RE: [Algorithms] Terrain performance comparisons

[Gareth L. <GL...@cl...>]

There is another issue with shader LOD (atleast on my current project) and
that's passes. By removing per-pixel lighting (or bumpmapping) you can
significantly reduce the number of passes required for a group of polys.

> -----Original Message-----
> From: Tom Forsyth [mailto:tom...@bl...]
> Sent: 30 July 2003 08:33
> To: gda...@li...
> Subject: RE: [Algorithms] Terrain performance comparisons
> 
> 
> Ah, gotcha. Yes, you and Charles have a good point that when you
> simplify shaders you can also merge previously different shaders into
> one. Which decreases the setup time and increases your batching and
> enables more LOD than you could have had otherwise.
> 
> There's still annoyingly hard limits on how much you can use 
> batching to
> improve speed - you still have to get those diffuse textures and
> orientations and bone matrices to the hardware somehow. But 
> at least you
> can render the whole person with one shader rather than the 
> 20 you would
> use up-close.
> 
> Very true. I'll be over here eating my humble pie.
> 
> TomF.
> 
> > -----Original Message-----
> > From: gda...@li... 
> > [mailto:gda...@li...] On 
> > Behalf Of Jonathan Blow
> > Sent: 30 July 2003 08:00
> > To: gda...@li...
> > Subject: Re: [Algorithms] Terrain performance comparisons
> > 
> > 
> > Tom Forsyth wrote:
> > 
> > > Shader LOD is less compelling to me than geometry LOD. If you have
> > > pretty much any sort of LOD - even static levels - then 
> > you're going to
> > > be drawing (very) roughly constant-area triangles. So your
> > > vertex-processing load per pixel is roughly constant. And 
> > obviously your
> > > pixel load per pixel is constant (excepting overdraw, which 
> > you can't do
> > > much about using LOD - you need occlusion algos, which is 
> > orthogonal to
> > > the problem).
> > 
> > I think we are thinking about shader LOD for different reasons!
> > 
> > It sounds to me like you're saying, the purpose of shader LOD 
> > is to make
> > far-away pixels cheaper to draw, and that's not really so 
> > important in the
> > grand scheme of things.  (Correct me if I am 
> misunderstanding you).  
> > 
> > That is not at all why I think shader LOD is important.  
> > 
> > The phenomenon I am worried about is that if you don't 
> merge shaders,
> > then there is an artificial limiter on how much you can LOD 
> > your geometry,
> > because you can't merge triangles that use different shaders. 
> >  So what you
> > end up with is, past a certain distance, a lot of small batches of
> > triangles that just won't reduce any further.  Your LOD is prevented
> > from operating.  And it's really slow to submit all those 
> > batches to the 
> > hardware, due to all the context switching.
> > 
> > I am not worried about the amount of time it takes to 
> > actually render a
> > single pixel (which is why I was dreaming about a world in 
> > which we use
> > one shader for everything, and just data-drive its parameters).
> > 
> >    -Jonathan.
> 
> 
> 
> 
> 
> 
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