clipka wrote:
> Ive schrieb:
> 
> I don't get it. On one hand, you are saying that reference white does 
> not matter for sRGB as a working color space (unless I misunderstand 
> you, which I guess I do), on the other hand yo're saying some adaption 
> /must/ be applied?
> 

Three cases for given xyY values.

These values refer to a reflective spectrum (as it is the case with 
Munsell color definitions or one of the many "real world" material 
databases out there, one of those is e.g. the Aster spectral library at
http://speclib.jpl.nasa.gov/ containing among other things data for all 
moon stones collected at the various Apollo mission landing sides).
All these databases and all measurement hardware that I'm aware of is 
using D50 as reference white (but as scott pointed out there are also 
others).
Now, assuming we are using scRGB (sRGB primaries and whitepoint but 
without gamma correction) as the POV-Ray internal RGB *working* color 
space we have to apply chromatic adaption for these xyY values to make 
them consistent with RGB values from other sources and especially with 
the RGB values for light sources that are used within POV-Ray to 
illuminate them.

Now, speaking of light sources within POV-Ray, the second case are xyY 
values that refer to those and then must *no* chromatic adaption be applied.

The third case (IMO not relevant for POV-Ray anyway) is the usage of 
sRGB as output device color space (as opposed to working color space)
where the sRGB standard implies an environment lighting/viewing 

assumed to be done by the human visual system and therefor no chromatic 
adaption has to be applied when calculating RGB values from given xyY.

and within the business I'm working in it is ignored and so does also 
e.g. Adobe - not that I'm saying what they are doing is always right;)


> I'm pretty sure somehow that the /spectrum/ of the whitepoint should 
> matter for such things as dispersion, but where and why adaption come 
> into play still eludes me.
> 
Well, in fact there is no chromatic adaption needed and I think that the 
whitepoint of the POV-Ray internal RGB working color space shouldn't 
matter at all for calculating dispersion samples (besides that it is 
needed for the xyz->rgb conversion). I seem to remember in some quick 
response I did state otherwise and in case this is true I'm sorry for 
the confusion this might have caused.


> Let me think aloud for a moment to try to sort this out, and kick me 
> where I'm wrong:
> 
> So the starting point is the /tristimulus/, which (basically) models how 
> strongly the three different color receptors ("cones") in the human eye 
> react to different wavelengths. (To my knowledge we can count the "rod" 
> receptors out, probably because they only contribute in dim conditions, 
> right? Otherwise we should be able to distinguish four different 
> primaries, as the rods' spetral response is yet again different than the 
> cones'.)
> 
Within the CIE standard observer experiments the rods are just ignored.


> Experiments were conducted to measure this per-wavelength response (a 
> bit indirectly) in a manner that, to my understanding, only yielded 
> /relative/ results: Conclusion could be drawn how much stronger a 
> particular cone type is stimulated by wavelength A compared to some 
> other wavelength B, but there was no way to infer how much stronger a 
> particular wavelength stimulated cone type A as compared to cone type B. 
> Thus, the immediate conclusions drawn from these experiments left open 
> the question what "white" is (which comes as no surprise, given that it 
> depends on the viewing conditions, i.e. the eye's "calibration").
> 
There is such a thing as the Grassmann law. Google it for more details.


> However, I'm a bit worried at this point already: I guess the 
> wavelengths to test were generated from a "white" light source by means 
> of a prism; did they actually measure the physical light intensity of 
> the light source at that particular wavelength, to compensate for any 
> nonlinearities in intensity in their results? Or is there a hidden 
> "whitepoint" in the original data already, due to the light source used?
> 

No. They did use mercury (and other metals) vapor lamps to produce 3 
different monochromatic light beams at three different wavelengths. Out 
of my head theses have been around 435nm, 545nm and 700nm and as a side 
note these values are not completely willingly chosen, they had also to 
deal with the kind of vapor lamps that where available in 1931.
The "observer" could then adjust the intensity of the three beams until 
the color that did result from the mixture did match a given one. So 
there is no "hidden" whitepoint and no dealing with "what is white" at all.


> Duh - I haven't even really /started/ thinking about the problem of 
> whitepoint, and it gets in my way already...

Don't worry ;)

-Ive

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