"Bruno Cabasson" <bru### [at] alcatelaleniaspacefr> wrote:
> Hi there!
>
> ...I discovered recently
> something that puzzles me unexpectedly. Two media-filled objects that I was
> expecting to render the same do not. Run the following fragment of code. You
> can notice that the spheres on the right (UnionedSpheres) are significantly
> brighter than those on the left (DistinctSpheres).
>

I played around with your code in lots of different ways, trying to see what
might happen by changing this or that. (My usual problem-solving method!)  My
own conclusion is that the spheres on the right--the ones made from a single
interior and multiple shifted medias--are the problem spheres, not the ones on
the left. (I guess that's kind of obvious, eh?) IMO, what you're seeing is yet
another scattering-media-and-transparency problem. Not that I know the
solution. Out of curiosity, I added another media sphere to both of your
'schemes'--and it makes the spheres on the right even brighter! What that tells
me is that the multiple medias in that union are 'adding
up' in some way...even though they are properly translated and supposedly don't
overlap. OR, that the medias' (default) extinction values are not behaving
properly...something I've noticed in some scenes of my own. (OR, it just might
be a strange by-product of your coding method--multiple unioned spheres sharing
a single interior with multiple shifted medias. That's new to me; very
interesting!)

But crazy experimenter that I am, I kept messing around with your code and came
across a quasi-solution to the problem that actually seems to fix it! I arrived
at this through...sheer luck. :-) It doesn't 'explain' the problem, but may give
some insight into it. In your code for the right-hand sphere grouping, I first
commented-out the density functions (which, as you say, are optional) just as a
way to eliminate one possible problem. Then, since your union has two spheres in
it, I reduced the color_map brightness of each media to .5 at index list .001
(i.e., 1/2 of its original value), then changed extinction in each media to
2--twice its value. Each of these 'fixes' has a reason behind it: Reducing the
brightness to half makes the media half as dense--and kind of 'watered down,'
color-wise--yet doubling the extinction compensates for that color weakening,
like a contrast control. Again, this was just some wild experimenting--but the
result happens to look identical to the spheres on the left; I'm hard-pressed
to see any difference. (The only thing I DO see is the lack of that subtle
shadow-line on the bottom/right sphere...which *might* actually be a
coincident-surface-like shadow problem on the bottom/LEFT sphere, from the
parallel light source.)

BTW, with THREE spheres (and three medias) in the right-hand union, the same
visual match is created by using extinction 3 and a color_map value of .333.
Strange but fascinating!

I can't say this 'scheme' of mine makes any real sense, but it does seem to be a
workaround to the problem.

Here's the re-worked code (with slightly different media values that work
better, I think).

Ken W.

-----------

#declare UnionedSpheres = // grouping to RIGHT
union{
    sphere {0, 1 translate 1.1*y}
    sphere {0, 1 translate -1.1*y}
    hollow
    pigment {rgbt 1}
    interior
    {
        media{
           method 3
           intervals 1
           samples 40
           scattering {1, Blue extinction 2}
            density {
              spherical
              color_map {
               [0 rgb 0]
               [0.001 rgb .5]
               }
              translate 1.1*y
                 }
             }
        media{
           method 3
           intervals 1
           samples 40
            scattering {1, White extinction 2}
            density {
              spherical
              color_map {
               [0 rgb 0]
               [0.001 rgb .5]
               }
              translate -1.1*y
                 }
             }
         }
     }

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