[Idea] SDR -> HDR "upconversion"

[spoRv]

Before shoot me, let me say I'm not that HDR expert, so please forgive me if I'll write stupid/useless/impossible things...

Said so, I'm curious to know if there is a way to "upconvert" SDR to HDR.
I'm pretty sure there is - like we upscale SD to HD, or upmix stereo to multichannel etc. - sure, quality will never be as the "real thing", but until a certain movie will be available in HDR, could be an alternative - can't say how valid.

Bear with me: lately I (and others) am experimenting with high contrast versions, to get that "movie look" so nice; of course, it's far to be considered HDR, but I thought about this: with only 220 "steps" in luma in rec.709, we have limited ability for dynamic range; in rec.2020 10bit, we would have 4 times those steps; I'm aware this is not strictly HDR, still there is more space to "expand" the dynamic range. The next step would be to add the right metadata to have HDR. And here I'm completely lost...

But I'm starting to think about this, because, sooner or later, we will be "forced" to upgrade to UHD - as we were from SD to HD (glad I started at the right moment!)

Ideas, opionions, proposals, experiences?

[OogieBoogie]

I forget their tag/username, but someone created their own HDR system a while back that activates HDR systems within screens. I'm fairly certain it was Rogue One that they did, or at least the one I saw and they were fairly descriptive in their methods too

[spoRv]

visionplushdr-x?

[OogieBoogie]

visionplus rings a bell, could be, what made it unusual is that they were honest about it

[deleted user]

Do you mean HDR or simply 10 bit? You can encode SDR in 10 bit and have more of those finer steps. But since most displays are not 10 bit one may question the use of that. One use scenario obviously would be material that is meant to be further color-graded.

HDR typically refers to Rec2020/Rec2100 and its color spaces which you need compatible display devices for (else you just see a low contrast picture). There are two variations of that color space as I understand it - PQ and HLG. PQ means Perceptual Quantizer. Don't ask me what that means but that is the most commonly used one and the one used for UHD Blu Rays. Afaik this always involves additional metadata per clip. Then there's HLG which means Hybrid Log-Gamma and was meant to be a kind of backwards-compatible mode that still looks OK on SDR devices color- and contrast-wise while having more headroom for highlights (by compressing them). Afaik HLG involves no metadata. I personally like the HLG concept but it doesn't seem to have caught on.

The obvious problem with upconverting SDR to HDR is that SDR is very often clipped in both shadows and highlights and you aren't getting that information back. Yeah you can totally convert SDR into a HDR colorspace but that will only make your SDR picture make look like an SDR picture on a HDR display and not come even close to filling up the entire spectrum of the HDR colorspace. The whole appeal of HDR is that you can have details in both shadows and highlights while also having high contrast. In SDR you have to typically compromise - lose out on contrast while retaining detail or get proper contrast while clipping (or strongly compressing or tonemapping).

Now if you had a movie that has a very low contrast look to begin with and no clipping you could convert it to HDR and then do some further color correction to fill up the HDR colorspace with all those colors that were "squeezed together". With enough skill involved you may end up with a very good result I think and a very dynamic image. The question is how accurate this would be to the movie's "intended" look. But that will be a whole new discussion with HDR to come...

There's a website that can create 3D LUTs to convert between the various color spaces for you but I can't currently find it. AviSynth+ may have functions that do that too. But you gotta be careful with PQ there I think because as mentioned it incorporates the metadata and I am not sure how without taking them into account you can end up with an accurate result (though you can certainly end up with an approximate one!)

[CSchmidlapp]

HDR confuses me. Especially the meta tagging element.
A 10bit panel able to produce all the variations included in rec2020 should be a standard?

EDIT Posted before i read TomArrows post

[OogieBoogie]

HLG was never created as a competing standard, it was made by Nippon TV and the BBC to allow for their broadcasting infrastructure which wouldnt have been able to broadcast conventional HDR everywhere, it was only ever a compromise.

[Jetrell Fo]

I can't imagine there being any good coming from an SD to HDR up conversion.

[Doctor M]

HDR is already an upconversion in any film not shot for it in the past few years. Directors and cinematographers never intended their films to look like what HDR releases look like. Any that supervise the new transfers are just doing it for the paycheck.

It is the modern equivalent of converting 2D films to 3D or colorizing black and white. I'm really surprised more purists aren't complaining about it.

The problem is, in theory, they shine a light source of different intensities through the film and then combine the results. A bright light will pull more detail from the shadows, a weaker one allows the faint detail in the brightest areas to show up. With HDR the combined result can be shown all at once.

Seriously, are we supposed to believe they had no idea what details were being hidden in shadows or washed out by the sun when they made movies older than three years?

Anyway, post conversion is sketchy. Unless you have a film source, it's unlikely you'll be able to pull anything out of the brightest or darkest areas. Most TV 'enhancement' features (aka torch mode) do the best they can already along those lines.

EDIT: ALTHOUGH, since we know we can apply curves to correct colors, in THEORY we could apply the same technique and attempt to extrapolate into the wider gamut of HDR. It would be completely bogus, but maybe possible.

[Stamper]

Um what? HDR is just an expanded color space. What you say It's like we're all typing on computers for the money instead of on typewriters as originaly intended.