a size parameter between 0.0 and 1.0 that gives a angle of reflection between
0° and 90°, and a smooth parameter that gives and angle over which a smooth
transition from full to no reflection happens.
These work with global illumination and do importance sampling of the area within
the angle. Note that unlike most other BSDF's these are not energy conserving in
general, in particular if their weight is 1.0 and size > 2/3 (or 60°) they will
add more energy in each bounce.
Diffuse: http://www.pasteall.org/pic/show.php?id=42119
Specular: http://www.pasteall.org/pic/show.php?id=42120