Originally Posted by mark haflich
How can you convert from a smaller space to a wider space without introducing gross errors?
Simple, you just don't use the whole gamut. It's like how you fit a scope (2.39:1) movie in OAR on a Blu-ray disc that's specified to have a resolution of 1920x1080 (1.78:1), you just don't use the whole thing, you fill out the 1920x800 or so pixels that the 2.39:1 movie fills and leave the rest unused.
To further the analogy, later then, when you crop it back to 2.39:1, you don't lose anything since you just cropped off the unused area.
The same thing applies to gamut, Rec 2020 specifies an area (gamut) that is larger than Rec709 or DCI-P3, so when you go to create your UHD Blu-ray, you just run it through a toll that "pads" the higher saturations so they go unused. It's no different than a black and white movie, or a movie with muted colors today in Rec709.
Nobody here (and I hope nobody at the studios) is talking about stretching P3 or 709 content to fill out a 2020 gamut, that would be terrible and cause lots of errors.
If you are converting down from a wider space, there will also be losses 9all those colors outside the smaller space) but at least the colors within both spaces won't be lost or in error. The perimeter spaces of the smaller space would also have the colors outside that space moved to it.
It's actually a lot harder to reduce the gamut than to pad a smaller one out to fit a larger one. You run into problems like, do you clip hard at the edge, or do you try to do it softly. Like clipping white/black. If you clip hard you can end up with large areas of solid color that could be unseamly. But SMPTE is working on (or already has defined) how to remap wider gamuts to smaller ones.