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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_constraint.c
+4 -1
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@@ -743,7 +743,10 @@ void mj_instantiateEquality(const mjModel* m, mjData* d) {
mjtNum* refpos_c = mjSTACKALLOC(d, 3*npc, mjtNum);
for (int n = 0; n < npc; n++) {
int gn = gindices[n];
if (m->flex_centered[f]) {
if (m->flex_centered[f] ||
(m->flex_node[3*(gn + nstart)+0] == 0 &&
m->flex_node[3*(gn + nstart)+1] == 0 &&
m->flex_node[3*(gn + nstart)+2] == 0)) {
mju_copy3(xpos_c + 3*n, d->xpos + 3*bodyid[gn]);
} else {
mju_mulMatVec3(xpos_c + 3*n, d->xmat + 9*bodyid[gn], m->flex_node + 3*(gn + nstart));
src/engine/engine_core_smooth.c
+14 -16
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@@ -564,16 +564,13 @@ void mj_flex(const mjModel* m, mjData* d) {
// 0: vertices are the mesh vertices, 1: vertices are interpolated from nodal dofs
if (m->flex_interp[f] == 0) {
// centered: copy body position
if (m->flex_centered[f]) {
for (int i=vstart; i < vend; i++) {
for (int i=vstart; i < vend; i++) {
if (m->flex_centered[f] ||
(m->flex_vert[3*i+0] == 0 &&
m->flex_vert[3*i+1] == 0 &&
m->flex_vert[3*i+2] == 0)) {
mji_copy3(d->flexvert_xpos+3*i, d->xpos+3*m->flex_vertbodyid[i]);
}
}
// non-centered: map from local to global
else {
for (int i=vstart; i < vend; i++) {
} else {
mji_mulMatVec3(d->flexvert_xpos+3*i, d->xmat+9*m->flex_vertbodyid[i], m->flex_vert+3*i);
mji_addTo3(d->flexvert_xpos+3*i, d->xpos+3*m->flex_vertbodyid[i]);
}
@@ -585,13 +582,14 @@ void mj_flex(const mjModel* m, mjData* d) {
int nodenum = nend - nstart;
mj_markStack(d);
mjtNum* nodexpos = mjSTACKALLOC(d, 3*nodenum, mjtNum);
if (m->flex_centered[f]) {
for (int i=nstart; i < nend; i++) {
mji_copy3(nodexpos + 3*(i-nstart), d->xpos + 3*m->flex_nodebodyid[i]);
}
} else {
for (int i=nstart; i < nend; i++) {
int j = i - nstart;
for (int i=nstart; i < nend; i++) {
int j = i - nstart;
if (m->flex_centered[f] ||
(m->flex_node[3*i+0] == 0 &&
m->flex_node[3*i+1] == 0 &&
m->flex_node[3*i+2] == 0)) {
mji_copy3(nodexpos + 3*j, d->xpos + 3*m->flex_nodebodyid[i]);
} else {
mji_mulMatVec3(nodexpos + 3*j, d->xmat + 9*m->flex_nodebodyid[i], m->flex_node + 3*i);
mji_addTo3(nodexpos + 3*j, d->xpos + 3*m->flex_nodebodyid[i]);
}
src/engine/engine_core_util.c
+25 -17
View File
@@ -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);
}
}
}
src/engine/engine_core_util.h
+1 -1
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@@ -130,7 +130,7 @@ MJAPI void mj_local2Global(mjData* d, mjtNum xpos[3], mjtNum xmat[9],
//-------------------------- miscellaneous ---------------------------------------------------------
// gather global node positions and velocities
MJAPI void mju_flexGatherState(const mjModel* m, mjData* d, int f, mjtNum* xpos, mjtNum* vel);
MJAPI void mju_flexGatherState(const mjModel* m, const mjData* d, int f, mjtNum* xpos, mjtNum* vel);
// extract 6D force:torque for one contact, in contact frame
