Graeme Gill wrote:
Bill Spitzak wrote:
The Y of the primaries can be used as alternative method of specifying the
whitepoint. (convert the
3 Yxy colors to XYZ, add them, then convert back to Yxy and the xy is the
whitepoint, I think).
Correct, but this assumes the display is perfectly additive. Real world ones
mightn't be, so
Yxy or XYZ x RGBW does provide extra information.
I suppose that is possible if the primaries and whitepoint were all Yxy
triples, because that provides 12 degrees of freedom. However I have
always seen them specified as xy pairs thus giving 8 degrees of freedom,
which is less than the 9 degrees of freedom from three Yxy triples.
Also non-additiveness is usually considered a second step after
conversion to additive primaries. I have doubts it can be specified by
just one extra point in the color space.
I think the reason rgb sets are specfied as 4 pairs of xy (the three primaries
and the whitepoint)
instead of 3 triples is to remove the arbitrary multiplier that makes there be
9 numbers instead of 8.
A lot of display folk are very chromaticity diagram oriented - they are used to
just
specifying xy, and it's nice to have the white point be explicit so you can
more easily
check what the white point color temperature is.
xy of the whitepoint does define the color temperature. Possibly you are
thinking of X and Z?
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