Alpha blending ...

[Gautam Mehrotra]

In article <1990Sep5.171056.23011 at odin.corp.sgi.com> robert at sgi.com writes:
>
>In article <1990Sep5.151623.12906 at ux1.cso.uiuc.edu>,
>gautamm at 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 at sgi.com
>

thanks,
gautam