MJAPI void mj_contactForce(const mjModel* m, const mjData* d, int id, mjtNum result[6]);
src/engine/engine_derivative.c
+62 -36
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@@ -880,17 +880,62 @@ static void mjd_flexInterp_kernel(const mjModel* m, mjData* d, mjtFlexOp op,
const int* dof_indices, int ndof, int nband) {
int nv = m->nv;
// build global2local map for ADDH
int* global2local = NULL;
// compute upper bounds across all interpolated flexes
int max_nodenum = 0;
int max_npc = 0;
for (int f = 0; f < m->nflex; f++) {
if (!m->flex_interp[f]) continue;
if (m->flex_rigid[f]) continue;
int order = m->flex_interp[f];
int npc = (order+1)*(order+1)*(order+1);
if (npc > max_npc) max_npc = npc;
if (m->flex_nodenum[f] > max_nodenum) max_nodenum = m->flex_nodenum[f];
}
// nothing to do
if (max_npc == 0) {
return;
}
int max_dim_c = 3 * max_npc;
// single unconditional markStack
mj_markStack(d);
// global2local map for ADDH
int* global2local = mjSTACKALLOC(d, nv, int);
if (op == mjFLEXOP_ADDH) {
mj_markStack(d);
global2local = mjSTACKALLOC(d, nv, int);
mju_fillInt(global2local, -1, nv);
for (int i=0; i<ndof; i++) {
global2local[dof_indices[i]] = i;
}
}
// per-flex node positions (upper bound)
mjtNum* xpos = mjSTACKALLOC(d, 3*max_nodenum, mjtNum);
// per-cell arrays (upper bound)
mjtNum* xpos_c = mjSTACKALLOC(d, 3*max_npc, mjtNum);
mjtNum* K_rot_cell = mjSTACKALLOC(d, max_dim_c*max_dim_c, mjtNum);
// sparse Jacobian for one cell (upper bound)
int* J_rownnz = mjSTACKALLOC(d, max_dim_c, int);
int* J_rowadr = mjSTACKALLOC(d, max_dim_c, int);
mjtNum* J_val = mjSTACKALLOC(d, max_dim_c*nv, mjtNum);
int* J_colind = mjSTACKALLOC(d, max_dim_c*nv, int);
// temp allocations for chain
int* chain_colind = mjSTACKALLOC(d, nv, int);
mjtNum* blk_jac = mjSTACKALLOC(d, 3*nv, mjtNum);
// ADDH-specific allocations (upper bound)
mjtNum* J_reduced = NULL;
mjtNum* KJ = NULL;
if (op == mjFLEXOP_ADDH) {
J_reduced = mjSTACKALLOC(d, max_dim_c*ndof, mjtNum);
KJ = mjSTACKALLOC(d, max_dim_c*ndof, mjtNum);
}
// loop over flexes
for (int f=0; f < m->nflex; f++) {
// only process flex_interp
@@ -899,10 +944,10 @@ static void mjd_flexInterp_kernel(const mjModel* m, mjData* d, mjtFlexOp op,
}
// get stiffness and damping
mjtNum* k = m->flex_stiffness + m->flex_stiffnessadr[f];
mjtNum* K = m->flex_stiffness + m->flex_stiffnessadr[f];
// skip if rigid or no stiffness
if (m->flex_rigid[f] || k[0] == 0) {
if (m->flex_rigid[f] || K[0] == 0) {
continue;
}
@@ -926,27 +971,9 @@ static void mjd_flexInterp_kernel(const mjModel* m, mjData* d, mjtFlexOp op,
int cy = m->flex_cellnum[3*f+1];
int cz = m->flex_cellnum[3*f+2];
int nodenum = m->flex_nodenum[f];
int* bodyid = m->flex_nodebodyid + m->flex_nodeadr[f];
// standard stack allocation
mj_markStack(d);
mjtNum* xpos = mjSTACKALLOC(d, 3*nodenum, mjtNum);
// per-cell arrays
int dim_c = 3 * npc;
mjtNum* xpos_c = mjSTACKALLOC(d, 3*npc, mjtNum);
mjtNum* K_rot_cell = mjSTACKALLOC(d, dim_c*dim_c, mjtNum);
// sparse Jacobian for one cell
