From: Angus Dorbie (dorbie++at++sgi.com)
Date: 03/21/2000 10:49:21
Hmm...
on reflection you'd need a unique transformation to position the sphere
anyway.....
Cheers,ANgus.
Angus Dorbie wrote:
>
> I suspect that billboarding wouldn't be a very optimal approach to this.
>
> Ideally you don't want a transformation node per sphere and doing the
> vertex transformation for billboards on many complex spheres would be a
> huge overhead. The hardware sprite mechanism may come to the rescue on
> iR.
>
> What I'd suggest is doing a host/application based billboarded vertex
> transformation of a few geometries, if you have orthographic projection
> then just one will do, but for perspective you'll need several to cover
> the different view vector angles. Maybe you'd render slightly more than
> a hemisphere to reduce this work by reducing the number of unique
> billboards you need to give an accurate silhouette. Maybe you could
> cache lot's of different pretransformed versions of the sphere. Anyway,
> as you move through the scene or move the mollecule you'll have to
> switch the geometry associated with each atom or particle.
>
> I'd also suggest that you can tesselate more for lighting where you
> predict the phong highlight will be on the sphere (or use texture) and
> tesselate for a detailed silhouette of the sphere. I don't think
> intersections require as much tesselation as the shilouette and you may
> not even have any sphere-sphere intersections, in which case a pure
> sprite might work well if you can light with a texture.
>
> Cheers,Angus.
>
> Allan Schaffer wrote:
> >
> > On Mar 19, 7:59pm, Simon Hayhurst wrote:
> > > From some work we did a few years ago in SGI Europe, we also found a huge
> > > gain by only using hemi-spheres, and rotating the hemi-sphere to face the
> > > user. There's a fair bit of App side workload in doing this, but since
> > > molecules are typically moving anyway, the net win was considerable.
> >
> > Yes this would be a tremendous gain. Place the hemisphere geometry
> > under pfBillboard nodes so that they'll automatically rotate to face
> > the viewer.
> >
> > Someone might be thinking, "what's the difference, why not just let
> > automatic backface culling remove the back-facing side of the
> > spheres?": It's because the backface calculations are done on a
> > per-triangle basis down in the graphics pipe. Not sending the
> > geometry down in the first place would be a big performance win.
> > And on MP systems, the billboard calculation is done in the CULL,
> > where there's quite typically plenty of extra time each frame.
> >
> > Allan
> >
> > --
> > Allan Schaffer allan++at++sgi.com
> > Silicon Graphics http://reality.sgi.com/allan
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