[Radiance-general] RGB to Radiance definition

[Greg Ward]

If you don't know the RGB values of a material, you can just use gray.  It won't hurt your accuracy as much as shifting *all* surfaces to gray.  It may be misleading when looking at renderings, however.  People may assume what is shown as gray really is gray, when it's actually unknown.

Cheers,
-Greg

> From: Germán Molina Larrain <germolinal at gmail.com>
> Date: June 24, 2016 6:59:57 AM PDT
> 
> Thanks, Greg and Mark for your answers! 
> 
> I studied this more, and decided not to follow Mark's procedure. Groundhog is meant to be a Lighting tool, thus I do not really want to offer "very unscientific" methods... I guess I will keep the eyedropper and colorpick just for SketchUp representation. By default, the Radiance material will be gray (based on just the reflectance), unless the user decides otherwise.
> 
> Would it be a problem if I mixed gray and non-gray materials in the same model? I mean... I might know the RGB transmittances of a glass, but only the reflectance of my carpet. Will that induce errors?
> 
> Best,
> 
> Germán
> 
> 2016-06-23 13:35 GMT-03:00 Mark Stock <mstock at umich.edu>:
> 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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