About HDR to SDR Quality

[jay123210599]

If I convert a 4K HDR video or movie to SDR, will it still have higher quality than the official 1080p SDR version?

[Selur]

Depends on how the official 1080p release and your conversion was made in details. (used tools and settings)

[GeoffreyA]

If I convert a 4K HDR video or movie to SDR, will it still have higher quality than the official 1080p SDR version?

Assuming UHD and Blu-ray for simplicity, it depends on a lot of things. How good the UHD is, how bad the Blu-ray is, and how good the tools are.

If one is looking at 2008-era Blu-rays, many were poor, ill encoded, and made from ageing masters. A more recent Blu-ray of the same movie, perhaps made from a 4K scan of the camera negative, will often be as close as one can get to best quality in SDR, if the grading was done faithfully and well. Often, but not always the case. Take the 2018 remastering of The Matrix: its 1080p Blu-ray was ill graded, blowing out terribly in certain scenes, such as the rooftop sequence; but the UHD—made from the same scan—is thought to be the best home presentation to date. Creating an SDR version from that UHD may well give better results than the Blu-ray.

Other times, it might be subjective. Criterion's Blu-ray of Mulholland Drive, made from a 4K scan and overseen by Lynch and Deming, is excellent. The UHD, made from the same scan, is thought to be a bit better. So, if you were to make an SDR version from the UHD, which I've done, it might be subjective which SDR version looks better, complicated by the fact that the UHD was graded with a darker picture than the Blu-ray. (Another phenomenon, at least on MD, is that the grain is finer and more "condensed" on the UHD, which carries over to the SDR-converted version's compressing better than the official Blu-ray, whose grain is coarser.)

I think the official Blu-ray, after review, should be considered the first choice of an SDR version. If it's not accessible, a rubbish old one, or done poorly, converting the UHD, with libplacebo for instance, will usually give solid results. Also, if you are targeting 10-bit depth, starting from the UHD, you've already got a slight advantage over the Blu-ray, mainly affecting banding. However, if the conversion is done with poor settings and tools, it can be disastrous.

Comparison: https://we.tl/t-Wr3w6EQDUV

[FranceBB]

It really depends on what you're targeting as an output, but in principle starting from the UHD BD will yield better quality.
You see, the UHD version is 3840x2160 4:2:0 10bit planar, which means that it has 3840x2160 luma and 1920x1080 chroma (which is upscaled on playback by your 4K TV during the RGB conversion).
What I usually do is to downscale the luma while preserving the resolution of the chroma (you're gonna have to align it as it's top_left originally) to get a perfectly valid 1920x1080 4:4:4 10bit.
From there, you can either decide to leave it as PQ and let the player convert it to BT709 SDR on the fly or make the RGB 16bit roundtrip and apply a LUT or perform dynamic tonemapping with an XYZ 16bit roundtrip. It's your call.
The official FULL HD BD however will be 4:2:0 8bit, so in theory it *should* be inferior.

[GeoffreyA]

FranceBB, yes, that point has been on my mind now and then and I've wanted to do it. On the UHD, we've already got 1920x1080 chroma, and in the usual conversion, that is being scaled up to 2160p, merged with the luma, and converted to RGB linear light. Tone mapping, etc., are done and it is scaled down. So the chroma is going, so to speak: 1080p > 2160p > RGB > 1080p > YUV 4:4:4 > 540p.

I haven't worked out how to avoid that properly in my encoding "pipeline." (I did it once but it was coming out over-saturated, likely because gamut mapping wasn't done.) It would require some thought. Also, using libplacebo, for instance, does complicate things because it abstracts the process, and outside of it, dynamic tone mapping is not easily accessible in the common tools, as far as I'm aware.

[huhn]

downscale to 1080p and then tonemap?

upscaling the chroam first and downscaling next is not a really big issue.

[GeoffreyA]

downscale to 1080p and then tonemap?

upscaling the chroam first and downscaling next is not a really big issue.

Please elaborate. I'm a bit lost.

[Asmodian]

Downscale the luma to match the chroma for YUV 4:4:4, do the convertion to RGB and tonemap, and then back to YUV 4:4:4.

[GeoffreyA]

Downscale the luma to match the chroma for YUV 4:4:4, do the convertion to RGB and tonemap, and then back to YUV 4:4:4.

Thanks! I'll try to implement that.

[jay123210599]

Selur, GeoffreyA, and FranceBB, the movies I'm talking about are The Incredibles and Incredibles II 4K UHD Blu-Ray Movies. My target output would be HDR to SDR 4K with black bars removed.

[GeoffreyA]

Yes, your 4K SDR version will have higher quality than the 1080p SDR version. In the script on the other page, just remove the 1080p scaling part, "w=1920:h=-1".

[jay123210599]

Yes, your 4K SDR version will have higher quality than the 1080p SDR version. In the script on the other page, just remove the 1080p scaling part, "w=1920:h=-1".

You mean this? https://forum.doom9.org/showthread.php?p=2013487#post2013487

Do I remove crop=1920:804:0:138 from the script?