Roberto A. wrote:
> 
> There are Renderman compliant renderers with raytracing support that do true
> displacement without problems (BMRT being one of them, and Pixie a more
> current example). Of course, true displacement is best used with Reyes
> renderers, since you have almost infinite precision with that particular
> algorythm.

There is no raytracer that can apply displacement to an analytically 
solved shape.  That's not too bad of course because all non-raytracers 
can't even render those shapes.  At the risk of repeating myself - 
isosurfaces are as close to 'true displacement' as you can get.

> Displacement can be applied to any primitive that takes shaders on a
> Renderman compliant renderer. The original object retains the same
> geometry - it's only displaced by the shader when rendering.

This is simply wrong (see above).  What the renderer you have in mind 
probably does is to approximate the shape with a mesh and displace that 
- this is of course much less a 'true displacement' than an isosurface.

> However, with
> isosurfaces, you have to modify the original function, thus the original
> object is modified before parsing or rendering. As David said, picky, but
> true.

No, you don't have to modify the function as ABX already pointed out:

#declare fn_Shape = function { ... }

#declare fn_Iso = IC_Displace (fn_Shape)

isosurface {
   function { fn_Iso(x,y,z) }
   ...
}

> Both approaches have their weak and strong points. However, that doesn't
> change the fact that I'd like to apply displacement on pure primitives on
> POV,

Sorry, this will never happen, simply because it is not possible (at 
least not without an approximative root solver like in the isosurface 
shape).

Christoph

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