RE: [Algorithms] Bullets on walls

[Stephen J B. <sj...@li...>]

On Fri, 21 Jul 2000, Mark Atkinson wrote:

> We do it this way:
> 
> If the decal does not cross a polygon boundary with a high enough crease 
> angle, draw it as a quad.  Use ZBIAS if supported and/or move it along the 
> normal to lift it off the surface.
> 
> Otherwise draw it as decal, as per Jaimi's method.  Remember to enable 
> ALPHATEST so there's no overdraw or fill-rate hit even if the base triangle 
> is large and has several decals.
 
Eh?

Alpha Test (presuming it's the same mechanism as glAlphaFunc in OpenGL)
won't eliminate (all of) the overdraw/fillrate issues.  The renderer
still has to scan-convert all those pixels, do the texture calculation
on each one and compare the alpha value against the threshold.

* On some hardware, that'll be no cheaper than drawing the entire polygon.

* On other hardware, you'll only pay *some* per pixel cost (maybe half).

* It'll never be zero though - that's impossible - it has to consume
  some texture bandwidth per-pixel of the entire polygon.

Steve Baker                      (817)619-2657 (Vox/Vox-Mail)
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