[opendtv] Re: 1080P Question

Hello,

Kilroy Hughes @ Microsoft wrote:

Ø  I reviewed the deck and am newly inspired to advocate allowing xvYCC 
(negative coefficient encoding in Y'CbCr).

I prefer if you say: (encoding of negative R'G'B' values in Y'CbCr). But let's 
not forget
that xvYCC can (and probably will) also encode larger-than-unity values of 
R'G'B'.

BTW, if Y'CbCr stay within 16..235(240) and add only the previously illegal
combinations of legal Y'CbCr values then (I think) the standard is called sYCC.


Ø  Any recommendation on how to correctly represent that in H.264 encoded 
content?

I'm not sure what you mean, or if I did that I would know the answer, but I 
think that
it makes no difference to the (video) content, only to the metadata that 
describes the
way the content is encoded. Is that what you meant ? In that case I can only 
refer you
to the HDMI 1.3 and 1.4 specs, where they have tackled the same problem. I think
that there are similar references in MPEG-2. It's all in the metadata.

The only purpose for the "this is wide gamut" metadata bit that I can think of 
is for
switching between gamut extension (for standard gamut content) and gamut mapping
(for wide gamut content). And this is not even very essential to have.

I think that even in the absence of any metadata, xvYCC would still work quite 
well.
Of course you do need information about the R'G'B'<->Y'CbCr matrix (Rec.601
for SD, Rec.709 for HD), but that applies also to standard YCC.


Ø  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).

Ø  Some inaccuracy there, but Transfer Characteristics different from Rec 709 
should encourage some guessing as to what real primaries were encoded.

True, basically you just need a few bits to signal that a wide-gamut standard 
has been
used, and perhaps distinguish between a few variations. Nothing of it is 
essential IMO.
On top of that, HDMI also allows to send "Gamut Boundary Metadata", which can
describe very accurately what the source gamut is. This can be simplified to the
original color primaries, i.e. the source gamut is an RGB cube with primaries 
that are
different from Rec.709 (a.k.a. sRGB). But it is also possible to encode an 
irregular 3D
shape in all its detail. I think that very few people really ever want to use 
this, so I don't
know if this will ever be picked up by broadcasters or receivers. It's optional 
anyway.

The real camera primaries themselves do not matter, of course, since they have 
been
replaced by (color space converted to) the Rec.709 (sRGB) primaries for xvYCC
encoding. The color space that is actually used will be smaller than an RGB 
cube, it
is described by a 2D contour like the "Pointer color set" or the 3D "Munsell 
cascade"
variant. Knowing the edges of the source gamut is supposed to help you with 
tuning
of the gamut mapping or gamut extension.

Note that HDMI 1.4 distinguishes a few of the professional (photo) color spaces,
meaning that the data is referred to the actual (non-standard) color primaries. 
This
avoids the conversion to the Rec.709 referred xvYCC color space, and permits a
single conversion to the target (display) color space. There is no 
compatibility with
Rec.709 standard displays, but if the target refuses to accept it then the 
source will
have to convert the data to an acceptable format (xvYCC or even standard gamut).
I suppose that these color spaces were added for connecting professional cameras
directly to a display (via HDMI), and transfer the image data as in the JPEG 
files.

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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