General collision body

[Eydcao]

The ultimate goal for ti-MPM in Houdini should be compatible with varying geometries. The particle emission can be handled by Houdini built-in SOPs. However, a general sdf/volume based collision is needed.

Ti-element end:

• Support representing the SDF/volume as sparse grid (as it's free in Ti)
• Algorithm for the collision between a particle and a sparse volume; get relative velocity, treatment for different surface type; and, projection for in-body particles to the nearest surface.

Houdini end:

• Support reading volume/VDB information (position and velocity) and convert to the data structure for ti-element back end.
• UI designs: volume operator path; velocity (volume) attribute name(string)