scott schrieb:

> That isn't very scientific though, usually "white" in a strictly 
> scientific way means equal energy across all relevant wavelengths (see 
> "white noise" in audio).  The CIE colour matching functions were 
> designed to give equal XYZ values when presented with a spectrum of 
> "white" light such as this.

Okay, I think I get that.

So in /that/ context, there is no "whitepoint" involved, right? Or am I 
getting something wrong here?

And when you add two spectral colors, still the whitepoint would not 
come into play - presuming that color perception is linear, which seems 
to be the case from all that scientists have found out. (Stop me when 
I'm talking nonsense).

And with the sRGB color model, the same principle applies, because it is 
just another choice of the coordinate axes in 3D color space (leaving 
the transport function aside for now).

So as long as we're talking about some light color which we intend to 
convert from XYZ to sRGB, we can happily forget about whitepoint: If we 
shove a XYZ color into the transformation matrix that represents "white" 
in the physical sense, then the sRGB color we get will just as well 
represent "white" in the physical sense - right?

Thus, in order to "render" light of a certain color with know XYZ 
coordinates on an sRGB "output channel" (be it a device, a file, or 
whatever), we should just take the XYZ value we have, shove it through 
the transformation matrix as defined in the sRGB standard, and live 
happily ever after. As for viewing conditions, I would expect these to 
be taken care of automatically by a properly calibrated display.

I just toyed around with the sRGB transformation matrix, leading me to 
the conclusion that the XYZ color model must be using illuminant *E* (!) 
(that is, equal physical light intensity) as its native "white" (i.e. 
<1,1,1> - heck, I could have guessed that from the x,y coordinates of 
the various illuminants), while "the" sRGB "white" (again <1,1,1>) 
matches D65. (Ah-hah! So that's what the "display whitepoint" is 
denoting in the sRGB specs.)

Okay, so I think I got it, as far as light goes. Now for the color of 
surfaces:

In my naive mind, I would have presumed that to specify the color of a 
surface via the CIE XYZ color system, one would specify the XYZ 
coordinates of the diffusely reflected light when the surface in 
question is subject to physically white light (i.e. illuminant E); 
however, from your explanations I gather that this is /not/ the case, right?

Which raises the (possibly trivial) question, what exactly /is/ then 
specified for color pigments? Is it, as I now tend to presume, the XYZ 
coordinates of the diffusely reflected light when subject to illuminant 
D65 instead?


Gee, that doesn't make it easier to come up with a sensible way of 
proper color handling in POV-Ray...

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