int* J_rownnz = mjSTACKALLOC(d, dim_c, int);
int* J_rowadr = mjSTACKALLOC(d, dim_c, int);
mjtNum* J_val = mjSTACKALLOC(d, dim_c*nv, mjtNum);
int* J_colind = mjSTACKALLOC(d, dim_c*nv, int);
// temp allocations for chain
int* chain_colind = mjSTACKALLOC(d, nv, int);
mjtNum* blk_jac = mjSTACKALLOC(d, 3*nv, mjtNum);
// gather raw node positions (unrotated)
mju_flexGatherState(m, d, f, xpos, NULL);
@@ -956,6 +983,16 @@ static void mjd_flexInterp_kernel(const mjModel* m, mjData* d, mjtFlexOp op,
for (int ci = 0; ci < cx; ci++) {
for (int cj = 0; cj < cy; cj++) {
for (int ck = 0; ck < cz; ck++) {
// get cell stiffness
mjtNum* k_cell = K + cell_idx * 3*npc * 3*npc;
// skip empty cells: stiffness buffer is zero-initialized at compile time
// (user_model.cc), and non-empty cells have strictly positive diagonal
if (k_cell[0] == 0) {
cell_idx++;
continue;
}
// gather cell-local node positions
int gindices[125]; // max npc = 125 for quadratic
mjtNum quat[4];
@@ -967,9 +1004,6 @@ static void mjd_flexInterp_kernel(const mjModel* m, mjData* d, mjtFlexOp op,
mju_quat2Mat(R, quat);
mju_transpose(RT, R, 3, 3);
// get cell stiffness
mjtNum* k_cell = k + cell_idx * 3*npc * 3*npc;
// compute K_rot_cell = RT * K_cell * R (block-wise)
mju_zero(K_rot_cell, dim_c*dim_c);
for (int a = 0; a < npc; a++) {
@@ -1025,9 +1059,7 @@ static void mjd_flexInterp_kernel(const mjModel* m, mjData* d, mjtFlexOp op,
addJTBJ_mulSparse(m, d, res, vec, J_rownnz, J_rowadr, J_colind,
J_val, K_rot_cell, dim_c);
} else if (op == mjFLEXOP_ADDH) {
mj_markStack(d);
// H -= J_cell^T * K_rot_cell * J_cell (banded format)
mjtNum* J_reduced = mjSTACKALLOC(d, dim_c*ndof, mjtNum);
mju_zero(J_reduced, dim_c*ndof);
for (int i = 0; i < dim_c; i++) {
@@ -1043,7 +1075,6 @@ static void mjd_flexInterp_kernel(const mjModel* m, mjData* d, mjtFlexOp op,
}
// KJ = K_rot_cell * J_reduced (dim_c x ndof)
mjtNum* KJ = mjSTACKALLOC(d, dim_c*ndof, mjtNum);
mju_mulMatMat(KJ, K_rot_cell, J_reduced, dim_c, dim_c, ndof);
// H[i,j] -= J_reduced[k,i] * KJ[k,j], store lower triangle in banded format
@@ -1056,20 +1087,15 @@ static void mjd_flexInterp_kernel(const mjModel* m, mjData* d, mjtFlexOp op,
res[i*nband + nband-1-(i-j)] -= val;
}
}
mj_freeStack(d);
}
cell_idx++;
}
}
}
mj_freeStack(d);
}
if (op == mjFLEXOP_ADDH) {
mj_freeStack(d); // free global2local
}
mj_freeStack(d);
}
src/engine/engine_passive.c
+22 -8
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@@ -273,6 +273,15 @@ static void mj_springdamper(const mjModel* m, mjData* d) {
for (int ci = 0; ci < cx; ci++) {
for (int cj = 0; cj < cy; cj++) {
for (int ck = 0; ck < cz; ck++) {
// get cell stiffness matrix
mjtNum* k_cell = k + cell_idx * 3*npc * 3*npc;
// skip empty cells (zero stiffness)
if (k_cell[0] == 0) {
cell_idx++;
continue;
}
// gather cell-local node data
mjtNum quat[4];
mju_flexGatherCellState(order, cy, cz, ci, cj, ck, xpos_g, vel_g, xpos0,
@@ -289,9 +298,6 @@ static void mj_springdamper(const mjModel* m, mjData* d) {
