Re: [Openexr-devel] On YUV support

[Kevin Wheatley]

Drew Hess wrote:
> Billy, re: your YUV comments, I tend to agree with you.  The OpenEXR
> format doesn't require that files store linear intensities (though that's
> what applications expect for RGB triples, anyway), so "YUV" could be
> stored gamma-corrected 0-1.  There might be a few advantages to using
> OpenEXR over PNG or other formats due to its ability to store channel
> attributes, but overall, yeah, there's no compelling reason to use OpenEXR
> for YUV.  I don't know of an application that does, currently.

it's perfectly valid to have values outside of 0-1 in "YUV" (Y'CbCr,
Y'PbPr, etc etc) if they come from another colour space by means of a
3x3 matrix transform, they represent items outside the gamut of "YUV".
(Just like its valid not to have subsampled CbCr when dealing with
"4:4:4" material.) 

Storing 'gamma corrected' data means you also would ideally want to
store the encoding method somehow, e.g. like SMPTE 268M (DPX) v2.0 as
a { gamma, black level, black gain, break point, reference white level
} tuple. as well as some reference to the colour space they represent
(ITU-R 709-5, 601-5 system B or G, 601-5 system M, SMPTE 274M, etc
etc).

> There are also some new HD cameras which use log curves to encode HDR
> information in the signal, but whether these operate in some funky
> over-range YUV space or log-encoded RGB, I don't know.  I'll try to find
> out.

If you mean the Viper from Thomson, then no they are RGB CCDs and they
log encode them (with a suitable offset and gain to balance them at
their chosen CCT) one of their advantages is that they don't have to
go through the matrixing of a "YUV" like colour space.

representing the log data from these sorts of sources would probably
also need some form of parameterised equation representation.

Kevin

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| Kevin Wheatley                   | These are the opinions of |
| Senior Do-er of Technical Things | nobody and are not shared |
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