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Implement multi-cell finite element method for interpolated flexes.

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This change introduces a `flex_cellcount` field to `mjModel` to specify the number of cells in each dimension for interpolated flexes. The stiffness computation, passive force calculation, and Jacobian derivatives are updated to operate on a per-cell basis, significantly improving performance by localizing computations to the nodes within each cell.

PiperOrigin-RevId: 901216393
Change-Id: Ic23132e609de11e71bb7fef8d1f139daad2ec264
This commit is contained in:
Alessio Quaglino
2026-04-17 04:10:54 -07:00
committed by Copybara-Service
parent 8415dff307
commit 6c7ed66781
36 changed files with 1777 additions and 1039 deletions
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src/engine/engine_vis_interact.c
+16 -10
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@@ -863,24 +863,30 @@ int mjv_select(const mjModel* m, const mjData* d, const mjvOption* vopt,
flexdist = newdist;
if (m->flex_interp[i]) {
mjtNum* coord = m->flex_vert0 + 3*(m->flex_vertadr[i] + vertid);
int order = m->flex_interp[i];
int npc = (order+1)*(order+1)*(order+1);
// cell lookup: get local coords and node indices
mjtNum loc[3];
int nodeindices[27]; // max npc for quadratic: 3^3 = 27
mju_cellLookup(coord, m->flex_cellnum+3*i, order, loc, nodeindices);
// find node with largest weight in this cell
int nodeid = -1;
int nstart = m->flex_nodeadr[i];
int nend = nstart + m->flex_nodenum[i];
mjtNum w = 0;
for (int j = nstart; j < nend; j++) {
if (mju_evalBasis(coord, j-nstart, m->flex_interp[i]) > w) {
w = mju_evalBasis(coord, j-nstart, m->flex_interp[i]);
nodeid = j;
for (int j = 0; j < npc; j++) {
mjtNum ww = mju_evalBasis(loc, j, order);
if (ww > w) {
w = ww;
nodeid = nodeindices[j];
}
}
if (nodeid < 0) {
mjERROR("flex %d: node closest to vertex %d not found", i, vertid);
}
flexbodyid = m->flex_nodebodyid[m->flex_nodeadr[i] + nodeid];
flexbodyid = m->flex_nodebodyid[nstart + nodeid];
if (m->flex_centered[i]) {
mju_copy3(flexpnt, d->xpos + 3*flexbodyid);
} else {
mju_mulMatVec3(flexpnt, d->xmat + 9*flexbodyid, m->flex_node + 3*nodeid);
mju_mulMatVec3(flexpnt, d->xmat + 9*flexbodyid, m->flex_node + 3*(nstart + nodeid));
mju_addTo3(flexpnt, d->xpos + 3*flexbodyid);
}
} else {