Add passive collision mode for flexes.

PiperOrigin-RevId: 801754907
Change-Id: I4147af2ee2596519d42cb9a03cf8d5488f0bc3e8

doc/includes/references.h

@@ -123,7 +123,7 @@ struct mjContact_ {                // result of collision detection functions
   int     vert[2];                 // vertex ids;  -1 for geom or flex element
 
   // flag set by mj_setContact or mj_instantiateContact
-  int     exclude;                 // 0: include, 1: in gap, 2: fused, 3: no dofs
+  int     exclude;                 // 0: include, 1: in gap, 2: fused, 3: no dofs, 4: passive
 
   // address computed by mj_instantiateContact
   int     efc_address;             // address in efc; -1: not included
@@ -1220,6 +1220,7 @@ struct mjModel_ {
   mjtByte*  flex_internal;        // internal flex collision enabled          (nflex x 1)
   int*      flex_selfcollide;     // self collision mode (mjtFlexSelf)        (nflex x 1)
   int*      flex_activelayers;    // number of active element layers, 3D only (nflex x 1)
+  int*      flex_passive;         // passive collisions enabled               (nflex x 1)
 
   // flexes: other properties
   int*      flex_dim;             // 1: lines, 2: triangles, 3: tetrahedra    (nflex x 1)
@@ -2104,6 +2105,7 @@ typedef struct mjsFlex_ {          // flex specification
   mjtByte flatskin;                // render flex skin with flat shading
   int selfcollide;                 // mode for flex self collision
   int vertcollide;                 // mode for vertex collision
+  int passive;                     // mode for passive collisions
   int activelayers;                // number of active element layers in 3D
   int group;                       // group for visualizatioh
   double edgestiffness;            // edge stiffness