From: Kevin Smeaton (ksmeaton++at++sgi.com)
Date: 08/06/2001 15:47:17
Hi Performer Users,
I am passing the attached questions that I received from one of my past
Performer students to the group in the hope that they may be answered. What
he asks is beyond my current knowledge of Performer internals.
Thanks
Kevin S.
----- Original Message -----
From: "Stephen Jeffrey" <Stephen.Jeffrey++at++maths.uq.edu.au>
To: <kevin.smeaton++at++sgi.com>
Sent: Monday, August 06, 2001 9:53 AM
Subject: NURBS hardware acceleration
> Dear Kevin,
>
> I have a question regarding NURBS and evaluators. If you could
> help me it would be really appreciated. If not, could you please
> suggest who might.
>
> I wish to construct NURBS curves and surfaces so that I can take
> advantage of NURB's invariance with respect to perspective
> transformation. In other words, I want the perspective transformation to
>
> be applied to the control points, and then the NURBS curve/surface
> constructed from the transformed control points. (As opposed to
> constructing the curve and then applying the perspective transformation
> to every approximating line used to represent the curve/surface)
>
> Would the GLU implementation (of the NURBS routines) on an
> SGI Infinite Reality 3 machine (which I assume has hardware acceleration
>
> for evaluators) take this approach? How does the GLU implemetation
> proceed? Does it construct the NURBS curve/surface, convert it to Bezier
>
> form and then use the OpenGL evaluator commands? If so,
> where in the rendering pipeline is the perspective transformation
> applied?
>
> Effectively, I need to know:
>
> 1. Do the GLU NURBS routines convert everything to a Bezier form?
> If this is case, would the invariance to perspective transformation
> (possessed by NURBS, but not Bezier curves) be lost?
> If so, the perspective transformation would have to be applied
> to every approximating line used to represent the curve/surface
> and not just the control points.
>
> 2. What do the hardware evaluators actually compute? Are
> they computing the Bernstein polynomials, or the Bezier curves?
> If they only compute the Bernstein polynomials, would the
> CPU then be used to contruct the Bezier curve from the
> Bernstein polynomials?
>
> cheers
> steve
>
> --
>
> Dr Stephen Jeffrey
> Advanced Computational Modelling Centre
> University of Queensland
> St. Lucia Queensland 4072 Australia
>
> Phone: +61 (7) 3365 6130
> Fax : +61 (7) 3365 6136
> Email: Stephen.Jeffrey++at++acmc.uq.edu.au
>
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