[xcrysden] Problems with transparency

[Filipe Ribeiro]

Hi,
I came across what looks like either an OpenGL bug or an XCrysDen bug with
transparencies. I'll try to describe the problem step by step.

1. Take a wave function of a simple molecule or polymer.

2. Select a reasonable iso-value (but don't check the "render +/- isovalue"
box. The problem is clearer this way). The lobes of the wave function should
be visible and not transparent (default) as expected. So far so good.

3. Now, turn the transparency on. The lobes appear transparent and the
overall picture looks fine.

4. Click on the "Set TRANSPARENCY parameters" box and go to the "Front Side
Color" tab. In my default configuration the only parameter with an alpha
component lower than 1.00 is the "Diffuse color" (0.60). Move it to 1.00 so
that all the alpha components are 1.00. Click "Update". This should - if I
understand it correctly - make the system completely opaque. (You can also
set to 1.00 the alpha of the diffuse color on the back side color tab, but
nothing changes).

5. Now the problem: rotate the molecule. From directions with positive x, y,
z the lobes look opaque as they should. However as you look at the system
from the opposite direction the lobes in the front appear in the back and
vice versa, as if the "painter algorithm" is getting the objects sorted
backwards.

So, has anyone seen this before, and is this something really simple to
correct? I got into this because I want transparent wave functions but not
too transparent, so I need to play with the alpha settings.

Thanks in advance.

Filipe


-- 
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Filipe J Ribeiro
University of California, Berkeley
Department of Physics
366 LeConte Hall # 7300
Berkeley, California 94720-7300
USA
Phone: 510.643.3374
Fax: 510.643.9473
http://civet.berkeley.edu/~fribeiro
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