Add boolean flags for gravity compensation and surface velocity in mjModel.

These fields (`flg_gravcomp` and `flg_surfacevel`) replace the fast-path checks originally guarded by `ngravcomp` and (recently) `nsurfacevel`. Since the engine uses these integers only as flags (zero vs non-zero), migrating them to actual booleans makes them writeable from the Python bindings at runtime without violating size/dimension constraints.

The legacy integer field `ngravcomp` is marked as deprecated and will be removed in a future release.

PiperOrigin-RevId: 949779204
Change-Id: Ifab1f026063a4239302e6ad689663b611b59dda8

src/engine/engine_core_constraint.c

@@ -3130,7 +3130,7 @@ void mj_projectConstraint(const mjModel* m, mjData* d) {
 // add relative surface velocity of contacting geoms to contact rows of efc_vel
 static void mj_addSurfaceVel(const mjModel* m, mjData* d) {
   // no surface velocity on any geom: quick return
-  if (!m->nsurfacevel) {
+  if (!m->flg_surfacevel) {
     return;
   }
 

src/engine/engine_forward.c

@@ -794,7 +794,7 @@ void mj_fwdActuation(const mjModel* m, mjData* d) {
                        d->moment_rownnz, d->moment_rowadr, d->moment_colind);
 
   // actuator-level gravity compensation
-  if (m->ngravcomp && !mjDISABLED(mjDSBL_GRAVITY) && mju_norm3(m->opt.gravity)) {
+  if (m->flg_gravcomp && !mjDISABLED(mjDSBL_GRAVITY) && mju_norm3(m->opt.gravity)) {
     // number of dofs for each joint type: {mjJNT_FREE, mjJNT_BALL, mjJNT_SLIDE, mjJNT_HINGE}
     static const int jnt_dofnum[4] = {6, 3, 1, 1};
     int njnt = m->njnt;

src/engine/engine_io.c

@@ -250,7 +250,7 @@ void mj_makeModel(mjModel** dest,
   // CHECK SIZE PARAMETERS
   {
     // dummy variables for MJMODEL_SIZES set after mjModel construction
-    int nnames_map = 0, nJmom = 0, ngravcomp = 0, nsurfacevel = 0, nemax = 0, njmax = 0, nconmax=0;
+    int nnames_map = 0, nJmom = 0, ngravcomp = 0, nemax = 0, njmax = 0, nconmax=0;
     int npolygonmax = 0, nmeshdegmax = 0;
     int nuserdata=0, nsensordata=0, npluginstate=0, nhistory=0, narena=0, nbuffer=0;
 
@@ -270,7 +270,7 @@ void mj_makeModel(mjModel** dest,
 #undef X
 
     // suppress unused variable warnings
-    (void)nnames_map; (void)nJmom; (void)ngravcomp; (void)nsurfacevel; (void)nemax; (void)njmax; (void)nconmax;
+    (void)nnames_map; (void)nJmom; (void)ngravcomp; (void)nemax; (void)njmax; (void)nconmax;
     (void)npolygonmax; (void)nmeshdegmax;
     (void)nuserdata; (void)nsensordata; (void)npluginstate; (void)nhistory; (void)narena;
     (void)nbuffer;
@@ -543,6 +543,8 @@ void mj_saveModel(const mjModel* m, const char* filename, void* buffer, int buff
   bufwrite((void*)&m->opt, sizeof(mjOption), buffer_sz, buffer, &ptrbuf);
   bufwrite((void*)&m->vis, sizeof(mjVisual), buffer_sz, buffer, &ptrbuf);
   bufwrite((void*)&m->stat, sizeof(mjStatistic), buffer_sz, buffer, &ptrbuf);
+  bufwrite(&m->flg_gravcomp, sizeof(mjtBool), buffer_sz, buffer, &ptrbuf);
+  bufwrite(&m->flg_surfacevel, sizeof(mjtBool), buffer_sz, buffer, &ptrbuf);
   {
     MJMODEL_POINTERS_PREAMBLE(m)
     #define X(type, name, nr, nc)  \
@@ -644,13 +646,16 @@ mjModel* mj_loadModelBuffer(const void* buffer, int buffer_sz) {
   }
 
