Tetrahedral Inversion

[nithinp7]

The FEM-based tetrahedral, strain-limiting constraint is at the core of our soft-body simulation. It has undergone many revisions and is quite stable and robust now. Even then, there are still cases where tetrahedrons invert under large forces or timesteps. After some recent improvements, the tetrahedral strain constraint itself attempts to “pop” the tetrahedron back into its original uninverted state - however, there are often situations under extreme deformation where the surface collisions can cause an inverted tetrahedron to explode. Special handling should be given to these cases, to either disable collision on triangles from inverted tetrahedrons, adaptively timestep to avoid such states, or to provide some sort of hard-constraint that can fix inverted tetrahedrons after each PD timestep (a similar thing is currently done to stabilize collisions).