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Old 17th May 2022, 16:19   #161  |  Link
wswartzendruber
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Quote:
Originally Posted by ErazorTT View Post
So then you say that 90% of the SDR signal should map to 75% of the HLG signal?
That is my present philosophy until someone can argue why it shouldn't be.
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Old 17th May 2022, 23:33   #162  |  Link
ErazorTT
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That is my present philosophy until someone can argue why it shouldn't be.
I found a sentence in 2408-5 supporting your view. In section 7.7:
Quote:
The difference in perceived mid-tone contrast arises because of the difference in the diffuse white level of an SDR image (~100 cd/m2) and that of an HDR image (~200 cd/m2).
Thus in order to compare the pixel signal values of SDR and HLG, we have to carry out the comparison under the assumption that SDR and HLG have the same diffuse white level of ~200. Making the 100% signal of SDR going above 200 in that comparison.

PS: i found an even better quote from 2408-5. Table A8.2 states the following numbers:
90%SDR = 75%HLG = 203nits
100%SDR = 79%HLG = 260nits

Last edited by ErazorTT; Yesterday at 00:03.
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Old Yesterday, 02:58   #163  |  Link
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Uh, wow. Appendix 8 is new for the fifth revision. And it's all about how the PRC does HDR/SDR simulcasting.

And...we independently came to similar conclusions about mapping HLG to SDR.
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Old Yesterday, 09:23   #164  |  Link
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You know, I had the impression that HLG not only has more headroom in the highlights but also in blacks ("bottomroom"..? ). At least when reading through the BBC documentations.
But now our HLG streams have basically the exact same pixel signal values in the darks and midtones as SDR. So how to reconcile this with the BBC statement?
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Old Yesterday, 21:57   #165  |  Link
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Wait, what does the BBC say that counters this?
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Old Today, 01:25   #166  |  Link
ErazorTT
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this here for example at around minute 8:
https://www.smpte.org/webcast/dr-pq-...-presented-bbc
Granted, at this stage he talks about the camera curve (OETF) and not the display curve (EOTF), but I would have thought that there should have been also some repercussions in the display curve if the camera curve is different.

here are the slides for that talk:
https://www.dutchguild.nl/31jan2017/...0published.pdf
Especially slide 14, where it zooms into the curve, and one sees that the SDR curve does actually not match the HLG curve.

And then there is Figure 18 in 2390-10, again the OETF, the red HLG line is not exactly on top of the blue SDR line in the lower half, but slightly above it. Thus bringing home the same message as the plot on slide 14.

Or to put it differently: Section 5.1 and section 5.2 of 2408-5 do the SDR to HLG conversion differently. 5.1 produces an HLG signal that follows the same curve as SDR and can be matched exactly like we're doing, and 5.2 generates a HLG signal which looks more like the OETF plots I mention above.


I am aware however that the PQ to HLG conversion does not have any of these ambiguities and has only the luma-scale as a free parameter. So all this does not contradict your LUT generator, it's a question about the expectation of what curve the birghtness of the resulting HLG will follow. And this is not clear to me. Should I expect to get basically an SDR curve in the lower half, which would match the section 5.1 SDR to HLG conversion, or should I expect to get something looking like the OETF which does not match the SDR curve and would reproduce the section 5.2 SDR to HLG conversion.

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