Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!mailrus!uflorida!stat!vsserv!loligo!pepke
From: pepke@loligo (Eric Pepke)
Newsgroups: comp.graphics
Subject: Re: Z-Buffer Edge effects
Message-ID: <147@vsserv.scri.fsu.edu>
Date: 1 Sep 89 15:18:01 GMT
References: <1972@diamond.warwick.ac.uk>
Sender: news@vsserv.scri.fsu.edu
Reply-To: pepke@loligo.UUCP (Eric Pepke)
Organization: Supercomputer Computations Research Institute
Lines: 36

In article <1972@diamond.warwick.ac.uk> arthur@flame.warwick.ac.uk (John Vaudin) writes:
>If I render a cube from an isometric view then the edges of the
>back faces are visible. This is because the Z value for the pixels in
>the edge of the front face and the edge of the back face are equal so the
>visibility of the edge pixels is determined by the order in which the
>drawn.
>
>I do not understand how to get round this problem. If two pixels have
>the same Z value what do you do ? How do real systems get round this ?

I don't know about all real systems, but most of the ones I have seen get
around it by not documenting the problem and letting you find out after
you buy the machine.  :-)

For the particular problem that you describe, however, when the polygons 
share vertices, there is a relatively simple solution: don't draw the edges.
Use the Macintosh model where lines around polygons are considered infinitely
thin, and only render the pixels that are honest-to-Chthulu in the interior
of the polygon.  You can pretty easily whip together a Bresenham system that
does this in integral screen coordinates and guarantees that adjacent polygons
will never overlap or leave gaps.

If you do this, the normal case of filled abutting polygons will work quite
nicely.  If the user really wants to mess up the system by having objects 
that are coincident in 3-space and of different colors, well, life's rough.

Now, if you really want to have fun, precompute the Bresenham error terms
based on subpixel values of the transformed coordinates.

Eric Pepke                                     INTERNET: pepke@gw.scri.fsu.edu
Supercomputer Computations Research Institute  MFENET:   pepke@fsu
Florida State University                       SPAN:     scri::pepke
Tallahassee, FL 32306-4052                     BITNET:   pepke@fsu

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