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Old 19th February 2024, 20:44   #18  |  Link
ErazorTT
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Join Date: Mar 2003
Location: Germany
Posts: 215
@wswartzendruber, yeah sorry I was all over the place with my thoughts.

In the meantime I have sorted my thoughts and came to the following conclusion: Having one LUT for the process of tonemapping the PQ down to 1000 nits AND converting to HLG is proably not the most optimal way of doing it.
The main argument here is that all the different maxcll and lumascales are part of the tonemapping, which can be done with a 1D-LUT (one for each combination of maxcll and lumascale) while the conversion to HLG can be done with a static 3D-LUT, which absolutely never changes.
Since that 1D-LUT can be generated extremenly fast for each combination of maxcll and lumascale, this opens the possibility for dynamic tonemapping.

So I went and implemented exactly that in DoViBaker. It now provides the possibility to have dynamic tonemapping, based on a 1D-LUT generated in memory on the fly. And then on top of that the user can apply the conversion to HLG with a static 3D-LUT. Of course there is also the possibility to have static tonemapping in case the input is just PQ without DolbyVision.

I think that this is an impoved workflow since on top of the addition of dynamic tonemapping, I can also circument the issue with the maximal output brightness of the HLG conversion, by allowing the intermediate output after the tonemapping to be scaled such that it fills the whole singal range. The then adjusted HLG conversion 3D-LUT uses the whole input range, which solves that problem. And also that adjusted 3D-LUT can be much smaller since now all LUT-points are used, and not only those below maxcll.
So instead of having a 65x65x65 LUT where only the lower 50x50x50 points are used, I can now directly use an adjusted 50x50x50 LUT, which has the same fidelity as the usual 65x65x65 LUT but now also hits the absolute max output brightness perfectly, while being less than half the file size (and CPU cache size).

Last edited by ErazorTT; 19th February 2024 at 21:10.
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