VMD-L Mailing List

From: John Stone (johns_at_ks.uiuc.edu)
Date: Fri Jan 30 2004 - 00:57:40 CST

Charles,
  I think I've tracked this down to a weakness in the way VMD exports
a particular type of triangle to the ray tracers. I did some tests
in OpenGL and as I expected the coloring of both types of ribbons reps
matches up almost exactly. The main difference between the way they are
drawn is the type of geometry they use for rendering and the way that
geometry is interconnected. The difficulty in getting this to export
to ray tracers is that many ray tracers don't have direct support for
the particular variant of per-vertex coloring that we're using in
VMD via OpenGL. I could add it for Tachyon, and the newest versions
of POV-Ray may have it, but many of the other renderers would still
lose out. That said, I think I can improve this "fallback" code
that's being run in this case so that the coloring you get would
still be much closer to what you see in OpenGL.

For VMD 1.8.3, I do plan to add new triangle code, volume textures, and
a couple other VMD-centric features to Tachyon so that people can get
identical results in Tachyon to what is done in VMD via OpenGL.

I'll look at the the other ray tracers and external renderers to see
if their newest versions support some of these things, but I can't make any
promises about those until I investigate further.

As soon as I have a patch that ameliorates this ray tracer
coloring issue I'll let you guys know so you can test it out.

  John

On Thu, Jan 29, 2004 at 10:31:23PM -0500, Charles_at_Schwieters.org wrote:
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>
> Hi John--
>
> > I could address your request in one of several ways.
> > I'll take a look and see what's causing the NewRibbons code to
> > generate fewer colors. A cursory examination didn't immediately
> > reveal the answer, but it should be easy to fix. In any case,
> > with the NewRibbons code I could add color interpolation
> > at each "section" which would end up with color gradients that
> > are smoother than even the old ribbons code. There are times when
> > you want a sharp color change and times when smooth is preferable,
> > so the interpolation solution isn't as good as tying the color more
> > closely to individual atoms along the ribbon. I'll look at this some
> > more and let you know when I have something you can try out. It should
> > be a minor change.
>
> I agree that one might not always want the interpolation- maybe it
> could be a toggle, or a tunable parameter. Interestingly, the opengl
> representation of the newRibbons drawing mode seems to have the
> interpolation.
>
> anyway, thanks for looking into it--
>
> Charles
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-- 
NIH Resource for Macromolecular Modeling and Bioinformatics
Beckman Institute for Advanced Science and Technology
University of Illinois, 405 N. Mathews Ave, Urbana, IL 61801
Email: johns_at_ks.uiuc.edu                 Phone: 217-244-3349              
  WWW: http://www.ks.uiuc.edu/~johns/      Fax: 217-244-6078