AVIF lossless is not accurate

[tomByrer]

Are you using the latest version? Is the version currently in use as reported by npm ls sharp the same as the latest version as reported by npm view sharp dist-tags.latest?

v0.28.1

What are the steps to reproduce?

.avif({ quality:50, speed:2, lossless:true })

What is the expected behavior?

The diff to be equal to source .png

Are you able to provide a minimal, standalone code sample, without other dependencies, that demonstrates this problem?

I'm not sure if you want my entire testing program copy/pasted here? (I use sharp to diff ;) )

Are you able to provide a sample image that helps explain the problem?

src .png: (note: highly lossless compressed via FileOptimizer)

sharp lossless CPU2 (note: file is really AVIF, but added .png at end to hack, urr I mean upload here)

diff:

diff with the low levels increased to see artifacts easier; note the blocks of red, blue, & green show lossless shifts more than edges:

avif.to is much closer, though a few blocks & some edges still are not perfect:

squoosh.app has truelly perfect lossless. Uses libavif also, but unsure how they encode lossless. Diff would be blank (useless), so here is their AVIF lossless output:

What is the output of running npx envinfo --binaries --system?

System:
OS: Windows 10 10.0.19042
CPU: (8) x64 Intel(R) Core(TM) i5-8300H CPU @ 2.30GHz
Memory: 2.28 GB / 31.85 GB
Binaries:
Node: 16.0.0 - C:\Program Files\nodejs\node.EXE
Yarn: 1.22.1 - C:\p\n\yarn\bin\yarn.CMD
npm: 7.10.0 - C:\Program Files\nodejs\npm.CMD

Thanks for your interest & sharp!

[hdmr14]

same here

[lovell]

Hi, this looks like the effects of chroma subsampling - please see #2562