Originally Posted by tgm1024
There's something going on here besides a larger color gamut.
There is a bit more to it, indeed.
Wide Gamut is nothing new to start with. Before LED-lightened LCDs, there were CCFL lightened LCDs. And with WG-CCFL, wide gamut was often used, especially in PC monitors. Thus, the ability of wide gamut was already there, then with (blue-) LED lightened LCDs it got temporary lost (gamut of these is only slightly larger than Rec.709).
Next, it is a misleading assumption that the colors of a Rec.709 video would be "ultimately correct". They aren't, for the simple reason that the real-world, which has been captured by a camera, has a much wider gamut than Rec.709. Given that the capturing device can capture a gamut wider than Rec.709, what happens is that the wider realworld gamut gets compressed, so that it fits into the smaller Rec.709 gamut. This compression is not done linearily, but rather so that colors of weaker saturation stay mostly unchanged, and only the area of highly saturated colors get more of the necessary compression. Then, Sony's color expansion algorithms work in the opposite way: they leave the weaker saturations alone, and only the higher saturated colors are target to expansion.
That's the rough idea. Of course the result isn't always correct, because there is quite some variety about the actual color handling (and capturing gamut) on the input/production side. And a TV studio camera will act different than a handheld consumer camera will act different than a professionel film camera. However, Sony as a content producer has good knowledge about the "typical ways" how colors are handled on the input side ... and based on this knowledge, they did a reasonably good job implementing algorithms that make the appropriate reversion of the original reduction, on average
The situation without
the Triluminos-shebang (ie plain Rec709) is: most colors are correct, but some things stay under-saturated. (Because Rec.709 does just not allow.)
The situation with
Triluminos+expansion is: most colors are correct, but some things may come out over-saturated.
You win here, you lose there. The old game.
It should be noted that Sony did include some degree of defensive-ness particular to the skin-tone color range, in order to not overenhance skin tones too much. This works out quite well with natural
skin tones. But it sure can fail when skin tones are already overenhanced to begin with (orange&peal look of many modern films, think e.g. of the yellow-orange faces in Transformers.)