> 
> Until YOU are going to provide EACH pov object with a function which
> provide the derivative at any point of the object, including at the
> surface, but also inside and outside, the only approach would be
> to make a kind of guess by casting more rays.
> Thus it would require more rewrite of the pov-engine, and a serious
> slowdown. (I remind you that some object provide a normal only
> for a previously computed intersection... so you cannot 'sample' around
> the intersection to get a feeling about the variation of the normal...)
> 
[Snip]
Yes, I know all these problems.
Additional problem, which comes with use of RenderMan shaders, is the 
fact, that in RenderMan all surfaces are parametric (with respect to uv) 
and surface derivatives are usually taken with respect of u and v (this 
is done 'analytically' in BMRT (don't know about PRMan), other 
derivatives are made by 'supersampling': running shader in u+du or v+dv 
points and calculating required expression). This means, that using 
RenderMan shaders with surface derivatives could be done to small subset 
of POV-Ray objects (sphere, plane, box, mesh, ??).
I intend to create method for POV-Ray objects, which returns surface 
derivatives and implement it for 'parametrizable' objects (as you put 
it: 'safe approach'). For other objects dummy implementation (or 
approximation??) could be made. This is not 'perfect' solution, but it 
allows to use some advanced shaders with POV-Ray (current implementation 
of shader support is limited and it will not be fully supported anyway, 
for it would require too much work, easier would be to use RenderMan 
compliant renderer in such case).

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