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Chroma subsampling is a terrible idea in the digital era. You should just assign a different compression factor to the chroma channels instead. I don't understa

Chroma subsampling is a terrible idea in the digital era. You should just assign a different compression factor to the chroma channels instead. I don't understand why we keep using this system of discarding 75% of the color data BEFORE applying the lossy compression algorithm. (Well I do know of one argument - apparently it reduces the amount of CPU time required to compress and decompress the data, but I think that's a pretty lame reason)

kevin_thibedeau 7y ago
There is a physiological reason that you can't resolve the chroma at the same resolution as the luminance.
- blt 7y ago
  But even after you decide to allocate << 50% of your information bandwidth to chroma, there may still be ways to use that allocation than are more effective (perceptually) than 2x downsampling.
  - pingucrimson 7y ago
    This. Subsampling is kind-of the most primitive method of "lossy compression" there is.
blt 7y ago
Good point. When compressing a full video to a size target, I've always felt the 1080p, squashed version was still better than the 720p, less-squashed version.
plorkyeran 7y ago
x264 supports encoding 4:4:4 (i.e. no chroma subsampling) video. There's some very obvious cases where it produces better results, but it's typically just slower with no upside.
- ttoinou 7y ago
  The GP is talking about using 4:4:4 but having lower bitrates for chroma channels than luminance
  - WAHa_06x36 7y ago
    That is still slower than subsampling.
    - zeroimpl 7y ago
      But it can produce a result which looks better at the same bitrate. It’s not all about speed.
      - WAHa_06x36 7y ago
        "but it's typically just slower with no upside" was the original statement.
Ace17 7y ago
> You should just assign a different compression factor to the chroma channels instead [of subsampling chroma]. The good news is that we don't need to change the H.264 standard for this. Supposing that: - most of the already deployed embedded H.264 decoders support 4:4:4 profiles - non-chroma-downsampled input content is available Then, it's only a matter of changing encoder implementations to do exactly as you say.
- zeroimpl 7y ago
  I'm pretty sure there is plenty of 444 content. Movies are likely filmed/edited at 444, they are provided to theaters at 444, but they are generally only made available to consumers at 420. I believe the reason is that most hardware decoders sold to consumers do not support anything above 420. As Blu-rays and such are all currently 420, the chip makers don't have much incentive to support 444. It's a shame because your TV fully supports 444, and the difference can be huge on some content.
ComodoHacker 7y ago
Subsampling actually is a form of quantization, which in turn is a form of lossy compression.
- ttoinou 7y ago
  A bad form of it given that you already have the ability to reduce bitrate per channel on the encoder side. It leads to a lot of issues like pixel format conversion and decoders incompatibilities for example
ttoinou 7y ago
Fully agree. Seeing all the color conversion errors it can lead to also.. In the future it would be more simple for everyone to always use 4:4:4
brandmeyer 7y ago
Chroma subsampling is a natural way to leverage the information content produced by Bayer filters in most cameras. https://en.wikipedia.org/wiki/Bayer_filter https://en.wikipedia.org/wiki/Bayer_filter
verall 7y ago
Chroma subsampling is nice because it lets you "delay" some of the upsampling to later device in the pipeline, like your GPU outputs YUV4:2:0 and your monitor hardware does the chroma upsampling. If you don't want that, use 4:4:4. This feature you're talking about where you assign different compression factor to different color planes is already implemented in x264: https://en.wikibooks.org/wiki/MeGUI/x264_Settings#chroma-qp-offset https://en.wikibooks.org/wiki/MeGUI/x264_Settings#chroma-qp-...

kevin_thibedeau 7y ago

There is a physiological reason that you can't resolve the chroma at the same resolution as the luminance.

blt 7y ago
But even after you decide to allocate << 50% of your information bandwidth to chroma, there may still be ways to use that allocation than are more effective (perceptually) than 2x downsampling.
- pingucrimson 7y ago
  This. Subsampling is kind-of the most primitive method of "lossy compression" there is.

blt 7y ago

Good point. When compressing a full video to a size target, I've always felt the 1080p, squashed version was still better than the 720p, less-squashed version.

plorkyeran 7y ago

x264 supports encoding 4:4:4 (i.e. no chroma subsampling) video. There's some very obvious cases where it produces better results, but it's typically just slower with no upside.

ttoinou 7y ago
The GP is talking about using 4:4:4 but having lower bitrates for chroma channels than luminance
- WAHa_06x36 7y ago
  That is still slower than subsampling.
  - zeroimpl 7y ago
    But it can produce a result which looks better at the same bitrate. It’s not all about speed.
    - WAHa_06x36 7y ago
      "but it's typically just slower with no upside" was the original statement.

Ace17 7y ago

> You should just assign a different compression factor to the chroma channels instead [of subsampling chroma]. The good news is that we don't need to change the H.264 standard for this. Supposing that: - most of the already deployed embedded H.264 decoders support 4:4:4 profiles - non-chroma-downsampled input content is available Then, it's only a matter of changing encoder implementations to do exactly as you say.

zeroimpl 7y ago
I'm pretty sure there is plenty of 444 content. Movies are likely filmed/edited at 444, they are provided to theaters at 444, but they are generally only made available to consumers at 420. I believe the reason is that most hardware decoders sold to consumers do not support anything above 420. As Blu-rays and such are all currently 420, the chip makers don't have much incentive to support 444. It's a shame because your TV fully supports 444, and the difference can be huge on some content.

ComodoHacker 7y ago

Subsampling actually is a form of quantization, which in turn is a form of lossy compression.

ttoinou 7y ago
A bad form of it given that you already have the ability to reduce bitrate per channel on the encoder side. It leads to a lot of issues like pixel format conversion and decoders incompatibilities for example

ttoinou 7y ago

Fully agree. Seeing all the color conversion errors it can lead to also.. In the future it would be more simple for everyone to always use 4:4:4

brandmeyer 7y ago

Chroma subsampling is a natural way to leverage the information content produced by Bayer filters in most cameras. https://en.wikipedia.org/wiki/Bayer_filter https://en.wikipedia.org/wiki/Bayer_filter

verall 7y ago

Chroma subsampling is nice because it lets you "delay" some of the upsampling to later device in the pipeline, like your GPU outputs YUV4:2:0 and your monitor hardware does the chroma upsampling. If you don't want that, use 4:4:4. This feature you're talking about where you assign different compression factor to different color planes is already implemented in x264: https://en.wikibooks.org/wiki/MeGUI/x264_Settings#chroma-qp-offset https://en.wikibooks.org/wiki/MeGUI/x264_Settings#chroma-qp-...