proposal: x/image/webp: add animation support to the decoder

[bep]

I did search for this, as I expected someone to have raised this issue before.

I tried to decode a webp image with animation and got:

webp: invalid format

The behaviour of gif.Decode in this situation is to decode the first frame. And then there is gif.DecodeAll which gives you a struct with all frames and the timing etc. information. It would be nice to have a webp.DecodeAll or similar.

[DeedleFake]

Animated WebP has depressingly poor support, even on Android, despite being vastly superior to GIF in many, many different ways. It would be fantastic if Go could add it.

[ianlancetaylor]

CC @nigeltao

I doubt that anybody is opposed to doing this, but somebody has to do the work. I'm not sure that this needs to be a proposal at this stage. Accepting a proposal won't cause the work to be done.

[nigeltao]

Yeah, I agree with what @ianlancetaylor said. I certainly don't have the spare time myself to work on this.

[silverwind]

A sample image can be found here: https://mathiasbynens.be/demo/animated-webp

[patricsss]

I have a working implementation of this, I just need to clean it up and submit a PR.

[patricsss]

https://go-review.googlesource.com/c/image/+/513295

Still needs some cleanup and documentation, but that's the gist of it.

webp/vp8 is kind of funky. I'm torn between two things and would love feedback:

1. Keep the decoder as parsing vp8/webp and returning the appropriate image data/metadata

This has a major pain-point/drawback which is that the image frames are offset and aren't necessarily what you want from trying to decode the data or

2. Re-write the image data into new images, taking into account the Frame{X,Y} and Frame{Width,Height} values from the ANMF frame headers.

This would make the library more useful, but also is more expensive to do.