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From: jfoust@well.UUCP (John Foust)
Newsgroups: comp.sys.amiga
Subject: Re: Interchange program for Sculpt 3D and Videoscape3D
Message-ID: <4447@well.UUCP>
Date: Sat, 14-Nov-87 17:57:39 EST
Article-I.D.: well.4447
Posted: Sat Nov 14 17:57:39 1987
Date-Received: Sun, 15-Nov-87 20:34:31 EST
References: <3557@ccicpg.UUCP> <4382@well.UUCP> <4386@well.UUCP> <4391@well.UUCP>
Lines: 27
Summary: correction


I must comment on something I said in this message.  I said:

> What about the way VideoScape clips polygons?  It uses a very crude
> method.  It first checks the centroid of the polygon.  If the centroid
> is not visible in the viewing window, then the polygon is not drawn. So
> if you create a table top, and zoom in to an object on a corner of the
> table, the table top disappears.  Also, it fails the most primitive
    
This is not true.  I got this description from someone who uses VideoScape
a lot, but in retrospect they had it confused.  I hadn't tested it myself
before I posted.  I apologize for the error, and clarify:

While "centroid" implies a point that is the average of all coordinate
triplets, VideoScape instead uses the center of the bounding box of the
object, that is, the center of the max and min of all points in the object.
This might be more properly called the center of extent.  It does render
polygons if their center of extent is off-screen.  (This should have been
obvious to me.)  VideoScape uses the distance of the center of extent as
the determiner of the "depth" of the polygon when drawing the scene.
Farther polygons are drawn first, layering the image.

A correct example of a failure of this rendering technique is an object on
a tabletop.  In a view with the center of the table between the object and
the camera, the object may disappear because the tabletop will be drawn
after the object, thus obscuring it.  How do you get around this?  Break
the up the tabletop into several polygons, instead of one big polygon.