mji_addScl3(displ_c+3*n, xpos_c+3*n, xpos0_c+3*n, -1);
}
// get cell stiffness matrix
mjtNum* k_cell = k + cell_idx * 3*npc * 3*npc;
// compute force in corotational frame
if (enbl_spring) {
mju_mulMatVec(frc_c, k_cell, displ_c, 3*npc, 3*npc);
@@ -332,12 +338,20 @@ static void mj_springdamper(const mjModel* m, mjData* d) {
// apply accumulated forces to bodies
for (int i = 0; i < nodenum; i++) {
mju_scl3(dmp_g+3*i, dmp_g+3*i, m->flex_damping[f]);
if (m->flex_centered[f]) {
if (enbl_spring) mji_addTo3(d->qfrc_spring + m->body_dofadr[bodyid[i]], frc_g+3*i);
if (enbl_damper) mji_addTo3(d->qfrc_damper + m->body_dofadr[bodyid[i]], dmp_g+3*i);
int bid = bodyid[i];
int nidx = i + m->flex_nodeadr[f];
// fast path: node at body origin (not pinned), direct DOF write
if (m->body_dofnum[bid] > 0 &&
(m->flex_centered[f] ||
(m->flex_node[3*nidx+0] == 0 &&
m->flex_node[3*nidx+1] == 0 &&
m->flex_node[3*nidx+2] == 0))) {
if (enbl_spring) mji_addTo3(d->qfrc_spring + m->body_dofadr[bid], frc_g+3*i);
if (enbl_damper) mji_addTo3(d->qfrc_damper + m->body_dofadr[bid], dmp_g+3*i);
} else {
if (enbl_spring) mj_applyFT(m, d, frc_g+3*i, 0, xpos_g+3*i, bodyid[i], d->qfrc_spring);
if (enbl_damper) mj_applyFT(m, d, dmp_g+3*i, 0, xpos_g+3*i, bodyid[i], d->qfrc_damper);
if (enbl_spring) mj_applyFT(m, d, frc_g+3*i, 0, xpos_g+3*i, bid, d->qfrc_spring);
if (enbl_damper) mj_applyFT(m, d, dmp_g+3*i, 0, xpos_g+3*i, bid, d->qfrc_damper);
}
}
src/engine/engine_vis_visualize.c
+11 -4
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@@ -1459,11 +1459,18 @@ static void addFlexBvhGeoms(const mjModel* m, mjData* d, const mjvOption* vopt,
for (int i=0; i < NX; i++) {
for (int j=0; j < NY; j++) {
for (int k=0; k < NZ; k++) {
int offset = 3*(i*NY*NZ + j*NZ + k);
int n0 = i*NY*NZ + j*NZ + k;
// skip if this node is pinned (no joints on its body)
if (m->body_jntnum[bodyid[n0]] == 0) {
continue;
}
int offset = 3*n0;
int offset1 = 3*((i+1)*NY*NZ + j*NZ + k);
int offset2 = 3*(i*NY*NZ + (j+1)*NZ + k);
int offset3 = 3*(i*NY*NZ + j*NZ + (k+1));
if (i < NX-1) {
if (i < NX-1 && m->body_jntnum[bodyid[(i+1)*NY*NZ + j*NZ + k]] > 0) {
mjvGeom* thisgeom = acquireGeom(scn, i, mjCAT_DECOR, mjOBJ_UNKNOWN);
if (!thisgeom) {
return;
@@ -1472,7 +1479,7 @@ static void addFlexBvhGeoms(const mjModel* m, mjData* d, const mjvOption* vopt,
mjv_connector(thisgeom, mjGEOM_LINE, 3, xpos+offset, xpos+offset1);
releaseGeom(&thisgeom, scn);
}
if (j < NY-1) {
if (j < NY-1 && m->body_jntnum[bodyid[i*NY*NZ + (j+1)*NZ + k]] > 0) {
mjvGeom* thisgeom = acquireGeom(scn, i, mjCAT_DECOR, mjOBJ_UNKNOWN);
if (!thisgeom) {
return;
@@ -1481,7 +1488,7 @@ static void addFlexBvhGeoms(const mjModel* m, mjData* d, const mjvOption* vopt,
mjv_connector(thisgeom, mjGEOM_LINE, 3, xpos+offset, xpos+offset2);
releaseGeom(&thisgeom, scn);
}
if (k < NZ-1) {
if (k < NZ-1 && m->body_jntnum[bodyid[i*NY*NZ + j*NZ + (k+1)]] > 0) {
mjvGeom* thisgeom = acquireGeom(scn, i, mjCAT_DECOR, mjOBJ_UNKNOWN);
if (!thisgeom) {
return;