U3D: Transparency bug (?)

Question

Dear All (and Supportpersons Adobe in particular),
I believe I have come across a bug that surfaces
when one part of a transparent mesh has to be seen thru another.
Please, take a look at the following examples:
"Real-world"
http://www.iaas.msu.ru/tmp/u3d/surf_alpha.pdf
- if you rotate it around, at some camera positions the mesh is transparent,
at other not.
"Test-case" (simplest sample from Intel U3D Sample library)
http://www.iaas.msu.ru/tmp/u3d/box.pdf
if "doube-sided rendering" is enabled in 3D Preferences and Model Render Mode is set to Transparent via Toolbar some triangles differ in color and are not transparent.
Does the effect appear on your systems?
It looks like a bug to me, but I may miss something obvious about 3D graphics
(i.e. may be it is the case like "it does not work the 'natural' way you expect it to - but it should not and can't"). May be there are known workarounds?
Oh mighty people of Adobe, please, confirm my report or diffuse my skepticism (end reeducate me if necessary)!
Sincerely, Michail
PS.
I have tried both OpenGL 1.4 and Software rendering in Adobe Reader 9 on my MacBook.
It does not look like it is related to Lights, Textures, Vertex Colors etc.
It may be correlated with double-sided rendering and looking on the other part of the same mesh as opposed to another mesh.

Answer 1

Dear Michail,
The surf_alpha file contains one mesh for the complex surface, and Acrobat sorts by object. Sorting by triangle would give visually fewer artifacts, and would also be incredibly slow as every triangle in the scene would be sorted for every frame. Z buffer rendering does not deal well with multiple simultaneous transparent objects, and even sorting the triangles does not always work if there is self intersection within or between objects. Some algorithms do deal with this issue properly, but would require significant pre-processing, hardware or time per frame.
One partial solution you could try is to break the mesh into multiple objects. Minimizing concavity within each mesh will minimize artifacts due to transparency.
Regards,
David