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SDR to HDR (Inverse Tone Mapping)

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jmone:
My only other wish, is for SDR to HDR conversion to bypass whatever Windows 11 is doing when in full time HDR Mode.  Not that Win11 is doing a "bad" SDR --> HDR conversion but, I presume that JRVR would be better. 

Hendrik:
There is really no ambiguity when doing a naive SDR to HDR conversion, because you are not converting the image to HDR, you are just fitting the image into a larger box. There is a perfect answer to this task, and its not so hard to achieve it.

bogdanbz:
Regarding the "SDR to HDR conversion that Windows is doing".

Windows is doing a sRGB color space with a sRGB gamma function conversion to BT2020 colorspace with ST2084 transfer function. Which means Windows considers the sRGB content to have the sRGB "gamma" function applied, and then maps it to ST2084 absolute luminance values, with the white in the sRGB content reaching the value you configure with the slider in the "Windows HD color settings", where the slider at zero means a sRGB white mapped to 80 nits.

Mapping the sRGB "gamma' to ST2084 is correct for PC content (SDR PC displays are supposed to be used in their sRGB mode for accurate display, which is not the same as a "2.2 gamma mode").

But this mapping will not give the expected results for video content, which is encoded with a 2.4 or 2.2 gamma function instead of the sRGB gamma. The end result of the Windows mapping is that lower luminance values in video content will appear brighter than they should be, giving a washed out appearance.

jmone:
There seems to be two main ways to do SDR-->HDR and the following is from Dolby on the approaches.  I presume that Windows is doing the first, but I'm more interested in the 2nd (Inverse DRT with White Point Adaptation).  I use this method in Resolved when combining 709 and LOG footage on a single HDR project and it works really well.


--- Quote ---Up-mapping SDR Archive Content to HDR

There are two fundamentally different ways to approach this issue:

The accurate (mathematical) conversion from the SDR to HDR format is defined in the document “MovieLabs Best Practices for Mapping BT.709 Content to HDR10 for Consumer Distribution” https://www.movielabs.com/ngvideo/MovieLabs_Mapping_BT.709_to_HDR10_v1.0.pdf
This preserves the original look of the SDR image within HDR. This means that the resulting HDR watched on an HDR display should look identical to the original SDR watched on an SDR display.  However, it does not change the look of the image and therefore may not blend well with other native HDR content. Such conversions are also referred to as direct mapping conversions, a terminology used in the Ultra HD Forum Guidelines.
 
To perform an inverse tone-mapping of the SDR content to create an “HDR look”.  The Ultra HD Forum refers to this type of conversion as up-mapping. This is often the desired approach as it will enable SDR clips to more seamlessly blend into an HDR production, or in the case of a complete SDR archive production creating this “HDR look” is often the primary reason for performing the remastering.
--- End quote ---

jmone:
If I'm reading some of the libplacebo notes correctly, looks like hassn has already implemented RTM.


--- Quote ---haasn commented on Feb 19, 2022
This was added in libplacebo v4.192, using the BT.2446A curves (among some others)
--- End quote ---

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