[Help] "glue" HDR luma to SDR2HDR chroma

[spoRv]

Working on Mad Max, I thought if it's feasible to "glue" HDR luma from UHD to chroma from the regraded UHD HDR2SDR (using DVD as color reference), converted back to HDR.

Now, I could do everything with AviSynth, and finally convert to HEVC, what I'd like to know is how to preserve the HDR data of the UHD... is there any way to let the new encoded HEVC file retains original UHD HDR metadata?

[deleted user]

You just enter the metadata into x265 while encoding and that's all you need to do afaik. With that said, I don't think the metadata is actually all that important, it's more important that metadata exists at all (so the TV knows it's HDR) than for it to be exactly the correct metadata. The metadata doesn't really change the image data, afaik it's there as some kind of orientation for automatically reducing the dynamic range of the HDR material for TVs that have a lower dynamic range/not enough nits. Hence no urgent need to take it into account I think. At least that's how I understand it, someone correct me if I'm wrong.

I think the bigger challenge here would probably be to get correct looking colors, since I think they don't work 1:1 the same way that Rec709 colors work (may need some matrix conversion or whatever?). I could be wrong, but I tried grading HDR footage a few times raw just by playing with curves and the colors never came out quite right. With that said, you could probably just generate an SDR to HDR conversion LUT and then apply that to the color reference and use that. Also would have to see if just combining luma and chroma works well in HDR, certainly curious to find out if you try.

[spoRv]

You just enter the metadata into x265 while encoding and that's all you need to do afaik.

As the second setting I posted here? https://fanrestore.com/thread-2674.html
Do you think it's that easy?

...you could probably just generate an SDR to HDR conversion LUT and then apply that to the color reference and use that.

It is what I'll try to do ASAP!

[deleted user]

I don't know the precise settings because I never really do this kind of stuff myself (only did it once), but yeah it's not very difficult. I think there's a thing where the metadata can change from scene to scene, I wouldn't know how to do that, but again, I don't conjecture it's of immense importance.

For example, most HDR2SDR projects out there are doing tone-mapping that completely ignores the metadata, so how important can it really be?