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List:       freedesktop-xorg
Subject:    Re: 4 corner gradient
From:       Allen Akin <akin () pobox ! com>
Date:       2007-06-28 17:46:29
Message-ID: 20070628174629.GZ26575 () arden ! org
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On Thu, Jun 28, 2007 at 11:29:04AM +0100, Keith Whitwell wrote:
| Glynn Clements wrote:
| > Tom De Man wrote:
| > 
| >> Using the i915 I try to create a 4 corner gradient rectangle using 2 3D
| >> primitive triangles. I get a visible diagonal line between the two
| >> triangles ...
| > 
| > Gouraud shading uses linear interpolation. Essentially, each of the
| > two triangles will be mapped to a planar (flat) triangle in RGB space. 
| > As the 4 vertices (red, green, blue, white) aren't co-planar, there
| > will be a discontinuity (corner) along the diagonal where the two
| > triangles meet.
| > 
| > OTOH, bilinear interpolation between the vertices will produce a
| > continuous surface (specifically, a ruled surface) with no
| > discontinuities.
| > 
| > AFAIK, you cannot achieve bilinear interpolation using OpenGL
| > primitives...

I have a vague recollection that you can do this in old-style OpenGL
using texture mapping, but I can't find sample code at the moment.

| You could do it pretty simply with a pixel shader.

Yep, that would be the way to go with current hardware.

Allen
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