[Canvaskit] How to speed up "non trivial blending" in CPU canvas?

[Marcus Stenbeck]

I've been rendering video with Skottie, and I'm noticing that if there's transparency or some blending mode turned on my render FPS goes from ~60fps down to ~4fps. I've tried to triage what it could be, and it looks like there's a concept of "non-trivial blending" that I'm enabling by using blending or opacity.

Since I'm not in a web browser everything falls back to the CPU-based raster renderer which as expected doesn't have as good performance as the WebGL-backend one.

However, I saw that there's a render flag that's not exposed via CanvasKit. Would that speed up the rendering, and if so, could we expose this via the JavaScript API?

Skottie: https://github.com/google/skia/blob/master/modules/skottie/include/Skottie.h#L182

CanvasKit: https://github.com/google/skia/blob/master/modules/canvaskit/skottie_bindings.cpp#L177

Any other suggestions on what I could try to improve render speeds?

I haven't yet gotten to the point of building the CanvasKit wasm on my own to try this out, and I'm hoping to avoid learning the whole process of building myself if I can avoid it.

[Marcus Stenbeck]

Here's a fiddle with an example of a slow animation: https://jsfiddle.skia.org/canvaskit/be956f5c6a7aa8909bc50d5956b35da5eb3bd3979f9a09c72c399d1e8f8c242c

The image layer has a multiply blending mode (the "bm: 1" in the JSON). Toggle that prop between value 0 and 1 and you should be able to see the performance hit when using the CPU renderer.

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