yuv: Drop the LUTs, do integer-only (fixed-point), SIMD-capable arithmetic instead #15
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Let me once again link to some high(er(ish)) resolution video content to benchmark on: |
adrian17
approved these changes
Dec 29, 2021
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FYI, the crate references in main repo will need to be updated after this. |
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This, somewhat surprisingly, already speeds up the conversion function by a factor of about 10% on the web target in itself.
But the more important thing is that it's SIMD-capable. And that target feature (SSE2) is enabled by default on desktop targets, AFAIK.
All the tests still pass (in fact I had to add a couple more), and the sample videos I tested still look completely fine to me.
Once ruffle-rs/ruffle#5834 is merged, it should speed it up (on web, in capable browsers) by an additional factor of 2x. Why only 2x, and not 4x, you ask? Well, I don't know. Maybe we should ask our friend, Amdahl. I'm not complaining though, it's still a nice uplift.
I've also experimented with 16-bit intermediate precision, but it's just barely not accurate enough for my taste, and isn't any faster. And using
i32x4also allows the neat transpose trick at the end.Nor is doing a 2x2 group of pixels together faster, even though the chroma samples can be just splatted across all lanes, the additional shuffling in memory and more complicated iteration most probably negate that.
Additionally, if we were really serious about performance, we could also use
bytemuck::pod_align_to, but since the h.263 decoder chops off the luma samples to odd widths in some cases, it would make lining up the chroma samples to it kinda awkward. And the WASM VM might also not even JIT these loads into the faster aligned versions of the native instructions...