WebP Encoding Options - unclear example

[gl-bars]

https://imagemagick.org/script/webp.php uses -quality 50 with lossless. But if I understand correctly, -quality corresponds to "compression rate" in this case?

https://github.com/webmproject/libwebp/blob/main/doc/tools.md

quality parameter will in this case control the amount of processing time spent trying to make the output file as small as possible

It might make sense to give two examples (for lossy and lossless) and clarify -quality parameter for lossless case.
Also, 50 seems a bit low, maybe it should be a higher value or max value (99?).

[mrienstra]

I just documented some details re: ImageMagick's libwebp wrapper here:
ImageMagick/ImageMagick#7121 (comment)

I stumbled across a typo in the process, https://github.com/ImageMagick/Website/blob/518e77e/include/webp.php#L93 shows segment rather than segments, as per https://github.com/ImageMagick/ImageMagick/blob/f86cbbd/coders/webp.c#L1188 -- just opened a PR for that, since it's such a small change.

At any rate, yes, more examples would be neat, always helpful to have a few, and maybe some of the other behaviors of the wrapper could be documented as well? Here are three candidates:

1. ImageMagick sets pass=6 if target-psnr or target-size are specified -- see ImageMagick coders/webp.c#L1205-L1207 (cwebp has the same behavior)

2. If quality is specified, and libwebp is v1.0.0 or newer (v1.0.0 was released Apr 20, 2018), than ImageMagick sets near_lossless to the same value as quality, see ImageMagick coders/webp.c#L1145-L1150. near_lossless cannot be set directly (independently of quality).

3. If the quality is set to 100 or greater, ImageMagick sets lossless=1, see coders/webp.c#L1151-L1152. lossless is parsed after this, so you can force a lossy encode at quality 100+ if you wish: -quality 100 -define webp:lossless=false)

---

Some specific examples which might be helpful, the presets defined inside libwebp:

Picture (digital picture, like portrait, inner shot):

magick wizard.png -define webp:sns-strength=80 -define webp:filter-sharpness=4 -define webp:filter-strength=35 -define webp:preprocessing=0 wizard.webp

Photo (outdoor photograph, with natural lighting):

magick wizard.png -define webp:sns-strength=80 -define webp:filter-sharpness=3 -define webp:filter-strength=30 -define webp:preprocessing=2 wizard.webp

Drawing (hand or line drawing, with high-contrast details):

magick wizard.png -define webp:sns-strength=25 -define webp:filter-sharpness=6 -define webp:filter-strength=10 wizard.webp

Icon (small-sized colorful images):

magick wizard.png -define webp:sns-strength=0 -define webp:filter-strength=0 -define webp:preprocessing=0 wizard.webp

Text (text-like):

magick wizard.png -define webp:sns-strength=0 -define webp:filter-strength=0 -define webp:preprocessing=0 -define webp:segments=2 wizard.webp