Path: chuka.playstation.co.uk!news From: James Shaughnessy Newsgroups: scee.yaroze.programming.3d_graphics Subject: Re: Non-coplanar quads Date: Mon, 10 Aug 1998 21:08:23 +0100 Organization: PlayStation Net Yaroze (SCEE) Lines: 36 Message-ID: <35CF5337.79CB@manc.u-net.com> References: <01bccdd1$bd605fc0$LocalHost@default> NNTP-Posting-Host: manc.u-net.com Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 7bit X-Mailer: Mozilla 3.0 (Win95; I) Mike wrote: > I was wondering how the GS lib deals with non-coplanar quads, that is, > four-sided polygons whose vertices are not in the same plane. Does it > split quads into tris? If so, does it do this in a predictable way > (ie. always using the same combinations of vertices to form the tris)? I believe quads are actually drawn as two triangles by the GsLib, planar or otherwise -- so a non-coplanar quad would have an unpredictable fold/split direction (depending on the vertex order) I'm sure of this because when you get up right close to polygons they disappear when cutting the projection plane, and sometimes, even quads split diagonally into a triangle. It of course is faster than always handling triangles because of the way the lower level gslib handles the TMDs. I don't think you'll often come across non-coplanar quads though because a 3D editor would not export such a polygon in DXF format, but it could be forced manually of course. My problem is getting the camera to get right up close to the floor or walls without the projplane cutting huge sections of the road/wall out. Automatic poly subdivision reduces this but is logarithmically expensive (but then again its often necessary to reduce texture map warp). One thing I've noticed is drawing a single 4-vertex quad polygon with 4x4 subdivision (effectively 16 polys) is a LOT faster than a 25 vertex 16 polygon TMD with no subdivision. But alas at the expense of texture sizes. Later, Jim PS I realise your message was over 10 months old but I only just read it! (and no-one else has responded) -- ----------------------------------------- James Shaughnessy james@manc.u-net.com http://www.netyaroze-europe.com/~shaughnj -----------------------------------------