[Chimera-users] Chimera-users Digest, Vol 57, Issue 24

[Jonathan Hilmer]

POV-Ray also works extremely well as a distributed job.  This site has
some basic discussion of some of the distributed POV options:

http://columbiegg.com/httpov/?p=why


Jonathan



> Today's Topics:
>
>    1. (no subject) (jean-francois menetret)
>    2. Re: Flip handedness of map (Tom Goddard)
>    3. VRML alpha vs 1-alpha (Nils Becker)
>    4. Re: VRML alpha vs 1-alpha (Greg Couch)
>
>
> ------------------------------
>
> Message: 4
> Date: Tue, 22 Jan 2008 10:37:54 -0800 (PST)
> From: Greg Couch <gregc at cgl.ucsf.edu>
> Subject: Re: [Chimera-users] VRML alpha vs 1-alpha
> To: Nils Becker <nils.becker at ens-lyon.fr>
> Cc: chimera-users at cgl.ucsf.edu
> Message-ID: <Pine.OSF.4.63.0801220934340.1761700 at guanine.cgl.ucsf.edu>
> Content-Type: TEXT/PLAIN; charset=US-ASCII; format=flowed
>
> On Tue, 22 Jan 2008, Nils Becker wrote:
>
> > Hi,
> >
> > 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
> up).
>
> 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.
>
>         Greg Couch
>         UCSF Computer Graphics Lab
>
>
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>
> End of Chimera-users Digest, Vol 57, Issue 24
> *********************************************
>