RE: [Algorithms] Terrain performance comparisons

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

As Peter-Pike Sloan pointed out, naive mipmapping without renormalisation is
actually almost precisely what you want for diffuse normal maps. Consider
the two versions:

Calculate then mipmap - "proper": result = lerp ( dot(tex0,L), dot(tex1,L) )

Mipmap then calculate: result = dot ( lerp (tex0,tex1), L )

So if you multiply it all out, modulo some clamping of negative values, you
get the same result!

Of course with specular calculations it's complete rubbish :-)


Tom Forsyth - Muckyfoot bloke and Microsoft MVP.

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> -----Original Message-----
> From: Thatcher Ulrich [mailto:tu...@tu...]
> Sent: 30 July 2003 15:01
> To: gda...@li...
> Subject: Re: [Algorithms] Terrain performance comparisons
> 
> 
> On Jul 30, 2003 at 01:59 -0500, Jonathan Blow wrote:
> > 
> > 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.
> 
> There's yet another (related) point I would like to make about shader
> LOD: for decent fill-rate (and antialiasing), it's essential to have a
> way to scale your texture accesses & shader computations with pixel
> size.  For example, mip-mapping is a form of shader LOD.  But
> mip-mapping basically is good for diffuse lighting on a flat surface,
> it doesn't work quite right for normal maps etc.
> 
> I think to be compelling, the next frontier of geometric LOD has to
> confront the shader LOD issue and unify it with geometric LOD.  In my
> own LOD stuff in the past, I've handled this by using pre-baked
> mip-mapped textures (w/ surface normals based on the hi-res geometry),
> and unique texture mapping, which works for diffuse shading with
> static lights and static geometry, but obviously that's pretty
> limited.
> 
> -- 
> Thatcher Ulrich
> http://tulrich.com