[Radiance-general] RGB to Radiance definition

[Greg Ward]

By the way, I love your presentation, Mark.  Would it be OK to link it to radiance-online.org (rather than having it buried in an obscure mailing list thread)?

-Greg

> From: Mark Stock <mstock at umich.edu>
> Date: June 23, 2016 9:35:47 AM PDT
> 
> The slides referenced in that thread have moved (apologies!) to here:
> http://markjstock.org/doc/gsd_talk_11_notes.pdf
> 
> The particular slide with my very unscientific RGB-to-reflectance
> conversion contains:
> 
> Start with an RGB color from whatever color picker you use
> (249, 214, 172)
> Normalize 0..255 to 0..0.9
> (0.88, 0.76, 0.61)
> Square it (to move medium gray of 0.5 closer to grey card grey of 0.18)
> (0.77, 0.57, 0.37)
> 
> I hope this helps.
> 
> Mark
> 
> On 6/23/16,  <> wrote:
>> Hello Germán,
>> 
>> Have a look at Mark Stocks comment on converting RGB values
>> http://www.radiance-online.org/pipermail/radiance-general/2011-April/007743.html
>> 
>> Regards
>> 
>> Terrance
>> 
>> On 23/06/2016 4:00 am, Germán Molina Larrain wrote:
>>> 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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> 
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