Blurry animations without feature1_2

[walles]

Animation with feature 1_2

Animation with feature 0_5

Description

Both files were generated using this exact code, just clone it and cargo run --bin=screenshot:
https://github.com/walles/loadviz/blob/b19b25e91687b274db487cb4f9daeee9ce82b580/libloadviz/src/bin/screenshot.rs

If you change the required feature from 1_2 to 0_5 without any changes to the Rust code, you'll get the blurry screenshot when running the program.

Expectations

I expected enabling features would give me things like "now you can encode animations!". And it does, this is good.

I did not expect enabling features heavily affecting the quality of the output. But it does, I think this was bad.

Also, I think this behavior is really un-obvious, so maybe 0_5 should be removed? Or fixed if there's something wrong with it messing up the quality?

Notes

Since GitHub doesn't support uploading WEBP images (please vote for it!) I used this trick for that. They are really WEBPs, no matter what the file extensions say.

[qnighy]

Yeah it's strange.

libwebp-sys2-rs is designed to be a handwritten translation of libwebp(C)'s public headers, and these public headers evolve between versions. The version-numbered features, such as 0_5, 0_6, and 1_2 are just meant to switch header versions. Therefore there are three possibilities for the behavioral inconsistency:

1. libwebp(C) behaves differently based on the difference in its header versions.
2. libwebp-sys2-rs does not correctly translate libwebp(C)'s public headers.
3. Some sort of Rube Goldberg machine, possibly involving undefined behaviors.

And technically, libwebp-sys2-rs defines slightly different interfaces based on its version-numbered features because the original headers do so. Especially,

1. Some of the structs, including WebPConfig, often have different fields of different sizes. I expect libwebp(C) to be designed so that the changes are backwards-compatible, but I'm not 100% sure.
2. It declares different ABI versions, which are passed to the library during initialization. libwebp(C) may be using it to switch behaviors, though I think this is fairly unlikely.