[Radiance-general] XYZ to RGB conversion - neutral color is not neutral

[Greg Ward]

Hi Kynthia,

Under what lighting does your wall look white?  Your Z value is larger than your Y value, indicating a blue tint under D65 (outdoor) illumination.  This is sometimes the case with white surfaces, where they add extra blue to make the wall appear whiter under interior lighting.  I don't think your color calculation is wrong, although the values are out of range for plastic by a factor of 100 because your measured values are in percent.

You should try measuring the XYZ values under EE (equal energy) white rather than D65.  This will avoid color shifts in your conversion, as xyz_rgb.cal assumes equal-energy white for the XYZ measurements, and this is the same illuminant Radiance assumes in its RGB color space.

Cheers,
-Greg

> From: Chamilothori Kynthia <kynthia.chamilothori at epfl.ch>
> Date: February 8, 2018 7:40:41 AM PST
> 
> Dear Radiance community,
>  
> I would like your feedback on -another- XYZ to RBG conversion.
> I have measurements in the XYZ colorspace from a CM-600d Konica Minolta Spectrophotometer with settings for a 2 degree standard observer and a D65 standard illuminant, both with the specular component included (SCI) and excluded (SCE).
>  
> First, I am assuming that I should use the SCE measurements for the plastic materials.
> As an example, my SCE measurements of a white wall are X 81.05, Y 85.61 and Z 86.95.
>  
> When using the xyz_rgb.cal, the resulting color is R 73.3782327 G 90.3295341 B 86.8467252 - a clearly blue/green color!
>  
> Same thing happens when using the Colour Picker for Radiance (http://www.jaloxa.eu/resources/radiance/colour_picker/index.shtml) for a CIE XYZ (1931) input; but not when using the Colour Picker from an older page (http://www.jaloxa.eu/resources/radiance/cp4r.shtml), where the result is R 0.944 G 0.934 B 0.901.
> The latter is a neutral color that looks correct, and is also the result when using easyrgb.com and specifying the observer and illuminant.
>  
> My guess is that I am missing a conversion regarding the illuminant - do you maybe have any input on what I am doing wrong with xyz_rgb.cal?
>  
> Thank you for your time and knowledge!
> Kynthia
>  
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