Hi,

I have just thought of a feature that could be patched into povray, and I
was just wondering if it would actually be possible to do. When I was doing
physics in school, and when we were covering the topic of Light, we studied
how different types of light sources give out different types of
frequencies. As a result, different materials react in different ways to
different frequencies.

The simplest example that I can think of is the difference between an
ordinary yellow lightbulb, and a yellow sodium street lamp. The standard
lightbulb gives out all kinds of frequencies, including red, green and blue
components (much like lights in Povray as they stand).  As a result, red,
green and blue components in the materials the light strikes all reflect
some of those frequencies back, enabling you to continue to discern colours.

However the light given out by a sodium based streetlamp is monochromatic.
While it might look like ordinary yellow light, it isn't. Only specific
wavelengths (ones that our eyes decode as being yellow) are given out - no
greens or blues or anything. So when this monochromatic light strikes
coloured materials, no blue or green wavelengths are reflected. You can
notice this when standing under a sodium streetlight, as the world seems to
be in greyscale (only a yellow-tinted greyscale ;) ).

It is the same with direct sunlight, or flourescent lights or whatever -
different types of lights give out different types of frequencies, giving
them a different "quality" (poor word, but a hard thing to describe).

So I was wondering - you it be possible to add this kind of feature to a
raytracer? Would it just be an addition to the light source code (like some
kind of colourmap, except for frequencies)? Or would it be necessary to
specify aswell in textures how they should react to different frequencies of
light? Would it be difficult to add this kind of thing?

Just pondering...

Element

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In article <3a5f7d93@news.povray.org>, "Equiprawn" 
<ele### [at] redbrickdcuie> wrote:

> So I was wondering - you it be possible to add this kind of feature 
> to a raytracer? Would it just be an addition to the light source code 
> (like some kind of colourmap, except for frequencies)? Or would it be 
> necessary to specify aswell in textures how they should react to 
> different frequencies of light? Would it be difficult to add this 
> kind of thing?

Full simulation of this type of thing would be extremely difficult, 
slow, and still not a perfect representation of reality...however, there 
is such a thing as "good enough". :-)
Colored lights are allowed...they are sufficient in many cases. If you 
want to model the splitting of wavelengths from prisms, MegaPOV has a 
dispersion patch (which works with photon mapping). I'm not sure what 
the exact effect you want is...do you want calculations done in spectral 
color space instead of RGB?

-- 
Christopher James Huff
Personal: chr### [at] maccom, http://homepage.mac.com/chrishuff/
TAG: chr### [at] tagpovrayorg, http://tag.povray.org/

<><

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Equiprawn wrote:
> 
> So I was wondering - you it be possible to add this kind of feature to a
> raytracer? Would it just be an addition to the light source code (like some
> kind of colourmap, except for frequencies)? Or would it be necessary to
> specify aswell in textures how they should react to different frequencies of
> light? Would it be difficult to add this kind of thing?
> 

The short answer is "Yes, it is possible".

Since visible light, by way of physics, is a continuum of wavelengths in
a certain range, you could conceivable define the spectrum for each
light source, and define the light reflective/absorptive properties of
each material using another spectrum.  This would more accurately model
the interaction of a physical object with physical light.

However, this would probably require a little more overhead in almost
all of the raytracer's calculations.  It is also much more simple when
working with Computer Graphics to define the color using a method that
is directly related to the way color is already represented on the
screen.

Try translating the spectrum that would be produced by such a
calculation (bouncing one spectrum of light off the
reflective/absorptive spectrum of the object) into a color to be
represented on the screen of your computer.  Another set of calcualtions
to be performed by the raytracer.

So, while it is possible, it may not be very practical.

Randy

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I believe this is called spectra color model. It would be nice in some cases but
I don't think implementing it would be at all easy or really all that practical
though.

What I wonder about spectra is how it would be specified and stored, you would
need several data for a color at intervals along the visible spectrum, it would
long to specify, large to store and slow to process compared to RGB model I
would think.

--
David Fontaine  <dav### [at] faricynet>  ICQ 55354965
My raytracing gallery:  http://davidf.faricy.net/

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Well the thing that has been keeping this feature in my mind was an
interview I read with one of the CG artists who worked on Riven
(http://www.riven.com). He said that the type of light they used to simulate
the sunlight outside wasn't a standard light, and that it worked more on
frequencies than standard RGB. That, he said, is what made it bleach out the
colours of whatever it hit, and made it feel more like really warm sunshine
than would other wise be possible.

Also, I just thought that such specialised light effects would be cool, and
possibly useful for certain scenes where flourescent lights or monochromatic
street lights are present.

Element

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