This change adds a new passive force computation for flexes with elastic2d="bend" and dof="trilinear". The bending energy is based on the squared difference of normals between adjacent face elements at their shared edge midpoint. The edge data is precomputed during model compilation and stored in flex_bending.
PiperOrigin-RevId: 910772638
Change-Id: I3b12c7b7f1ba6ac1875df495d89e8cfec921ca80
Modify EigendecomposeStiffness to project out rigid body translations and rotations from the stiffness matrix eigenvectors. This prevents "ghost damping" from rigid body modes being incorrectly constrained. Update hollow_vs_solid.xml with solimp parameters for edge constraints. Adjust engine_core_constraint_test.cc to reflect the reduced number of equality constraints due to the projection.
PiperOrigin-RevId: 910631207
Change-Id: Ibaaa59bc030cfc6ae657d8f0d1b2a51002cc8d4e
When elastic2d="stretch" is set on an interpolated flexcomp, treat the bounding box boundary as membrane elements rather than volumetric cells. This computes plane-stress stiffness over the boundary faces and updates the runtime force/derivative kernels accordingly.
Interior vertex tracking (moving vertices that follow the deforming shell) is not yet implemented so all mesh vertices need to be on the bounding box surface or the background grid should have no interior nodes (i.e. cellcount should be 1 on at least one axis).
PiperOrigin-RevId: 907654080
Change-Id: I51b90e2f6a1d1b036f9604e42de20e377dc5d3f9
Alessio Quaglino