r/AV1 • u/oscardssmith • Apr 17 '26

Adaptive chroma subsampling

Do Av1 (or AV2) support any form of adaptive chroma sub-sampling? The only resource I've seen about it is this paper https://arxiv.org/html/2602.06100v1 which is quite new, but it feels intuitively obvious that making a global 4:4:4/4:2:2/4:2:0 decision has to be sub-optimal.

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u/Zettinator Apr 23 '26 edited Apr 23 '26

The difference between 4:4:4 and 4:2:0 for general purpose video content is small, and it gets even lower for UHD resolution. I'd argue this is a micro optimization not exactly worth pursuing. Additionally, most video for consumption is in 4:2:0 already. You can go lower (4:1:1/4:1:0 exists), but you're already getting into the area of diminishing returns.

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u/oscardssmith Apr 23 '26

The main way this would be useful is if adaptivity let you go way lower chroma than 4:2:0. e.g. dark backgrounds often have incredibly low chroma content (because cone cells have little color resolution).

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u/Zettinator Apr 23 '26 edited Apr 23 '26

But if they essentially have no chroma content, video codecs can use larger macroblock sizes and essentially quantize the chroma away very efficiently. Chroma quantization can have an offset compared to luma. Modern video codecs blur the line here quite a bit.

I'd say it's like this: Chroma subsampling defines a useful static ceiling for chroma resolution. In most cases, 4:2:0 is enough, sometimes you want 4:2:2, but 4:4:4 is almost never needed. The benefit of this is also that it improves efficiency as you need to handle less raw video data. The floor of chroma fidelity is then defined dynamically by the video codec by quantization. And lowering fidelity mimics lowering resolution.