Clean-up flex assumptions.

Do not allow a mix of `elastic2d != none` with `dof = trilinear` since the latter assumes 3d elasticity.

Also, do not assume that `flex_interp > 0` in the engine. This will enable to use, e.g., `flex_interp = -1` to mean a linear surface finite element instead of a 3d finite element which is currently identified with `flex_interp = 1`.

PiperOrigin-RevId: 903852035
Change-Id: Ia6290b4a05e9e510ffb7f36d141cd525b40d3110

src/engine/engine_core_constraint.c

@@ -275,6 +275,7 @@ static int mj_vertBodyWeight(const mjModel* m, const mjData* d, int f, int* v,
   }
 
   int order = m->flex_interp[f];
+  order = order < 0 ? -order : order;
   int npc = (order+1)*(order+1)*(order+1);  // number of nodes per cell
 
   // cell lookup: get local coords and node indices
@@ -709,6 +710,7 @@ void mj_instantiateEquality(const mjModel* m, mjData* d) {
       int f = id[0];
       int nodenum = m->flex_nodenum[f];
       int order = m->flex_interp[f];
+      order = order < 0 ? -order : order;
 
       // skip if not interpolated (order == 0 or no nodes)
       if (!order || !nodenum) {
@@ -1676,6 +1678,7 @@ void mj_diagApprox(const mjModel* m, mjData* d) {
         int flex_id = m->eq_obj1id[id];
         int nstart = m->flex_nodeadr[flex_id];
         int order = m->flex_interp[flex_id];
+        order = order < 0 ? -order : order;
         int npc = (order+1)*(order+1)*(order+1);
 
         // per-cell constraint count
@@ -2296,6 +2299,7 @@ static int mj_ne(const mjModel* m, mjData* d, int* nnz) {
       // per-cell strain constraints: each equality is one cell
       int f = id[0];
       int order = m->flex_interp[f];
+      order = order < 0 ? -order : order;
       if (!order || !m->flex_nodenum[f]) {
         break;
       }

src/engine/engine_core_smooth.c

@@ -595,7 +595,8 @@ void mj_flex(const mjModel* m, mjData* d) {
         }
       }
 
-      int order = m->flex_interp[f];
+      int interp = m->flex_interp[f];
+      int order = interp < 0 ? -interp : interp;
       int cx = m->flex_cellnum[3*f+0];
       int cy = m->flex_cellnum[3*f+1];
       int cz = m->flex_cellnum[3*f+2];
@@ -2624,7 +2625,8 @@ void mj_rnePostConstraint(const mjModel* m, mjData* d) {
     case mjEQ_FLEXSTRAIN: {
       // increment: trilinear uses 2 center (I1,J-1) + 3*ngauss shear, quadratic uses 6*ngauss
       k = m->eq_obj1id[id];
-      int order = m->flex_interp[k];
+      int interp_k = m->flex_interp[k];
+      int order = interp_k < 0 ? -interp_k : interp_k;
       int nodenum = m->flex_nodenum[k];
       if (order && nodenum) {
         int nquad = order + 1;

src/engine/engine_derivative.c

@@ -887,6 +887,7 @@ static void mjd_flexInterp_kernel(const mjModel* m, mjData* d, mjtFlexOp op,
     if (!m->flex_interp[f]) continue;
     if (m->flex_rigid[f]) continue;
     int order = m->flex_interp[f];
+    order = order < 0 ? -order : order;
     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];
@@ -966,6 +967,7 @@ static void mjd_flexInterp_kernel(const mjModel* m, mjData* d, mjtFlexOp op,
     }
 
     int order = m->flex_interp[f];
+    order = order < 0 ? -order : order;
     int npc = (order+1)*(order+1)*(order+1);
     int cx = m->flex_cellnum[3*f+0];
     int cy = m->flex_cellnum[3*f+1];

src/engine/engine_passive.c

@@ -236,6 +236,7 @@ static void mj_springdamper(const mjModel* m, mjData* d) {
 
     if (m->flex_interp[f]) {
       int order = m->flex_interp[f];
+      order = order < 0 ? -order : order;
       int npc = (order+1)*(order+1)*(order+1);  // nodes per cell
       int cx = m->flex_cellnum[3*f+0];
       int cy = m->flex_cellnum[3*f+1];

src/engine/engine_setconst.c

@@ -714,6 +714,7 @@ static void makeFlexBandwidth(mjModel* m, mjData* d) {
     for (int f = 0; f < m->nflex; f++) {
       if (!m->flex_interp[f]) continue;
       int order = m->flex_interp[f];
+      order = order < 0 ? -order : order;
       int nodeadr = m->flex_nodeadr[f];
       int nodenum = m->flex_nodenum[f];
       int cx = m->flex_cellnum[3*f+0];

src/engine/engine_vis_interact.c

@@ -864,6 +864,7 @@ int mjv_select(const mjModel* m, const mjData* d, const mjvOption* vopt,
         if (m->flex_interp[i]) {
           mjtNum* coord = m->flex_vert0 + 3*(m->flex_vertadr[i] + vertid);
           int order = m->flex_interp[i];
+          order = order < 0 ? -order : order;
           int npc = (order+1)*(order+1)*(order+1);
 
           // cell lookup: get local coords and node indices

src/engine/engine_vis_visualize.c

@@ -1452,6 +1452,7 @@ static void addFlexBvhGeoms(const mjModel* m, mjData* d, const mjvOption* vopt,
     int cy = m->flex_cellnum[3*f+1];
     int cz = m->flex_cellnum[3*f+2];
     int order = m->flex_interp[f];
+    order = order < 0 ? -order : order;
     int NX = cx * order + 1;
     int NY = cy * order + 1;
     int NZ = cz * order + 1;