With great interest, I've been following the recent posts about problems
with overlapping media objects.  I've been working on several scenes that
also show some subtle artifacts. But the most vexing (and not at all
subtle) has to do with media objects in a light_group. I thought I'd post
some experiments to show it. Took me awhile to figure out where the problem
was, so hopefully I'll save others some hair-pulling.  Haven't *solved* the
problem, unfortunately. :-[

The unit-sized spheres are filled with basic scattering media, using method
3 and extinction 0, with no density maps or varying transparency. No merge
or union, either. (I added a slight amount of emitting media, just to
delineate the sphere shapes; that isn't contributing to the problem.)

I made a light_group out of the left sphere and a spotlight (centered in the
sphere), then overlapped the second media sphere with that light_group,
everything positioned on the same z-plane.  The camera is looking
straight-on at the spheres. The result is odd: In image #1, the lens-shaped
overlapping section shows lit media on one side of its central "axis" but
not on the other. It's not a max_trace_level problem, and no amount of
fiddling with media intervals or samples min, max can get rid of it.
Perhaps it's just logical behavior; though to my way of thinking, BOTH
"halves" of the lens-shaped section should be lit (or both should be
un-lit.) I could be wrong, though. Can anyone out there explain what's
going on?

For image #2, I moved the spotlight outside of both spheres (although it's
still part of the light_group.) For image #3, I moved the camera to the
left; the oval-shaped outline is the edge of the overlapping spheres.

There are ways around this, of course: merging the spheres before combining
them in the light_group; or making light_groups out of both spheres,
duplicating the spotlight and its position for both of them.  But in my own
scenes, this isn't what I had planned on, and will take quite a bit of
reworking. Perhaps that's inevitable...

Ken

Post a reply to this message

Attachments:
Download 'media_tests.jpg' (37 KB)

Preview of image 'media_tests.jpg'