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Originally Posted by xooyoozoo
VP9's column-wise parallelism is effectively splitting a frame into several small frames. It's similar to and is about halfway between HEVC's tiles and slices. Like tiles, there's no signaling/encapsulation overhead for each element, but like slices, the long rectangular elements (as opposed to square'ish ones) result in more dependency breaks and decreased efficiency.
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Column parallelism is very strange, since there's a lot more correlation along the horizontal axis than the vertical. Plus motion vectors tend to be much more horizontal than vertical.
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The weird thing, though, is that Google's (and therefore Chrome's, Opera's, Firefox's) VP9 decoder needs the encoder to operate in `frame-parallel` mode, which additionally worsens entropy coding because that also disables the backward context update tool.
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Well, I guess it doesn't matter, then
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It's troubling if there are legal VP9 features that don't work in current decoders. There should be some sort of profile/level way to signal and constrain features for device compatibility. Doing it feature by feature is quite fraught.