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Old 17th October 2018, 15:43   #53335  |  Link
Warner306
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Join Date: Dec 2014
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Quote:
Originally Posted by mytbyte View Post
No, I want to work with measured nits but I am wandering why MadVR generated BT.2390 curve (as measured) drifts gradually (and significnatly) away from HCFR reference BT.2390 curve and then returns to being spot-on at 70% but then gets clipped at 80-90% stimulus if we know the "formula" is defined in the papers and should be the same in HCFR and MadVR. Is the 80-90% clipping part of the formula to mantain some HDR effect with low peak brightness?

If translation to SDR was the cause, I expect the PQ curve tracking would drift as well but it is not.
There is likely something different about the BT.2390 used by HCFR and the BT.2390 used by madVR. madVR does additional processing, but testing luminance alone should yield a similar curve, I would assume. Your display is tracking PQ values accurately.

The brightness response of BT.2390 is also very different than straight PQ clipping because tone mapping deviates far off the original PQ curve. The SDR gamma curve would have to be contorted a bit to make it look right.

It would be hard to say what would be causing the differences without knowing if there are differences in the tone curves. I would try testing different mastering peaks (lower than 10,000) with target nits values in madVR that are above your display brightness because that's how most would use pixel shader. If I set my display to 175 nits and the target nits to 175 nits, the image is washed out. I don't think that is intended by BT.2390.

Quote:
Originally Posted by mytbyte View Post
hmm...there is no bar 16, there is a clip with 64-80 bars and there is a clip with C64-C111 gradient (0.0-0.07 nits). In the former I can, with great effort and in pitch black room notice that bars 76-80 are flashing, almost un-noticebly. In the gradient clip I can see the gradient becoming VERY slightly lighter, from left to right, than the bottom black part, so I am guessing there is no crush but also that near black gradation is so barely noticable - don't know if that's a problem and it should be more noticable.
I would doubt you are crushing black given the accuracy of your calibration. The 8-bit bars are represented along the bottom of the 8-bit clipping pattern. They are hard to see, but they are there.

Last edited by Warner306; 17th October 2018 at 15:54.
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