[Radiance-general] RGB to Radiance definition

[Greg Ward]

Hi Germán,

While it's generally a bad idea to have reflectances (or transmittances) greater than 1 or less than 0, Radiance does not restrict you to this range.  In fact, if you are rendering colors from another (wider gamut) color space, such coefficients are sometimes necessary.  However, the Y sum (expressed by your R*0.265 + 0.67*G + 0.065*B equation) should stay between 0 and 1.  If you have an eyedropper that yields values outside this range, then it is not producing usable colors.

Cheers,
-Greg

> From: Germán Molina Larrain <germolinal at gmail.com>
> Date: June 22, 2016 4:00:43 PM EDT
> To: Radiance general discussion <radiance-general at radiance-online.org>
> 
> Dear All, 
> 
> I am trying to integrate easier ways of defining materials in Groundhog, for which I have the following doubt. How can I define a Radiance material (i.e. plastic, metal, glass, etc.) from an RGB combination and the global Reflectance (transmittance)? The purpose of this is to enable "eyedropping" a color, and assigning a reflectance or transmittance to it.
> 
> What I initially did was to find the combination of RGB reflectances that satisfied the rho = R*0.265 + 0.67*G + 0.065*B while maintaining the original proportions of RGB. However, since the RGB color that comes from the eyedropper has Black/White components (which intend to emulate shadows and different light exposures), my procedure often ends up returning an R, G or B channel that is greater than 1.
> 
> I guess my question is: Is there any way of reconstructing the R, G and B reflectances (or transmittances or whatever) from an RGB pixel? Maybe playing with HSL space or something?
> 
> Best,
> 
> Germán
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