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Optimize flex by pinning nodes in empty cells.

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This change introduces an optimization for flexcomp objects defined by a mesh. It identifies grid cells that do not contain any mesh vertices and marks them as empty. Nodes that are exclusively part of empty cells are pinned, preventing them from moving. Stiffness computations are skipped for empty cells, reducing computational cost. The total mass is now distributed only among the non-pinned nodes.

PiperOrigin-RevId: 902565735
Change-Id: Id0a9a685536d5e18a3e42124a25ab08ff3a918f2
This commit is contained in:
Alessio Quaglino
2026-04-20 04:40:34 -07:00
committed by Copybara-Service
parent fa7b36d111
commit 508e581ba9
12 changed files with 455 additions and 90 deletions
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src/engine/engine_core_util.c
+25 -17
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@@ -988,28 +988,36 @@ void mj_local2Global(mjData* d, mjtNum xpos[3], mjtNum xmat[9],
//-------------------------- miscellaneous utilities -----------------------------------------------
// gather global node positions and velocities
void mju_flexGatherState(const mjModel* m, mjData* d, int f, mjtNum* xpos, mjtNum* vel) {
void mju_flexGatherState(const mjModel* m, const mjData* d, int f, mjtNum* xpos, mjtNum* vel) {
int nodenum = m->flex_nodenum[f];
int nstart = m->flex_nodeadr[f];
int* bodyid = m->flex_nodebodyid + m->flex_nodeadr[f];
// compute positions
if (m->flex_centered[f]) {
for (int i=0; i < nodenum; i++) {
mju_copy3(xpos + 3*i, d->xpos + 3*bodyid[i]);
if (vel) {
mju_copy3(vel + 3*i, d->qvel + m->body_dofadr[bodyid[i]]);
}
// compute positions and velocities
for (int i=0; i < nodenum; i++) {
int bid = bodyid[i];
if (m->flex_centered[f] ||
(m->flex_node[3*(i+nstart)+0] == 0 &&
m->flex_node[3*(i+nstart)+1] == 0 &&
m->flex_node[3*(i+nstart)+2] == 0)) {
mju_copy3(xpos + 3*i, d->xpos + 3*bid);
} else {
mju_mulMatVec3(xpos + 3*i, d->xmat + 9*bid, m->flex_node + 3*(i+nstart));
mju_addTo3(xpos + 3*i, d->xpos + 3*bid);
}
} else {
mjtNum screw[6];
for (int i=0; i < nodenum; i++) {
mju_mulMatVec3(xpos + 3*i, d->xmat + 9*bodyid[i], m->flex_node + 3*(i+nstart));
mju_addTo3(xpos + 3*i, d->xpos + 3*bodyid[i]);
if (vel) {
mj_objectVelocity(m, d, mjOBJ_BODY, bodyid[i], screw, 0);
mju_copy3(vel + 3*i, screw + 3);
}
if (vel) {
mjtNum body_vel[6];
mj_objectVelocity(m, d, mjOBJ_BODY, bid, body_vel, 0); // returns [omega, v_CoM] in world frame
// linear velocity at CoM
mju_copy3(vel + 3*i, body_vel + 3);
// add omega x (xpos - xipos)
mjtNum r[3], cross[3];
mju_sub3(r, xpos + 3*i, d->xipos + 3*bid);
mju_cross(cross, body_vel, r);
mju_addTo3(vel + 3*i, cross);
}
}
}