Originally Posted by Doug Blackburn
Let's not forget that editing is done in RGB space... probably 14- or 16-bit, though it's possible there's some 18-bit equipment out there today that I don't know about. And it is EASY to constrain the editing station software to the 14- or 16-bit equivalent of 16-235 8-bit space or to edit in "full range" space and constrain the data to 16-235 later when the original data is converted from its original 14, 16 or maybe 18-bits to 8-bit 16-235.
Sure, that would
be easy to do. But that isn't done most of the time, and the proof is in the content that we have. If you look at the values in the content, you can tell whether it's been clipped off or not, and almost anything I've looked at on DVD isn't clipped off at 16-235 in RGB. I don't have the ability to examine BD, but Stacey does, and the images he just provided should be interesting for you to observe. There is no good reason to clip off the content at 235, particularly when the peak white range 236-254 is explicitly provided in Rec709 for video values above 235. So when you have a video standard that provides you room for transients or specular highlights or heavily saturated bright objects above your nominal reference white, what advantages do you get by clipping all that off? That's a rhetorical question.
Data doesn't start out as YCbCr... YCbCr is an intermediate step.
Obviously. Going back to square one doesn't really deal with the discussion thus far.
YCbCr data originates as edited RGB that has been downconverted and constrained to 16-235 during the downconversion process.
Except that it hasn't been constrained to 16-235 in RGB. There is no good reason to clip the content this way during mastering, and if you actually looked at the content you'd know that it hasn't been.
Using your example of 250, 181, 240 (RGB - I'm not checking the conversion math, I'll assume it is correct)... the point defined by RGB 250, 181, 240 will not exist in your original RGB data (before it is converted to YCbCr).
Why wouldn't it? It certainly could. It's a perfectly legitimate piece of video data. Unless a hard clip was applied at 235 (which as evidenced by numerous examples above, and in many threads previous, and from a variety of sources is not common), then that pixel could conceivably exist in a source without any difficulty or the sun spinning off its axis or the disc player exploding in a ball of fire. It happens all the time. Everytime you put a disc in your player you're probably looking at values like that. Unless of course you decide you know better and clip it off.
Assuming you are using YCbCr encode and decode matricies that aren't screwed up, you will never encode/create a YCbCr triplet that would decode to an out-of-gamut RGB value.
You would if that RGB value were in the content to begin with. And we have every reason to believe with near certainty that such values exist. The discussion about whether or not they exist in content is not an interesting discussion, it's fairly clear-cut: they exist. The interesting discussion is the relevance of that data, the impact of clipping it, and how one might choose to align their display's white point on a digital display. That's an intriguing discussion to have, but we can't get to that point if you're stuck in a world where completely reasonable video codewords are too much for you to accept even in the face of much evidence.
So... you should be able to encode RGB 100,100,100 to whatever the YCbCr equivalent is then decode that YCbCr triplet and get 100, 100, 100 again. If every RGB value you encode (and there is NO reason the data should EVER be outside that range) is between 16,16,16 and 235,235,235 you should never, ever, create a YCbCr triplet that decodes to any out of range R, G, or B coordinate.
Well, you're right that if you clip RGB to 16-235 during mastering, you won't really get much outside that during playback if you don't do any processing to the video. But that's not reality. The content isn't clipped, because there is no reason at all to clip it. The examples Stacey posted illustrate just how not-clipped the content is, in this case on BD.
[I want to re-iterate a concept you seem to have difficulty with - maybe I can make it simple enough... you claim when you apply a digital tool to data that the tool can/will somehow create out of range digital data.
You're changing the subject again. Lets leave behind digital "magic-math" or whatever it is you want to call them (since you have a problem accepting the existence of digital filters) since that's a couple steps beyond where we are right now which is merely the presence of actual image data, not post-processed samples which may occur. Shall we stick to just what's in the content, and not what happens when we process it later?
Let's use an example of 16-bit RGB color data being our original... either from a digital cinema camera or from scanned film. 0 is black, 65535 is peak white.
Except the content's black won't be right at 0 because if you're just a little bit off, then you're hosed when it all is clipped. When it's mastered then choices are made about where in the content you find the black point and the white point.
And our editing station knows we are working in 16-bit space. In this situation, you can apply any digital tool or filter or anything you want to do to an image and you are NEVER going to get data outside the 0-65535 range.
Obviously. But you could produce values outside the black/white range wherever that might be in the 16-bit range as it's being worked on.
Because the editing station is in 16-bit space and you just can't produce data outside the that range no matter WHAT the digital editing tool does. Now... you may manipulate the tool in ways that produce results you don't like - NO QUESTION. But the data will still be in the 0-65535 range. Now, let's say you are done editing and it's time to prepare your data for your digital master - DVD or Blu-ray. The workstation will handle the conversion for you... down converting from 16-bits to 8-bits AND changing from "full range" (i.e. 0-65535 or 0-255) to the limited 16-235 range used for consumer video. After this is done, no digital editing tools are used on the 16-235 data so there is no chance for it to be carrying data below 16 or above 235. Could somebody screw something up? Probably. Is it "normal" for "recent" (say since 2000) consumer video to have content below 16 or above 235. If it is "normal" it is only because somebody is going out of their way to get there.
All the movies Stacey just posted have been released since 2000. I don't recall Blu-ray being a format in the year 2000...
Or what about the Cars image posted previously. You earlier were hell-bent convinced that it was only possible to produce these "erroneous" "out of bounds" values with old analog equipment. I wonder where we find any analog film or analog workflow in producing Cars... I was under the impression that Cars was an imaginary world created digitally, but perhaps I was mistaken about that and you've actually seen talking cars in your day-to-day travels?
There is absolutely NO REASON for data to exist in modern masters that's below 16 or above 235 - somebody has to be forcing data to exceed the easily controlled limits - editing workstations are fully capable of producing 16-235 data without exceptions. Your presumption that out of range data exists seems predicated on 16-235 data being edited when that's not the case - high-bit data is edited and only converted to 16-235 8-bits once there is no more editing, no more filtering, no more changes at all.]
Sure, but you just have it backwards is all. The lack of reason exists for clipping content to 16-235 when Rec709 explicitly allocates the values outside of that reference range for video data. How do you get from "video data" to "out-of-gamut." That's why it's not clipped in the studio and why it's not clipped on the content we get in the home.
My presumption that such data exists is because I've actually looked at DVD grabs myself and measured them. Or because people like Stacey have posted images like the ones he just posted directly from D5 master tape or from BD that illustrate the same. In other words, my presumption is based on the examination of evidence both sought out by myself, and by evidence provided by others.
Your presumption to the contrary is based on no evidence whatsoever that I can discern. In fact, you explicitly describe your complete lack of exploration of any evidence, which is usually characterizes as a state of 'unknowing' or ignorance:
Originally Posted by Doug Blackburn
In fact, now that I think about it real hard, I never measured any data off any disc.
And yet you're still here arguing about what is actually on the disc or not on the disc. The disc could be filled with lime jello for all you know, but you don't know until you examine the evidence. And until you are actually willing to do that, that is engage in at least some basic
scientific endeavor, you're never going to get anywhere.
Minds are like parachutes, yes it's important how you pack them, but it's a lot more important when they open...