"Jim Charter" <jrc### [at] msncom> wrote in message 
news:42569e69@news.povray.org...
> I think the diffuse component is irrelevent for rgb 0.  I think it is 
> basically the product of the two values, rgb and diffuse, and the angle of 
> the lightray, that are used to calculate shadow.  So if you go rgb 0 you 
> loose all capacity to get non-lighted shadows and instead have to rely on 
> specular and/or reflection to mold the surface.  So, as you say, adding a 
> little color gives "diffuse" something to work with.  The trouble is that 
> just about the time that adding color starts to make a difference, it 
> starts to look like grey, not black, material.

Very true.  Though in some cases, I've had rgb 0 objects show up like black 
holes in the middle of a picture, and giving it a very small value still 
makes it appear black, without making it look like there's black hole in the 
picture.  But if it looks ok in relation to other objects, no reason to 
change it.

>
> Yes I remember Jaime's suggestion too, though I thought he was talking 
> about specular.  I also remember that a long time ago there were often 
> threads discussing such formulas. Alas, am I now just catching up? Well 
> not quite. For a long long time I typically adjusted 
> diffuse/specular/reflection as some sort of loosely conceived division up 
> of a unit whole.  Then it came as something of a revelation to me when I 
> freed myself from that discipline.  Now, while Jaime's statement comes as 
> a sobering remainder to keep it sane, I am still not convinced that a 
> formulaic relationship between these elements always makes sense.

I'm not convinced either, but Jaime's lighting is always better than mine, 
so I don't argue.  Plus it makes my life simpler (biggest reason). ;-)

Incidentally, I looked up a demo scene of his, and he goes by the 2nd 
component of reflection (which is usually the highest), which would make it 
diffuse 0.85 in your case.  But with rgb 0, I don't think it matters, as 
you've already stated.

>
> I was introducing my daughter to raytracing just the other night.  We were 
> giving a sphere, with no finish specified, different colors just to 
> demonstrate how the color vector worked.  I was trying to get her to guess 
> how to get yellow by introducing the concept that yellow is really 
> not-blue.  So first I had to demonstrate that white is <1,1,1>. We did the 
> render.  "It doesn't look white, it looks grey", was her immediate 
> reaction. :(

Some day, when you grow up, you'll realize that there is no such thing as 
rgb 0 and rgb 1, just various shades of rgb 1*A, where A is a value between 
0 and 1, non-inclusive*.  ;-)

>
> With a low rgb, radiosity also is neutralized.  So the only way to get any 
> tones into the shadows is reflection. But even the slightest amount of 
> reflection gives a hard-shiny look.
>
> About the bottom of the shoe. ("Bottom" is the correct term.) Yes, if it 
> was made of leather it would show wear and have tan patches.  If not, then 
> it would be some homogenous material other than leather and therefore show 
> different surface characteristics.  It is an important question because it 
> may be true that the solution to the problem has to do with providing 
> massive amounts of minute surface detail.  After all, shiny, homogenous, 
> black patent leather is favored by some precisely for its "dark liquid", 
> form-denying properties.

I was just thinking that the bottom might be made of a different material. 
...or it might not...

>
> So the hunt goes on.  But I was interested to peoples reactions to gross 
> adjustments of these factors across the scene in general.
>

If you have the time/patience/computing power, making rotating animations 
can help point out flaws, especially for things such as specular.  From 
certain angles, specular reflections may appear ok, but from others, you 
realize they are too high / too rough / too etc.

Just mentioning some tricks that I've learned.

*I'm thinking of making that my sig.  :-)

-- 
Jeremy
www.beantoad.com

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