if you could take 2nd deriv. of P as func. of U and V, then maybe 2nd deriv. < 0 in convex region and 2nd deriv > 0 in concave region.  

it would switch from concave to convex at 2nd deriv = 0.

for posersurface with centre (0,0,0) facing front.

gather caused colour to bleed between adjacent surfaces.  they left it in recent versions, but it was rather grainy and slow.

BB, if you like renderman, can you write python script that calls it from within poser?