[Chimera-users] VRML alpha vs 1-alpha
gregc at cgl.ucsf.edu
Tue Jan 22 10:37:54 PST 2008
On Tue, 22 Jan 2008, Nils Becker wrote:
> I found information on the web site that VRML models use
> 1-alpha blending for transparency, and that that looks ugly.
> I agree.
> Now I have externally generated VRML files including transparency.
> Is there a way to get nice (normal) transparency rendering of these
> models in chimera?
> e.g. editing the vrml files, converting them to another format or some
> option inside chimera?
> Thanks for any help, cheers, Nils
The VRML transparency is 1-alpha, because alpha is an opacity value. So
that is just an alternative way of saying that same thing, and is not the
reason it is worse.
The reason VRML transparency is worse is that there is no sorting within
the VRML shapes, so the blending of transparent objects has to be
drawing-order independent, so VRML uses additive blending.
The simpliest solution would be to live with chimera's current depiction
and use the Save Image dialog to generate a ray-traced version. The
ray-traced version will be much nicer (and take a lot longer to compute,
try using the POV-Ray 3.7 beta on a multi-core computer to speed things
The other solution, is to write some Python code and use the _surface
module's Surface_Model and Surface_Group classes. That is, convert each
VRML object into a triangulated surface, a Surface_Group, and put all of
the Surface_Groups into one Surface_Model. All surfaces within one
Surface_Model are sorted so that the transparency is correct for more than
one overlapping surface (unlike the transpareny in chimera between
models that only works for one level of transparency). You can see
examples that use the _surface module in the GraspSurface, IMOD,
Icosahedron, and Intersurf modules (and others) in the CHIMERA/share
directory of your installation.
If you have any questions about the _surface module, please ask them on
the chimera developers mailing list, chimera-dev at cgl.ucsf.edu.
UCSF Computer Graphics Lab
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