I did not analyze these images in detail, but I am just writing about
the experience with similar other images.
For images like sample_G_06.png, the 2d locality mapping for distances
in WebP is inefficient, and the way png backward reference distance
encoding happens to work is a better match with the image.
The difference on sample_G_01.png is likely due to the fact that the
current implementation of WebP does not do a shortest path
optimization after the entropy codes have been chosen (the shortest
path optimization is done using a single entropy code), and possibly
due to the differences between Paeth-predictor (PNG) and the Select
In general, WebP lossless does not have many technical improvements
for gray scale images in comparison to png, and often one sees similar
performance between the two.
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