AVS Forum Special Member
Join Date: Mar 2000
Location: Kirkland, WA, USA
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FWIW, I don't think that being able to see the above-reference values (235-254) is absolutely essential to enjoy a movie, but every disc we've ever looked at has had above-reference values in RGB, though for some of them it was just a few stray values here and there. Not all of them have had above-reference Y, Cb, or Cr, though many do. For this reason, clipping in RGB is slightly less problematic than clipping in YCbCr.
It's very easy to generate a "legal" YCbCr triple that produces an RGB triple that's outside 16-235 in one or two channels. And while a lot of video is run through a "legalizer", not all of it is, and some legalizers are more aggressive than others. Some films are "legal" in YCbCr but have not been clipped at all in RGB. And even when clipping is applied during mastering it's pretty common to do a "soft clip" that allows for peaks above reference in small patches.
It's very rare for a film to have highly visible picture content in the above-reference range, where you'd really notice the difference between clipped and not clipped in an obvious way. But then a lot of tweaks we do as enthusiasts are designed to make very minor differences in the image. I've never quite understood someone who will tweak every last little bit of their system for maximum picture quality and then deliberately clip the above-reference material. On the other hand, if you can't get the above-reference to show on your system, it's not a reason to junk the system.
For me, it comes down to first principles: no director, cinematographer, or mastering technician looks at the movie on a monitor that clips above-reference material. No BVM clips above-reference. If you want your display to be as close as possible to the displays they use for mastering, you will try to retain the above-reference values.
Below-black is a more controversial. Pretty much all discs have some below-black pixels here and there. Whether it matters to the final image is a matter of intense debate. As with above-reference, it is easy to generate "legal" YCbCr triples that generate RGB values where one channel or another strays below black. And it's easy to show that the data encoded on discs does in fact undershoot in places in one channel or more. To be honest, for me the main reason to want below-black to be passed to the display is that it shows that the engineers cared about these issues enough to do the right thing. Clipping below-black seems sloppy to me. If they can't be bothered to pass all the data that's on the disc, what else did they get wrong?