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I struggle to think of a situation where the blur of chroma subsampling is acceptable but very high quality or prores isn't good enough, I need lossless.
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4:2:0 is not lossless. 4:4:4 with quantizing down to 8-bit YCbCr isn't lossless either. But 10-bit 4:4:4 is reversible back to 8-bit RGB without loss.
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There's a distinction to be made between lossless (compression) and lossless (bit depth/gamma/yadda yadda); typically the format conversion isn't considered part of the "lossy/lossless" distinction.

Definitely not something to be forgotten if you're expecting to compress lossless RGB, although this is pretty rare outside of niche-ish use cases (afaik game capture is basically the only place where people are likely to be using RGB and not YCbCr? could be wrong!)

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Also (if you round correctly) there is no generation loss in the format conversion. You can round-trip between them as many times as you want and you only get the loss of the initial RGB->YCbCr.
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Animation production (2D or 3D) can also have raw RGB frames.
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Ah, I hadn't considered that but that totally makes sense!
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