Smoothly projecting curves or meshes onto the limit surface of other meshes.
Projecting curves or meshes directly onto other meshes results in faceted looking surfaces (left image). However, when projecting to the the limit surface using OpenSubDiv (OSD), we'll receive a smooth alignment to its Catmull-Clark subdivision surface (right image).
The main functions for accessing the limit surface are osd_lookuppatch() and osd_limitsurface(). To get started, we could either use the ray node, the intersect()-, xyzdist() or – like in this case –the uvdist()-function, which all return the closest primitive number and the UV position on that particular polygon face.
Next we feed in a primitive number and the UV position on the primitive into osd_lookuppatch() to receive a patch number and the U and V coordinates on the patch. Both, the patch number and the UVs, are then fed to the osd_limitsurface() which returns the smooth world positions that are located right on the subdivision limit surface.
int prim_hit;
vector uvw;
uvdist(1, 'uv', v@uv, prim_hit, uvw);
int patch;
float patch_u;
float patch_v;
osd_lookuppatch(1, prim_hit, uvw[0], uvw[1], patch, patch_u, patch_v);
osd_limitsurface(1, 'P', patch, patch_u, patch_v, v@P);
In depth technical explanations and more production-ready examples on this subject are available by Yunus 'animatrix' Balciouglu on the SideFX website.
