image-rs / imageproc / 36752224288

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fix: use inverse CDF for histogram matching (#808)

match_histogram_mut previously mapped each source level to the target
level with nearest cumulative fraction. That could land in empty bins
just before CDF jumps (for example 29 instead of 30), so the matched
image histogram did not align with the target.

Use the discrete inverse CDF instead: map source level i to the smallest
target level y with target_cdf[y] >= source_cdf[i]. This matches the
standard construction and fixes the example from issue #783.

Update the gradient-to-step unit test expectations, add a regression
test from the issue, and regenerate the elephant_matched truth image.

Fixes #783

4 of 5 new or added lines in 1 file covered. (80.0%)

2931 of 3820 relevant lines covered (76.73%)

1301823.45 hits per line