Re: lines or triangles

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Rémi Arnaud (remi++at++remi.engr.sgi.com)
Mon, 28 Jul 1997 10:08:26 -0700 (PDT)

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chris wrote:
>
>
> Hi pfPeople.
>
> I have written a data visualization program which displays time dependent
> vector field, isosurface, etc. data evolution. The time dependence is updated in
> a forked Dbase process. The machine is an indigo2 max impact.
>
> I'm finding the following hard to believe. Rendering a single timestep
> of vector field data requires either 22,000 triangles, or 4100 lines, depending
> on how I model the arrows. Both of these absolutely crush my frame rate. Draw
> stage is about 200-300+ milliseconds.
>
> What amazes me is that while the vector field scene modeled with 22,000 triangles
> gives me about 3Hz frame rate, the vector field scene modeled with 4100 lines only
> gives me 4-5Hz frame rate. This seems like a pretty negligible improvement!
> Is this normal? I thought lines would be much easier to draw than arrows... I'm
> wrong?

Yes, wrong.
Lines does not use the same drawing algorithm than polygons.
They have a glLineWidth, and a surface based aproximation antialiasing
algorithm, compared to no antialiasing on surfaces for indigo2 and brute
force multisampling on RE2 and IR. They also can have a glLineStipple...

So lines are more expensive than triangles.
In practice, when you have too many polygon in a database,
you may think about changing the Hardware (22000 triangles
is a lot, it will run at 60HZ on a IR class HW only), or reducing
the polygon count using LOD or/and replacing some complex polygons models
with less polygons and textures.

Regards

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