   // read options and buffer
-  if (ptrbuf + sizeof(mjOption) + sizeof(mjVisual) + sizeof(mjStatistic) > buffer_sz) {
+  if (ptrbuf + sizeof(mjOption) + sizeof(mjVisual) + sizeof(mjStatistic) +
+      sizeof(mjtBool) * 2 > buffer_sz) {
     mju_warning("Truncated model file - ran out of data while reading structs");
     return NULL;
   }
   bufread((void*)&m->opt, sizeof(mjOption), buffer_sz, buffer, &ptrbuf);
   bufread((void*)&m->vis, sizeof(mjVisual), buffer_sz, buffer, &ptrbuf);
   bufread((void*)&m->stat, sizeof(mjStatistic), buffer_sz, buffer, &ptrbuf);
+  bufread(&m->flg_gravcomp, sizeof(mjtBool), buffer_sz, buffer, &ptrbuf);
+  bufread(&m->flg_surfacevel, sizeof(mjtBool), buffer_sz, buffer, &ptrbuf);
   {
     MJMODEL_POINTERS_PREAMBLE(m)
     #define X(type, name, nr, nc)                                           \
@@ -700,7 +705,8 @@ mjtSize mj_sizeModel(const mjModel* m) {
     + sizeof(mjtSize)*getnsize()
     + sizeof(mjOption)
     + sizeof(mjVisual)
-    + sizeof(mjStatistic));
+    + sizeof(mjStatistic)
+    + sizeof(mjtBool)*2);
 
   MJMODEL_POINTERS_PREAMBLE(m)
 #define X(type, name, nr, nc)         \

src/engine/engine_passive.c

@@ -821,7 +821,7 @@ static void mj_springdamper(const mjModel* m, mjData* d) {
 
 // body-level gravity compensation, return 1 if any, 0 otherwise
 static int mj_gravcomp(const mjModel* m, mjData* d) {
-  if (!m->ngravcomp || mjDISABLED(mjDSBL_GRAVITY) || mju_norm3(m->opt.gravity) == 0) {
+  if (!m->flg_gravcomp || mjDISABLED(mjDSBL_GRAVITY) || mju_norm3(m->opt.gravity) == 0) {
     return 0;
   }
 

src/engine/engine_print.c

@@ -569,6 +569,12 @@ void mj_printFormattedModel(const mjModel* m, const char* filename, const char*
 #undef X
   fprintf(fp, "\n");
 
+  // flags
+  fprintf(fp, "FLAG\n");
+  printInt(fp, "  flg_gravcomp", m->flg_gravcomp);
+  printInt(fp, "  flg_surfacevel", m->flg_surfacevel);
+  fprintf(fp, "\n");
+
   // options
   fprintf(fp, "OPTION\n");
 #define X(type, name, sz)                                             \

src/engine/engine_setconst.c

@@ -210,16 +210,18 @@ static void setFixed(mjModel* m, mjData* d) {
     ngravcomp += (m->body_gravcomp[i] > 0);
   }
   m->ngravcomp = ngravcomp;
+  m->flg_gravcomp = (ngravcomp > 0);
 
-  // compute nsurfacevel: number of geoms with nonzero surfacevel
-  int nsurfacevel = 0;
+  // compute flg_surfacevel: whether any geom has nonzero surfacevel
+  mjtBool flg_surfacevel = 0;
   for (int i=0; i < m->ngeom; i++) {
     const mjtNum* sv = m->geom_surfacevel + 6*i;
     if (sv[0] || sv[1] || sv[2] || sv[3] || sv[4] || sv[5]) {
-      nsurfacevel++;
+      flg_surfacevel = 1;
+      break;
     }
   }
-  m->nsurfacevel = nsurfacevel;
+  m->flg_surfacevel = flg_surfacevel;
 
   // set jnt_actuatorid and tendon_actuatorid
   mju_fillInt(m->jnt_actuatorid, -1, m->njnt);