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From: dave@onfcanim.UUCP (Dave Martindale)
Newsgroups: comp.graphics
Subject: Re: AmigaWorld Ray-Tracing Article
Message-ID: <15299@onfcanim.UUCP>
Date: Sat, 2-May-87 12:26:00 EDT
Article-I.D.: onfcanim.15299
Posted: Sat May  2 12:26:00 1987
Date-Received: Sat, 9-May-87 18:06:24 EDT
References: <1514@sphinx.uchicago.edu> <804@elrond.CalComp.COM>
Reply-To: dave@onfcanim.UUCP (Dave Martindale)
Organization: National Film Board / Office national du film, Montreal
Lines: 33

In article <804@elrond.CalComp.COM> amamaral@elrond.CalComp.COM (Alan Amaral) writes:
>
>Uh uh... Everything that I have read on reflection models, including
>some of the more advanced ones, state that the color of a purely
>specular reflection is the color of the LIGHT, NOT THE OBJECT!  Now
>above you're describing apples and criticizing oranges.  You say that
>the color of a mirrored SPHERE is irrelevent, and then talk about a
>colored LIGHT...  Which is it? If the object is green and the light
>is blue, the specular reflection is blue, NOT green, black, OR some
>subtle mixing of the two.  There may be some diffuse reflection
>that will bias the TOTAL color of the object, but that's another story.

For plastic this is true - the colour of the object is produced by
pigment within a carrier which is more-or-less white.  So light
scattered diffusely is the light source colour filtered by the pigment,
but specular reflections off the surface are purely the colour of the
light source.

However, for ojbects that obtain their colour from their atomic structure,
like metals, specular reflections are coloured too.  A good example is
copper.  This is complicated by the fact that the colour of the
reflection depends on the angle of incidence of the light - spectral
reflectance is a 2-parameter function (wavelength and incidence angle).
And if you have non-white incident light, it isn't sufficient to
just have reflectances for "red", "green" and "blue" and multiply them
by the RGB components of the incident light if you want really realistic
looking colours; you have to split the spectrum up into more than 3 bands
and do the computation band-by-band.

Shading that takes reflectance characteristics of the material into
account has been discussed by Cook and Torrance in the SIGGRAPH
proceedings sometime in the last few years.  (Sorry, I'm moving and
can't get at my copy to check this).