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Originally Posted by kolak
No- source is 4:2:2 10bit (most likely YUV422P10 color space).
So I need to add line with conversion to 4:4:4 before matrix?
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Yes, just add css="444" in your resample line.
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Another question - how lossless is/can be YUV->RGB->YUV with your tool?
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It's not lossless. For the matrix operation, it will be bound by numerical error noise. So the higher the bitdepth, the better. Anyway a 16-bit chain should be enough to process a 10-bit input.
And if you add inverse and direct chroma subsampling, it will add more errors because of the aliasing and the limited kernel bandwidth. Even using a very large kernel, the losslessness will not be guaranteed.
If you want more chance to recover the original pixels, dither with dmode=1 at the very last step. But if you inserted some operations in your processing like color or level correction, use a real dithering and forget about losslessness.
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If source is YUV will it stay in YUV for all conversions?
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I'm not sure if I correctly understand your question. We now have real high-bitdepth and planar RGB colorspaces in Vapoursynth, so there is no need to use the same tricks as in the Dither tools for Avisynth. When you specify a RGB colorspace in matrix, the clip is really converted to this colorspace.