[Radiance-general] Color reflectance values and materials..

[iebele]

Hi Dave,

As Mark already said, these values seem very bright to me also.

Once I did some measurements on a Kodak grayscale chart (from the good 
old Q-14 color separation guide) using a photospectrometer from Kodak, 
which results were all below 1.0 for each Radiance RGB triplet (see the 
attachment, which contain the conversions of the measured xyz-values to 
radiance rgb ). Since then my idea has been that rgb triplet values for 
diffuse reflectances above 1.0 are invalid. Reading your mail I get 
confused a bit.

Another point I don't understand is why you suggest to use gamma 
correction within a material description. I always think of gamma to be 
applied after the simulation has been done. In other words, I think of 
gamma to correct an image for a certain output device. So, why should we 
correct reflection values with a gamma _before_ a simulation?

The idea of a public database for materials, paints etc. dedicated to 
Radiance would be very usefull. Imho such a database would be very 
valuable as a part of the Radiance distribution also. This instead of 
pointing to websites, which might confuse users. Measured samples from 
commercial color charts (like RAL, Pantone and other paints and 
materials) would extend the direct useability of Radiance a lot. It 
might be an idea to plan measurements using a photospectrometer on such 
standard commercial color charts, and discuss the conversion of the 
results within this group. Or has this already be done in such an 
extensive way?

-Iebele



David Smith wrote:

>Mark,
>
>That's where gamma values come in. Let's say the RGB values of a paint
>sample from EasyRGB are (125,150,175), a blue gray. Take each channel,
>divide by 256, take the result to the power of 2.2 (monitor gamma) and
>then multiply by 256.
>
>R = (125/256)^2.2 * 256 = 52.9
>G = (150/256)^2.2 * 256 = 79.0
>B = (175/256)^2.2 * 256 = 110.9
>
>If you put the resulting values (52.9,79.0,110.9) in as the R G and B
>material colors, you should get the correct paint color in your
>simulation that matches the physical paint color.
>
>--Dave
>
>_______________________________________________
>Radiance-general mailing list
>Radiance-general at radiance-online.org
>http://www.radiance-online.org/mailman/listinfo/radiance-general
>
>  
>

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