Comment by a-french-anon
3 years ago
An issue with lossless WebP is that it only supports (A)RGB and encodes grayscale via hacks that aren't as good as simply supporting monochrome.
If you compress a whole manga, PNG (via oxipng, optipng is basically deprecated) is still the way to go.
Another something not mentioned in here is that lossless JPEG2000 can be surprisingly good and fast on photographic content.
Just tried it on random manga page
- OxiPNG - 730k
- webp lossless max effort - 702k
- avif lossless max effort - 2.54MB (yay!)
- jpegxl lossless max effort - 506k (winner!)
Probably depends on the manga itself. Action manga probably don't compress as well as more dialogue-heavy works.
I would, at a first blush, disagree with that characterization? Dialogue equals more fine-grained strokes and more individual, independent “zones” to encode.
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IIRC the way you encode grayscale in WebP is a SUBTRACT_GREEN transform that makes the red and blue channel 0 everywhere, and then use a 1-element prefix code for R and B, so the R and B for each pixel take zero bits. Same idea with A for opaque images. Do you know why that's not good enough?
I made a mistake there with subtract green.
If I had just added 128 to the residuals, all remaining prediction arithmetic would have worked better and it would have given 1 % more density.
This is because most related arithmetic for predicting pixels is done in unsigned 8 bit arithmetic. Subtract green moves such predictions to often cross the 0 -> 255 boundary, and then averaging, deltas etc make little sense and add to the entropy.
Can you explain why?
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Thankfully the following comment explains more than I know, I was speaking purely from empiric experience.
Then you can't know that any difference you see is because of how WebP encodes grayscale.