Hi all.
Is it possible to catch normals interpolation and apply for example a sine function on it ?
Am not talkink about normal mapping… i’m talking about normal interpolation ( the one that
interpolates from edge to edge in fragment )
I’m trying to change the interpolation behaviour and apply a function on it.
Please help ! this would be nice !
The behaviour of vertex interpolators cannot be modified; they always interpolate linearly, with perspective correction.
However, if you specify more precisely what you would like (perhaps with a picture), someone might be able to help you achieve it using another method.
for sure 
suming up what i want to do is flat shading with rounded edges. This is damn simple to express and to imagine but it appears it has
never been done ( except with normalmaps ), and seems almost impossible to achieve with today’s hardware specs.
Of course i don’t want to add polygons for beveling my models edges, but i thought that it would be possible to achieve through a
shader bit of code a flat shading ( the good ol’ one ) and at pixel shader comes to a distance of the edge ( with the same technique used
in the ‘wireframe shader’ ) interpolate from the ‘flat shading’ normal to the edge normal…
In fact, i found no way to influence the basic ( the interpolated one on the face, not the ‘bumpmapping’ one ) normal interpolation. it appears
that whatever i do, it’s always interpolating…
Then i thought that i could do my own interpolation… Grab the normals used in lighting, stor them in my own vars and play with them
as i want. but… it appears that in surface shaders, the o.Normal is not initialized. It can only be written to…
8*(
i’m stuck here… i’m feeling that what i wanna do should be achievable, but i’m a gimp in shader science and have too few time to dive
in it.
If anyone have an idea… or at least a link to some paper talking about all this, this would be greatly appreciated
Anyway, thanks Daniel for your answer ! i’m happy to see there are still ppl in there for talking about shaders 
regards.