster would like our results to exactly match OpenCV. This PR brings Pillow closer, but doesn't exactly match it.

[homm]

We don't have an aim "to do everything like OpenCV". The real aim is to do everything correctly and OpenCV is wrong very often.

from PIL import Image
import cv2, numpy

im = Image.open('../imgs/56811035-ce7e0000-6805-11e9-9123-b297635e739e.png')
num_im = numpy.array(im)

ref = (num_im[:, :, 0] * 0.299 + num_im[:, :, 1] * 0.587 + num_im[:, :, 2] * 0.114 + 0.5).astype('uint8')
res_pillow = numpy.array(im.convert('L'))
res_cv2 = cv2.cvtColor(num_im, cv2.COLOR_RGB2GRAY)

print(numpy.count_nonzero(ref - res_pillow))
print(numpy.count_nonzero(ref - res_cv2))

61
224

So Pillow's result much closer to reference. I think this is because OpenCV uses fewer significant bits.

Anyway, now this is a question of precision, not rounding error which was truly an error.

[radarhere]

If you've changed ImageColor, shouldn't this be changed as well for consistency?

Pillow/src/PIL/ImageColor.py

Line 137 in fa1deca

color = (r * 299 + g * 587 + b * 114 + 500) // 1000

[homm]

Not the "L" macro itself should be fixed. This macro is used for F mode for example, where no rounding needed. Instead, every place where "L" is used should be analyzed. This is a bit more work that needs more attention and I haven't much time today to do this before the release.

[homm]

Actually, I've fixed ImageColor.getcolor implementation.