Subject: concave objects

From: Neal Rogers <[email-masked]>
Date: Thu, 27 Feb 92 17:29:59 WET

Dave Stampe wrote :

>BTW has anyone actually bothered to create new objects for REND386 yet?

I'm about to knock some walls down in my house, and rebuild the kitchen, 
so I thought I'd try to model this using rend386. This brought some problems
to light - maybe the problems are in my .plg file, or maybe they're in the
software. I don't think it's the .plg though. My conclusions are as
follows -

1. Some of the faces of concave objects are displayed when the normal is
_away_ from the viewpoint.

2. and vice-versa, some faces of concave objects are not displayed when the
normal is towards the viewpoint.

3. Clipping is sometimes a bit weird. Something like..if a vertex of a
polygon is behind the viewpoint, then the polygon is treated as though
that vertex isn't part of it (sometimes). Strange triangles appear.

Also, a couple of other things -

1. Where's the light source ? Surely this should be in the .plg file.

2. If the .plg file is incorrectly specified (e.g. the number of vertices/
polygons provided is less than the number specified on the first line),
rend either crashes or does nothing. Seems like no sanity checking of the
data is performed.

3. Holes (such as windows) cannot be created (at least I tried the usual
methods).

Admittedly the viewpoint is inside the object (the house) but surely
this is a prerequisite for VR. I also realise the docs don't mention
vertex ordering in concave polygons - I tried only the usual one of
ensuring the first three vertices are clockwise when viewed from 'outside'
(in this case).

-----
Neal Rogers
[email-masked]  BT Customer Systems, Brighton Systems Centre

---

From: Dave Stampe-Psy+Eng <dstamp@watserv1>
Date: Thu, 27 Feb 92 18:13:04 -0500

First, only convex polygons are supported.  Split concave polys up if
you have to.

Second, check that all polys are CLOCKWISE from the desired viewing 
side.  This is not the case for some methods of defining polygon strips.
To be inside an object and yet see it, reverse the "outside" polygon
vertex order, or have two polygons (inside and outside) for each wall.

Third, to create holes, DON'T add extra verteces to a poly: use 4 polys 
to make the wall (or whatever).  

If these don't get you going, e-mail me the file and I'll look at it.

- Dave Stampe