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Consider that your display can only do 0-100 brightness (not really but for sake of argument) In SDR, you map the full SDR range (also 0-100) to that 0-100. W

by dagmx 3y ago

Consider that your display can only do 0-100 brightness (not really but for sake of argument) In SDR, you map the full SDR range (also 0-100) to that 0-100. When you add HDR, you’re now adding levels above 100 (let’s say 0-200). If your display can only do upto 100, you now need to put all the 0-100 stuff in 0-50. Or you get a display that can also display 0-200. Very few computer displays can do over a standard SDR range of 500 nits.

kstrauser 3y ago
But why isn't SDR then scaled from 0-200, then? That is, why isn't "fully bright SDR" mapped to "fully bright HDR"?
- saltcured 3y ago
  Well, it's pretty arbitrary. What is missing from most image (or sound) data is metadata to say what physical intensity is represented by the signal. I.e. how many nits (or decibels) should be emitted for this signal. AFAIK, most encoding standards only define a relative interpretation of the values within one data set. And even if standards did have a way to pin absolute physical ranges, many applications and content producers would likely ignore this and do their own normalization anyway, based on preconceptions about typical consumers or consumer equipment. To have a "correct" mixing, you would need all content to be labeled with its intended normalization, so that you can apply the right gain to place each source into the output. And of course there might be a need for user policy to adjust the mix. I think an HDR compositor ought to have gain adjustments per stream, just like the audio mixer layer.
- dagmx 3y ago
  What context are you talking about? In single mode where it’s just SDR, it’s mapped to take the full range of the display up to whatever is deemed a comfortable cap. In a mixed HDR/SDR mode, the H is range above the S , so it doesn’t make sense to scale it up. I prefer Apple’s terminology of Extended Dynamic Range because it’s clearer that its range above the SDR range. Now you could say that you intend for that SDR to be treated as HDR, but without extra information you don’t know what the scaling should be. Doing a naive linear scale will always look wrong.

by dagmx 3y ago

kstrauser 3y ago

But why isn't SDR then scaled from 0-200, then? That is, why isn't "fully bright SDR" mapped to "fully bright HDR"?

saltcured 3y ago
Well, it's pretty arbitrary. What is missing from most image (or sound) data is metadata to say what physical intensity is represented by the signal. I.e. how many nits (or decibels) should be emitted for this signal. AFAIK, most encoding standards only define a relative interpretation of the values within one data set. And even if standards did have a way to pin absolute physical ranges, many applications and content producers would likely ignore this and do their own normalization anyway, based on preconceptions about typical consumers or consumer equipment. To have a "correct" mixing, you would need all content to be labeled with its intended normalization, so that you can apply the right gain to place each source into the output. And of course there might be a need for user policy to adjust the mix. I think an HDR compositor ought to have gain adjustments per stream, just like the audio mixer layer.
dagmx 3y ago
What context are you talking about? In single mode where it’s just SDR, it’s mapped to take the full range of the display up to whatever is deemed a comfortable cap. In a mixed HDR/SDR mode, the H is range above the S , so it doesn’t make sense to scale it up. I prefer Apple’s terminology of Extended Dynamic Range because it’s clearer that its range above the SDR range. Now you could say that you intend for that SDR to be treated as HDR, but without extra information you don’t know what the scaling should be. Doing a naive linear scale will always look wrong.