[Radiance-general] RGB to Radiance definition

[Germán Molina Larrain]

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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>
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