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From: gautamm@kandinsky.ncsa.uiuc.edu (Gautam Mehrotra)
Newsgroups: comp.sys.sgi
Subject: Re: Alpha blending ...
Keywords: Intersecting polygons, alpha blend
Message-ID: <1990Sep5.190659.14015@ux1.cso.uiuc.edu>
Date: 5 Sep 90 19:06:59 GMT
References: <1990Sep5.151623.12906@ux1.cso.uiuc.edu> <1990Sep5.171056.23011@odin.corp.sgi.com>
Sender: news@ux1.cso.uiuc.edu (News)
Reply-To: gautamm@kandinsky.UUCP (Gautam Mehrotra)
Organization: National Center for Supercomputing Applications
Lines: 33

In article <1990Sep5.171056.23011@odin.corp.sgi.com> robert@sgi.com writes:
>
>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 ...
>|> 
>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.  

        I am sorry I didn't make myself very clear -- I am trying to
do transparent( translucent ) surfaces -- therefore the alpha
blending.  The problem is that if I use the z-buffer ( which I am
using), parts of intersecting polygons are not rendered at all ( the
part which lies behind values in the z-buffer). I can get this 
portion if I re-render the polygon with the zfunction changed to
test for values of z GREATER than the values in the z-buffer.
However, I  haven't been able to get the alpha values right. Does
anyone have any experience with this ?

Thanks once again !
gautam


>Hope this helps,
>Robert Skinner
>robert@sgi.com
>

thanks,
gautam