> 
> Before wasting to much of your time patching and for my own enlightment, 
> is there any real interest for this kind of shader information ?

Don't know. Perhaps not, as POVMan itself is already niche product. But 
this is my way of wasting time and perhaps I can learn something during 
this process :-)

> What does it allow to make ?
Analytical anti-aliasing. Of course this can be achieved with POV-Ray 
means and probably more efficiently. Additionally, writing shaders which 
antialiase well ain't easy task (although one can use number of already 
created shaders).
But if analytical antialiasing is too complex for given shader, then 
shaders frequency could be limited, if we know sampling frequency (IOW 
surface derivatives). This approach could be applied to many shaders, 
which use noise or its variants. (Of course more-or-less similar results 
could be achieved by giving estimated derivatives as parameters, but 
this works only for fairly 'flat' shapes.)
Other application is nonphotorealistic shaders. Cel shading is one of 
them, other application could be shaders in 'Pen-and-Ink', woodblock or 
engraving style. (PDF document describing pen and ink rendering could be 
found at
http://citeseer.nj.nec.com/winkenbach96rendering.html
though not directly renderman related. ARM book contains shader, which 
gives similar results).


> For the time being, I even hardly see any interest even just for
> the edge detection/celluloid thing... 
> so I wonder if it is really worth something.
Yes, it could be discouraging. There is quite number of patches, which 
have very little recognition, either because they are hard to use or 
they are too specific or they copy already existing functionality.

But there are other areas in shader support, which could be implemented 
or improved. One of them is displacement shader, which could give nice 
support for displacement, other is light shader, which could be used for 
creation of user-specified light-sources. Or Imager shader, which could 
be used for post-processing image. Or support for transformations 
between camera and screen spaces, which give more flexibility to 
shaders. So possibilities for shaders are huge, if I had only time enough...

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