[Openexr-devel] On YUV support

[Billy Biggs]

  The website has some notes on YUV support in OpenEXR and asks for
suggestions.  I would like to offer some.

  EXR files currently store linear intensity values for RGB.  However,
the YUV used in video systems is always based on gamma corrected RGB:
the Y in a YUV system is not really luminance [1].  Because of this, I
do not see any way to convert from linear intensity RGB to the U, V
channels of a video system without clamping to [0,1], making the use of
EXR over a 16-bit per channel integer format like PNG seem useless.

  Because of this, I would recommend dropping the idea of a YUV mode,
and the channel names U and V, until a better defined use case is found.

  I do have an alternate suggestion for some more channel names in EXR
files.  Specifying colours as CIEXYZ triples is quite useful, and I
would recommend "X" and "Z" as channel names.  I would then recommend
changing this text:

  Y      luminance, used either alone, for gray-scale images, or in
         combination with U and V for color images.

  to this:

  Y      luminance, used either alone, for gray-scale images, or in
         combination with X and Z for color images in CIEXYZ.

  Which makes much more sense and seems a lot more useful.

  Thoughts?

  -Billy

  [1] http://www.poynton.com/PDFs/YUV_and_luminance_harmful.pdf