Mark Wagner wrote:
> 
> Mark VandeWettering wrote in message <379### [at] pixarcom>...
> >The Toro work at Stanford is a significant example of how to make this
> >kind of system work.   And it supports displacement maps too!
> 
> Have you seen the latest developments with the SuperPatch in p.b.i?

You are missing his point.  The raymarching technique used in the
"SuperPatch" would need many more evaluations of the noise function than
a basic tesselation.  Solid noise functions cost a lot to evaluate. 
Thus, for doing that sort of surface triangles can be a big win
(especially in these days of lots of RAM and simple mapping of files to
VM).

There are always going to be time/space tradeoffs.  By splitting into
triangles, you have the cost of storing vertices and triangle indices
(plus the bounding hierarchy).  For a raymarcher you have the cost of
evaluating a potential function at every step along the ray (and the
memory occupied by the displacement map).

There is also a pretty long tradition of making specific primitive types
for raytracing.  For the asteroid example, Mark could have implemented a
spherical heightfield prim (like in Polyray) that has been built
expressly for tracing.  With that approach you have the memory savings
of only storing the displacement map (plus some additional acceleration
information), and much better performance than a raymarcher.  Of course,
it would have taken time to write the prim...

Xander

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