RE: [Algorithms] Bullets on walls

[Tom F. <to...@mu...>]

If you are using a small texture with clamp-to-edge mode, the scanline
conversion still has to be done (consuming pipeline clocks), but it will
just read the same texels over and over again (the edge ones) for most of
the drawing. So it won't compete for memory bandwidth.

Older chips won't see much benefit - they were indeed core-bound most of the
time, not memory-bound (or at least the core wasn't far behind the memory),
but newer chips will love you silly, especially in 32bpp modes.

Tom Forsyth - Muckyfoot bloke.
Whizzing and pasting and pooting through the day.

> -----Original Message-----
> From: Stephen J Baker [mailto:sj...@li...]
> Sent: 21 July 2000 18:38
> To: gda...@li...
> Cc: ma...@ci...
> Subject: RE: [Algorithms] Bullets on walls
> 
> 
> 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)
> L3Com/Link Simulation & Training (817)619-2466 (Fax)
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