Michael Lundahl wrote:

> Sorry, but I think twyst is right, it is a
> coinciding surface-problem, his solution is just not quite right.

Yes he is.

> Try this:
>
>  // The 'L'
>  union {
>   box {<0,0,0>, <1,9,1>}
>   box {<0.00000001,0.00000001,0.00000001>, <6,1,0.99999999>}
>
> /Michael

 Thanks to you, I've noticed I made a mistake by describing the problem in my
answer to twyst. Indeed, the 'snow' doesn't appear at Z=0.5, as I was
thinking, but at Z=1, what is normal (this is not a bug, and will never be).
What is surprising is that I see a mixture of red and white points at Z=1.
Let's try to become POV:
"I launch a ray. Who does this ray intersect with ?
    1 It intersects with the bounding-box. We have to continue
    2 It intersects with the panel at Z=0.5. Mhh, this is in the L. I won't
keep this intersection
    3 It intersects with both L-boxes at Z=0. This is not in the panel. Won't
keep
    4a It intersects with the vertical box ( | part of the L ) at Z=1. Ok, I
keep it.
    4b It intersects with the horizontal box ( _ part of the L ) at Z=1. Ok,
I keep it.
    ...
I look for the nearest intersection..."
Here is the problem: both intersections are at the same location. So, will
the color be the first or the second box color ?
But there should'nt be any problem, as both colors are same. Where do the
white points come from ? This could eventually be the panel, if its bottom
had been at Z=1. But it's not the case.

                         0   0.5     1     1.5   Z->
                             ______________
                         ____|_______     |          ^
O ---------------------> |   |      |     |          |
                         -----------      |          Y
                         ^   |____________| <-
                         |                    |
camera           _  part of L               panel


Did I say something wrong ?
This is quite imortant. As you can guess, I'm making something a little bit
more complicated that a simple L. And the same problem appears at each corner
of my logo. I really would like not to have to put some 0.00000001,
0.999999999 everywhere. I should only have to be careful with using colors.

Sorry if my posts are a little bit long, but it is not easy to make clear
explanations (I'm not english and drawings are not easy to do in ascii !!!)

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