Does HDR content have anything to do with codecs?

Question

I am new to video codec(encoding/decoding) stuff. I am trying to understand the difference between H.264(AVC) and H.265(HEVC) HDR video content. I thought HDR video 'means' it has 10 bit color depth and H.264 does not support 10 bit HDR content. I would appreciate any clarification on this matter. Thank you.

Answer 1

You can do HDR with almost any modern codec. All it means in practice is that the boundary between the highest and lowest intensities in the image goes beyond what a non-HDR image could portray; if the colors linearly mapped to your monitor on a computer went from 0.0-1.0, then HDR could go above 1.0 (and potentially below 0.0) and be rendered in a convincing way with the right software.

H.265/HEVC (and its royalty-free competitor AV1/VP9) are more about encoding efficiency; H.265 can create files which are roughly half the size of H.264, but due to its licensing fees, the web and world-at-large has been very slow to adopt it.

The chief differences between H.264 and H.265 (the new stuff) are that H.265 supports up to 8K UHD as a resolution, which I don't believe H.264 did (and almost no one will use for the foreseeable future, if I'm reading the video market right); and has a more advanced compression scheme at the same quality level. This doesn't mean that the video will come through higher-quality, but that it will take less space to provide it with the same quality. It's a disk thing.

On the other hand, while H.264 is widely supported, H.265 is proprietary and supporting it requires paying a reasonably hefty licensing fee, which is part of why everyone isn't using it already. (It is quality, it's just proprietary.) For this reason, H.264 is still a very popular standard. While H.264 is generally 8-bit, either one can support a wide range of bit depths in theory.

H.264 can theoretically support up to 12-bit planar, which is usually plenty; unfortunately a lot of decoders (x264 included) have packed that into ten bits and sacrificed fidelity. I've seen people do 14-bit, I'm just not convinced that it mattered. This is still usually more than enough; but bit depth has little to do with HDR, as any range of bits can be HDR if it's just mapped to the appropriate intensities. If you need 14-bit, you will probably want to go with the more modern H.265, just on the basis of the reasons why you might need 14-bit... I think VP9 also supports this, but I'm not sure. The only drawback of VP9 that comes to mind is that you would have a marginally slower stream speed, but it's good enough for Netflix.

That said, a trained human eye can still only distinguish up to about 12 bits; for the everyman it's actually a lot lower. So, unless you're doing medical imaging or radio astronomy, this may be academic.

Answer 2

No, HDR has nothing to do with 10 bit, 8 bit can also be HDR, in both files and in displays, Nvidia/Windows supports that.

HDR is PQ or HLG transfer and those transfers are HDR.

H.264 supports even 14 bit. That is not a problem.

You should see that PQ transfer the file tagged with in Mediainfo. That means HDR is in use. Same about AVIF images.

Answer 3

Yes, it has to do with the codec. If you want to compress HDR10/10+ content, in H264 or HEVC, there are SPS & PPS markers that indicate what the HDR10 settings are within the elemental bitstream.

The metadata merely indicates standard being used : HDR10, HDR10+. While the video is simply 10-bit YUV 4:2:0 or YUV 4:2:2 typically.

After being received over the network, a decoder can extract this HDR metadata and pass it to the display for actual rendering (with full HDR effects).

The HDR10 is a specification that acts like an umbrella for the metadata coming from various properties like HLG/PQ, Brightness Nit Values, Color Primaries etc. etc. However, HDR10 is static metadata, while HDR10+ or Dolby Vision are dynamic in their metadata meaning it can change from one frame to another.