Steve Baker (steve++at++mred.bgm.link.com)
Fri, 24 Jan 97 12:11:05 -0500
>> What would happen to a red surface with
>> RGB values like (1.0,0.1,0.1) - presuming that the colour calculations
>> are done indepenantly in RED, GREEN and BLUE - what would happen? The
>> answer is that all three colour components could go over 1.0 and (presumably)
>> be clamped to 1.0 for display. This would mean that the predominantly red
>> surface would show up as white - but real-world diffuse reflectors don't
>> change colours like that. (Don't get confused with specular reflectors -
>> which are a different matter).
Angus W.S. Henderson <angus++at++despair.paradigmsim.com> replied:
> I disagree. As I understand it the original request was for VERY bright lights,
> i.e to emulate the effects of overexposure, blinding lights at night etc.
>
> In these circumstances the original colour of the database is secondary you
> just want to burn the light colour onto everything, and as Ken said it's the
> textured surfaces that limit the effect.
Well, I suppose the effect of extremely bright lights depends on what exactly
you are trying to display.
If (as simulation usually requires), you are simulating the effect of light
from the real world hitting your eyes directly, then the red surface will
presumably absorb the same percentages of the red, gren and blue parts of
the spectrum when the light is bright as it would when the light is dim.
I'm no physicist - but I'd bet that this would be true until the light gets
bright enough to vaporize whatever the red surface is.
That would suggest that the light reflected from the surface is still in
the same 1.0,0.1,0.1 proportions. When this hits the receptors in your
eye, you should *presumably* still see the same proportions of red, green
and blue - unless your red receptors saturate completely and cease to detect
the amount of incoming light correctly. It would have to be an EXTREMELY
bright light to do that. Once the light gets that bright, what dominates
what you see are some of the effects detailed in James Arvo's excellent paper
at SigGraph 95 ("Physically-based Glare Effects for Digital Images")
Things would be different if you were trying so simulate photographic media,
in that case, the film may well exhibit effects that are different from
your eyes.
This comes down to simulating what people *expect* an effect to look like
(because they see it on TV or whatever) - which is often a different matter
from what the effect *really* looks like.
> I bet the only answer he gets is "use a multi-pass technique"
>
> I hate multi pass techniques.
I agree.
Steve Baker 817-619-1361 (Vox-Lab)
Hughes Training Inc. 817-619-8776 (Vox-Office/Vox-Mail)
2200 Arlington Downs Road 817-619-4028 (Fax)
Arlington, Texas. TX 76005-6171 Steve++at++MrEd.bgm.link.com (eMail)
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"You can't destroy the Earth - that's where I keep all my stuff!" - The Tick.
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