On 08/04/2011 10:24 AM, Mike Garrity wrote:
From: [email protected]
[mailto:[email protected]] On Behalf Of Tueller,
Shayne R Civ USAF AFMC 519 SMXS/MXDEC
Sent: Tuesday, August 02, 2011 4:49 PM
To: [email protected]
Subject: Re: [osg-users] intersection with quad mesh: only 3 verticesreturned
It is true that QUAD is an OpenGL primitive but the GL driver tessellates it
into two triangles
underneath the hood.
Most (all?) cards today do break them into 2 tris, but nothing in the OpenGL
spec
says that they have to, or specifies exactly how they should do it. The reason
is
that the original SGI geometry engine did not split them. It drew quads directly
with an odd algorithm for interpolating values across the interior. Quad lives
on
as a historical quirk, like the panda's thumb.
Keep in mind that the intersection methods that OSG provides have little
to do with what OpenGL does or doesn't do with quads, triangles or other
primitives. OpenGL has no primitive intersection features. These are
all provided by OSG itself. The relevant code for separating larger
primitives into triangles is in osg::TriangleFunctor. These triangles
are then used by osgUtil::LineSegmentIntersector to do the actual
segment/triangle intersections.
--"J"
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