I’m curious as to whether this setting actually does what I think it does. What I would expect to happen is that the setting defines the highest luminance that the SDR TV is capable of producing (at full white); so you map the MaxCLL luminance for that video to that maximum value, and then compress the luminance range of the movie to lie between 0 and that value. So, for example, if the value is 350 and MaxCLL for this movie is 1000, then you map 1000 nits luminance to full white, 0 nits luminance to black, and values between 0 and 1000 nits to values between 0 and 350, then find the equivalent SDR value on the curve.
And tweaking the contrast (I guess!) changes the shape of the mapping curve in the middle without affecting the ends - a bit like adjusting the gamma?
EDIT: And does that maximum luminance parameter still affect subtitle brightness? And if so, will that be a permanent thing?
You have described it very well. The reason it’s subtle is we scale the luminance in the PQ domain then scale it back up to ‘full scale’ in linear light. Changing the value just makes the system use a different bit of the PQ curve.
Only when you are outputting HDR as HDR. It has a different role in that case to the HDR-SDR case.
Is that going to be a permanent feature?
Depends on user feedback. If it’s useful, we can add a separate setting for those needing both HDR and SDR output from the same device.
Even is it’s only in the interests of avoiding user-confusion, that seems like a good idea to me! No one is going to guess what that setting does in HDR mode.
If it were me, I might also have a new and differently named setting that does what the Contrast control currently does in HDR->SDR mode. “Contrast” implies that you’re adjusting the white level, whereas this (if I’m understanding correctly!!!) is more like adjusting the gamma value - it will have the most effect in the mid-range and also adjust the trade-off between shadow-detail and highlight-detail, while keeping the actual black- and white-level the same
Contrast means we are adjusting the gain, exactly as in a traditional CRT. It doesn’t affect the white level.
That was my point: if it doesn’t affect the white level, it shouldn’t be called “Contrast”. People interpret that word in the sense of what “Contrast” does on a modern flat panel display, not in terms of what it used to do on CRTs.
I mean contrast in general doesn’t affect the white level.