Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!wuarchive!decwrl!sgi!shinobu!odin!texas.esd.sgi.com!robert
From: robert@texas.esd.sgi.com (Robert Skinner)
Newsgroups: comp.sys.sgi
Subject: Re: Alpha blending ...
Keywords: Intersecting polygons, alpha blend
Message-ID: <1990Sep5.171056.23011@odin.corp.sgi.com>
Date: 5 Sep 90 17:10:56 GMT
References: <1990Sep5.151623.12906@ux1.cso.uiuc.edu>
Sender: news@odin.corp.sgi.com (Net News)
Reply-To: robert@sgi.com
Organization: Silicon Graphics Inc., Entry Systems Division
Lines: 36


In article <1990Sep5.151623.12906@ux1.cso.uiuc.edu>,
gautamm@kandinsky.ncsa.uiuc.edu (Gautam Mehrotra) writes:
|> Is there any way I can use the alpha blending hardware to display 
|> a possibly intersecting set of polygons ??  Since the polygons 
|> can intersect, I have no way of properly ordering them in z ...
|> 
|> 
|> thanks,
|> gautam

I can only assume from the phrase "no way of properly ordering them in z", 
that you are doing hidden surface elimination with the painter's algorithm,
where you paint every polygon from the farthest to the nearest.  

You should use Z-buffering instead.  The Z buffer does hidden surface
elimination at each individual pixel, so intersecting primitives are
no problem.  Its a snap to use, just do this 

	zbuffer( TRUE );
	gconfig();

after you open the window, and call zclear() after clear().
The GT does z-buffering as fast as regular polygons, and there is no 
need to sort your primitives by depth, so your program should run
faster.

Hope this helps,
Robert Skinner
robert@sgi.com

	They are your parent's worst nightmare --
	because they ARE your parents.

				- Radio add about the Jefferson Airplane
				  reunion concert, Berkeley, Sept '89