Compute convex hull for collision pairs.

PiperOrigin-RevId: 788074030
Change-Id: I3d4bbe71f824db82e998e374d95a5c49995af806

doc/changelog.rst

@@ -30,6 +30,8 @@ Bug fixes
 - Fixed a bug that caused object lists in the child to have missing elements after attaching an mjSpec. This was caused
   by adding to the lists only the objects that belong to the tree of the requested body, but this causes to skip objects
   that were attached, since they belong to the tree of the parent.
+- Fixed a bug where the convex hull of a collision mesh was not being computed if the mesh could only collide via a
+  :ref:`contact pair<contact-pair>`.
 
 Version 3.3.4 (July 8, 2025)
 ----------------------------

src/user/user_model.cc

@@ -4481,11 +4481,25 @@ void mjCModel::TryCompile(mjModel*& m, mjData*& d, const mjVFS* vfs) {
   // map names to asset references
   IndexAssets(/*discard=*/false);
 
+  // compile pairs for convex hull check
+  // TODO(quaglino): Consolidate the two calls to pair->Compile() in TryCompile.
+  for (auto pair : pairs_) pair->Compile();
+
   // mark meshes that need convex hull
   for (int i=0; i < geoms_.size(); i++) {
+    bool is_in_pair = false;
+    for (const mjCPair* pair : pairs_) {
+      if ((pair->geom1 && pair->geom1->id == geoms_[i]->id) ||
+          (pair->geom2 && pair->geom2->id == geoms_[i]->id)) {
+        is_in_pair = true;
+        break;
+      }
+    }
+
     if (geoms_[i]->mesh &&
-        (geoms_[i]->spec.type == mjGEOM_MESH || geoms_[i]->spec.type == mjGEOM_SDF) &&
-        (geoms_[i]->spec.contype || geoms_[i]->spec.conaffinity ||
+        (geoms_[i]->spec.type == mjGEOM_MESH ||
+         geoms_[i]->spec.type == mjGEOM_SDF) &&
+        (geoms_[i]->spec.contype || geoms_[i]->spec.conaffinity || is_in_pair ||
          geoms_[i]->mesh->spec.inertia == mjMESH_INERTIA_CONVEX)) {
       geoms_[i]->mesh->SetNeedHull(true);
     }

test/user/user_model_test.cc

@@ -218,6 +218,80 @@ TEST_F(UserCModelTest, NestedZeroMassBodiesFail) {
   mj_deleteModel(model);
 }
 
+TEST_F(UserCModelTest, ConvexHullForCollisionMeshes) {
+  static constexpr char xml[] = R"(
+  <mujoco>
+    <asset>
+      <mesh name="mesh_no_hull" vertex="0 0 0  1 0 0  0 1 0  0 0 1"
+            face="0 2 1  0 1 3  0 3 2  1 2 3"/>
+      <mesh name="mesh_with_hull_contype" vertex="0 0 0  1 0 0  0 1 0  0 0 1"
+            face="0 2 1  0 1 3  0 3 2  1 2 3"/>
+      <mesh name="mesh_with_hull_conaffinity" vertex="0 0 0  1 0 0  0 1 0  0 0 1"
+            face="0 2 1  0 1 3  0 3 2  1 2 3"/>
+    </asset>
+    <worldbody>
+      <geom name="geom_no_hull" type="mesh" mesh="mesh_no_hull" contype="0" conaffinity="0"/>
+      <geom name="geom_with_hull_contype" type="mesh" mesh="mesh_with_hull_contype" contype="1"/>
+      <geom name="geom_with_hull_conaffinity" type="mesh" mesh="mesh_with_hull_conaffinity" conaffinity="1"/>
+    </worldbody>
+  </mujoco>)";
+
+  std::array<char, 1024> error;
+  mjModel* model = LoadModelFromString(xml, error.data(), error.size());
+  ASSERT_THAT(model, NotNull()) << error.data();
+
+  int no_hull_id = mj_name2id(model, mjOBJ_MESH, "mesh_no_hull");
+  int with_hull_contype_id =
+      mj_name2id(model, mjOBJ_MESH, "mesh_with_hull_contype");
+  int with_hull_conaffinity_id =
+      mj_name2id(model, mjOBJ_MESH, "mesh_with_hull_conaffinity");
+
+  EXPECT_NE(no_hull_id, -1);
+  EXPECT_NE(with_hull_contype_id, -1);
+  EXPECT_NE(with_hull_conaffinity_id, -1);
+
+  // mesh_no_hull should not have a convex hull.
+  EXPECT_EQ(model->mesh_graphadr[no_hull_id], -1);
+
+  // mesh_with_hull_contype should have a convex hull.
+  EXPECT_NE(model->mesh_graphadr[with_hull_contype_id], -1);
+
+  // mesh_with_hull_conaffinity should have a convex hull.
+  EXPECT_NE(model->mesh_graphadr[with_hull_conaffinity_id], -1);
+
+  mj_deleteModel(model);
+}
+
+TEST_F(UserCModelTest, ConvexHullForPairCollisionMeshes) {
+  static constexpr char xml[] = R"(
+  <mujoco>
+    <asset>
+      <mesh name="mesh" vertex="0 0 0  1 0 0  0 1 0  0 0 1"
+            face="0 2 1  0 1 3  0 3 2  1 2 3"/>
+    </asset>
+    <worldbody>
+      <geom name="geom1" type="sphere" size="1"/>
+      <geom name="geom_mesh" type="mesh" mesh="mesh" contype="0" conaffinity="0"/>
+    </worldbody>
+    <contact>
+      <pair name="hello" geom1="geom1" geom2="geom_mesh"/>
+    </contact>
+  </mujoco>)";
+
+  std::array<char, 1024> error;
+  mjModel* model = LoadModelFromString(xml, error.data(), error.size());
+  ASSERT_THAT(model, NotNull()) << error.data();
+
+  int mesh_in_pair_id = mj_name2id(model, mjOBJ_MESH, "mesh");
+
+  EXPECT_NE(mesh_in_pair_id, -1);
+
+  // mesh_in_pair should have a convex hull because it is in a collision pair.
+  EXPECT_NE(model->mesh_graphadr[mesh_in_pair_id], -1);
+
+  mj_deleteModel(model);
+}
+
 // ------------- test automatic inference of nuser_xxx -------------------------
 
 using UserDataTest = MujocoTest;