Animated webp encoder

[Poker-sang]

Prerequisites

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• I have provided test coverage for my change (where applicable)

Description

Animated webp encoder
closed#2521

[Poker-sang]

How to start workflow?

[JimBobSquarePants]

Crikey you’re fast!!

Workflows need permission to run from the repository owner. I’ll run it now.

[Poker-sang]

I tried to play the encoded out image in my local image viewer and it plays fine. But I'm not sure how to write a good unit test to verify it.

[JimBobSquarePants]

I would look at our Gif unit tests. We test using individual frames plus do metadata tests to ensure consistency.

[Poker-sang]

@JimBobSquarePants To avoid creating bitWriter objects too early, I changed some methods to static. As a result, the scratchBuffer field does not have any references. Should we remove this field?

ImageSharp/src/ImageSharp/Formats/Webp/BitWriter/BitWriterBase.cs

Line 28 in 2c260b2

privateScratchBufferscratchBuffer;// mutable struct, don't make readonly

[JimBobSquarePants]

If it's not used yeah.

[Poker-sang]

I'm sorry I used Resharper to refactor to Specify created type. Forgetting it leads to SA1117.

[JimBobSquarePants]

We need to throw some test images at this now.

I would like to see some testing involving alpha blending as I think we've previously implemented it incorrectly. If it works anything like png the blend should take place per scanline where a temp buffer is used to fold the initial decode before blending it with the frame row. If you look at the latest changes to #2511 you can see how it is implemented there.

[Poker-sang]

Do you mean that I should modify the WebP decoder and encoder according to APNG? Especially to address issues that arise with Blending and Disposing. Then select some new test images?