[vtkusers] VTK vs. straight OpenGL performance (was Re: VTK wireframe performance)

[Dave Reed]

> From: Sebastien BARRE <sebastien.barre at kitware.com>
> 
> At 2/1/2002 04:27 PM, Christophe ATTANASIO wrote:
> 
> >Why VTK have so poor performance (less or egal  than shading) in
> >wireframe mode ??
> 
> Nothing to do with VTK, but with your OpenGL card and drivers.
> 
> Yes, it seems strange that what used to be a rock-solid assumption 
> "wireframe rendering is faster than surface rendering" is not true anymore 
> for today's graphics cards.

And in my experience the exact opposite (i.e., polygons are faster
than wireframe) is a good assumption for today's graphics cards.

On a related note, it does seem that rendering the same number of
polygons with VTK is signifcantly slower than with just plain
OpenGL. For example just using GLUT, I can use glutSphere with a
tesselation of over 1 million polygons and it runs quite nicely on my
Radeon 7500 card, but a VTK example with around 300,000 triangles (in
strips) is slower.

I certainly understand that all the capabilities of the VTK
pipeline leads to some overhead, but I wonder if some optimizations
can't be made without causing problems.

You may remember last fall I started a thread about some "missing
traingles" when using a triangle stripper. The original author of Mesa
found the problem (the triangles were actually being rendered, but as
backfaces in some situations and it's fixed - starting in Mesa 4.0.1 I
think). While Brian was looking at the problem using VTK (since that's
the only program anybody had found that showed the problem), he
mentioned that there were some "optimizations" that could be made in
VTK's OpenGL calls. Unfortunately, he didn't elaborate.

Has anyone ever looked into trying to optimize the OpenGL calls in
VTK? Or does anyone who knows about the VTK internals think there are
some changes that could be made to use OpenGL more efficiently? Is
there interest in this, etc.?

Dave