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

[A Ruppertsberg]

Hi,

for converting spectral or XYZ data to RGB see Appendix in:
Ruppertsberg, A, and Bloj, M (2006) Rendering complex scenes for psychophysics
using RADIANCE: How accurate can you get?, J. Opt. Soc. Am. A, 23, 759-768

download pdf at:
http://www.brad.ac.uk/acad/lifesci/optometry/uploads/Publications/2006xJOPTSOCAMAx23x4x759.pdf

In that paper we also used spectral data from a data base (ref 47) of Munsell
chips and natural materials:
47. See http://spectral.joensuu.fi/databases/index.html.

What I suspect you are after are surface reflectance descriptor RGB values. The
XYZ values provided by eg the macbeth color checker are colour signal data (ie
the product of surface reflectance with illumination). The illumination is most
probably D65, which is white, so it's not far off to correspond to the surface
reflectance. But a lot of people forget the distinction between colour signal
and surface reflectance.
Also remember that there are no generically valid RGB values. RGB values are
always only valid for a particular display device. In that paper up there we
have used the transformation matrix based on RADIANCE's 'built-in' primaries.
It is true that you can obtain RGB values beyond the [0 1] range when you
convert XYZ to RGB and that simply tells you that the gamut of your RGB-space
does not include that colour. Then, you have to do something, either clip it or
tone-map it....

happy colouring!

alexa


Quoting "Lars O. Grobe" <grobe at gmx.net>:

> While I agree that extending the material file in the radiance
> distribution would be useful for viz, I'd like to encourage those who
> did not try yet to use their digital cameras and a calibration tool such
> as macbethcal and the not too expensive chart as a measurement device.
> Today as almost everyone has the equipment for digital photography, this
> should be not too difficult. While one can certainly discuss the
> accuracy, obtaining an "inaccurate" sample of the material that is
> actually to be simulated should still be better then an accurate sample
> of a generic material.
>
> That said, the most important feature of a new material database must be
> detailed documentation of the samples, measurement equipment, conditions.
>
> CU Lars.
>
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> Radiance-general at radiance-online.org
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>


-- 
Dr. Alexa Ruppertsberg
Bradford School of Optometry and Vision Science
University of Bradford
Bradford, BD7 1DP
UK

Tel. ++44 1274 235378

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