Path: utzoo!mnetor!uunet!lll-winken!lll-tis!ames!pasteur!ucbvax!NPS-CS.ARPA!zyda
From: zyda@NPS-CS.ARPA (michael zyda)
Newsgroups: comp.sys.sgi
Subject: Z-buffering Problems...
Message-ID: <8804211723.AA25090@nps-cs.arpa>
Date: 21 Apr 88 17:23:15 GMT
Sender: daemon@ucbvax.BERKELEY.EDU
Organization: The Internet
Lines: 33

IRIS 4D/70G Z-Buffering Problems...

  One of my students is having some problems with the z-buffering
on the IRIS-4D. He gave me the following letter to post on info-iris.
Any similar experience or solutions would be greatly appreciated...

  "I had some unexpected and unwanted results from using Z-buffering
on the IRIS 4-D. I am modeling terrain from a digital database of 
elevation data. To cut down the number of polygons drawn, I start 
with a big blue polygon at 0 elevation, "ocean", and only draw 
polygons that have elevations > 0. 

  Problem: At the intersection of the sea and land you cannot 
rely on the Z-buffer to always draw the land over the water 
or even to consistently draw it the same. This is with near and 
far clipping planes set to 100yds and 120000yds respectively.

  I believe this problem is due to the resolution of the Z-buffer.
I can get an acceptable picture if I draw sea level at -10, close, to 
-100, far, and set the far clipping plane to 1200000, 10 times the area 
I'm drawing. This effectively decreases the resolution and wastes 90% 
of the Z-buffer resolution. With this solution, the coastline, drawn 
at a distance, is still erratic. By lowering the sea and increasing the 
area that will be mapped to a certain z value, I insure that land 
will be drawn over water in a consistent manner. 
  
  This solution is not very acceptable. Does anyone out there
have similar experiences?"

   Frank Harris
   fharris@nps-cs.arpa


Responses can be sent through zyda@nps-cs.arpa