[opendtv] Re: 1080P Question
- From: Kilroy Hughes <Kilroy.Hughes@xxxxxxxxxxxxx>
- To: "opendtv@xxxxxxxxxxxxx" <opendtv@xxxxxxxxxxxxx>
- Date: Tue, 15 Sep 2009 20:13:45 +0000
Thanks Jeroen.
I reviewed the deck and am newly inspired to advocate allowing xvYCC (negative
coefficient encoding in Y'CbCr).
Any recommendation on how to correctly represent that in H.264 encoded content?
It looks like Transfer Characteristics = 11 signals IEC 6966-2-4; and Primaries
and Matrix Coefficients are the same as Rec 709 (VUI value = 1).
Some inaccuracy there, but Transfer Characteristics different from Rec 709
should encourage some guessing as to what real primaries were encoded.
Kilroy Hughes
From: opendtv-bounce@xxxxxxxxxxxxx [mailto:opendtv-bounce@xxxxxxxxxxxxx] On
Behalf Of Stessen, Jeroen
Sent: Wednesday, September 09, 2009 4:53 AM
To: opendtv@xxxxxxxxxxxxx
Subject: [opendtv] Re: 1080P Question
Hello friends,
Kilroy Hughes @ Microsoft wrote:
Ø But, how bad does it look when content is encoded xvYCC and the display (or
something else in the signal chain) clips negative RGB coefficients to Rec 709
gamut?
I have given examples of that in my presentation at The Tech Retreat 2007.
Don't I remember you being there ?? Maybe you can still download the ppt:
http://data.memberclicks.com/site/hopa/2007_Presentation_Stessen.ppt .
The short version is: the problem is only applicable to the out-of-gamut colors,
so for the in-Rec.709-gamut colors it means nothing, they are fully compatible.
Then, there are two kinds of out-of-gamut colors:
- Wide-gamut, where MIN(R',G',B') goes < 0. Such colors are physically
impossible on a standard gamut display, and must be gamut-mapped.
Clipping to zero is one form of gamut mapping, and not even that bad.
- Bright-gamut, where MAX(R',G',B') goes > 1. This is merely a problem
of
scale, the colors can be brought in-gamut by simple attenuation, which can
also be done by the (xvYCC) source. However, failure to do so gives very
visible and annoying clipping artefacts, so this must be properly dealt with.
Ø I'm struggling with that chicken/egg problem for Internet delivered content.
It might be possible to write new rules for a new channel and new devices, but
if those files land on legacy devices or plug into legacy displays resulting in
people have green faces ... I don't see a bridge to transition from a 709 to
xvYCC world of content and devices.
Duh, xvYCC was specifically designed with that transition in mind ! The
artefacts
of rendering on legacy displays can be kept minimal. Unfortunately, this makes
the
added value of an xvYCC display also rather minimal. The market is not taking
it up. The Digital Cinema has its own wide-gamut standard of course, and it does
not have to be compatible with anything else. However, Mark is using standard
satellite equipment for his Opera broadcasts, and he has to use Rec.709 video.
Given that the content may be fed to some real Digital Cinema projectors, it
would
seem that xvYCC is a nice compromise. But with the negligible amount of wide
gamut colors in typical opera scenes, the benefit to the viewers would be zero.
Ø Well, if all render devices recognized xvYCC content and displays, and could
convert to 709 output if xvYCC isn't supported, then it would work ... but
requiring that of all devices (which eliminates most existing devices) isn't
likely.
Avoiding clipping to 1, and defaulting to clipping to 0, already solves 90% of
the
issue. A better receiver would decode xvYCC and then apply a more accurate
form of gamut mapping to a standard display gamut. As many displays are not
really that standard, there would still be a small benefit of more accurate
color
rendering. But whether the customer would appreciate the difference ? Not
likely.
For xvYCC (x.v.Color) to become a success, we need wide-gamut sources,
transmissions, and displays. Every camera is in principle suitable for
wide-gamut,
and some modern displays (new LCDs, DLP and laser projectors) can render
a decently wide gamut, but there are no transmissions (or discs). End of
story...
Groeten,
-- Jeroen
Jeroen H. Stessen
Specialist Picture Quality
Philips Consumer Lifestyle
Advanced Technology (Eindhoven)
High Tech Campus 37 - room 8.042
5656 AE Eindhoven - Nederland
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Jeroen.Stessen@xxxxxxxxxxx<mailto:Jeroen.Stessen@xxxxxxxxxxx>
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