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Old 16th March 2023, 05:28   #100  |  Link
benwaggoner
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Join Date: Jan 2006
Location: Portland, OR
Posts: 4,750
Quote:
Originally Posted by FranceBB View Post
Uhmmm, I'll tell you what, I think it could work even right now. H.264 can transport HDR info in SEI so it's very easy to create an 8bit BT2020 HLG file. Chrome also is able to recognize the desktop colorspace and the normal video stream info (chrome://gpu), so in theory it should be able to recognize an H.264 stream flagged as arib-std-b67 and bt2020-10 just like it's able to recognize it if it's an AV1. I mean, it should actually work already out of the box with the current technology without any further update, in theory, but no one is doing it.
HLG is HDR-lite, however. And nominally requires 10-bit. The only reason anyone ever uses HLG as the same bitstream can provide reasonably decent SDR and HDR. But bang-for-the-bit is always better doing native SDR or native PQ HDR if universal compatibility isn't required.

HEVC is the current standard for HDR because:
  1. HDR typically comes with 4K, and HEVC is >2x as efficient as H.264 at 4K resolutions. These are big files, so the lower bitrate is really helpful.
  2. Real (PQ) HDR needs some clever QP offset tuning for optimal results, which has only been well documented for HEVC. That's what x265's --hdr10opt implements.
  3. All HDR capable CE devices support 10-bit HEVC decode, even though many of those only support H.264 up to 8-bit.

Sure, whacky things can be done with full range mapping and dithering and all that. Heck, I could make HDR MPEG-2 with sufficient motivation and time. But in the end it's a lot of work and a lot more bits for worse results, and no real reason to bother. Essentially everything with an HDR tone mapper and display supports 10-bit HEVC decode. We'll upgrade to better thing in the future, like VVC and/or AV1. But I can't imagine why anyone would practically benefit from using a sub-HEVC codec for HDR.
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