From 253abad210823254f0f238ef3ad4cc8d975372d3 Mon Sep 17 00:00:00 2001
From: Robin Alazard <robinalazard@google.com>
Date: Fri, 2 May 2025 08:34:28 -0700
Subject: [PATCH] Add MeshCollisionAPI to collider Meshes with convexHull
 approximation.

PiperOrigin-RevId: 754014642
Change-Id: I44cff505a2b9b7120a38f8c896b2c6560ce51c43
---
 .../usd/plugins/mjcf/mujoco_to_usd.cc            | 15 +++++++++++++++
 .../usd/plugins/mjcf/mjcf_file_format_test.cc    | 16 +++++++++++-----
 2 files changed, 26 insertions(+), 5 deletions(-)

diff --git a/src/experimental/usd/plugins/mjcf/mujoco_to_usd.cc b/src/experimental/usd/plugins/mjcf/mujoco_to_usd.cc
index b89abc3c..ef5e9f71 100644
--- a/src/experimental/usd/plugins/mjcf/mujoco_to_usd.cc
+++ b/src/experimental/usd/plugins/mjcf/mujoco_to_usd.cc
@@ -883,6 +883,21 @@ class ModelWriter {
                           model_->geom_conaffinity[geom_id] != 0)) {
       ApplyApiSchema(data_, geom_path,
                      pxr::UsdPhysicsTokens->PhysicsCollisionAPI);
+      // For meshes, also apply PhysicsMeshCollisionAPI and set the
+      // approximation attribute.
+      if (geom->type == mjGEOM_MESH) {
+        ApplyApiSchema(data_, geom_path,
+                       pxr::UsdPhysicsTokens->PhysicsMeshCollisionAPI);
+
+        // Note: MuJoCo documentation states that for collision purposes, meshes
+        // are always replaced with their convex hulls. Therefore, we set the
+        // approximation attribute to convexHull explicitly.
+        pxr::SdfPath approximation_attr = CreateAttributeSpec(
+            data_, geom_path, pxr::UsdPhysicsTokens->physicsApproximation,
+            pxr::SdfValueTypeNames->Token, pxr::SdfVariabilityUniform);
+        SetAttributeDefault(data_, approximation_attr,
+                            pxr::UsdPhysicsTokens->convexHull);
+      }
     }
 
     mjsDefault *spec_default = mjs_getDefault(geom->element);
diff --git a/test/experimental/usd/plugins/mjcf/mjcf_file_format_test.cc b/test/experimental/usd/plugins/mjcf/mjcf_file_format_test.cc
index abdf6a23..22b8ac52 100644
--- a/test/experimental/usd/plugins/mjcf/mjcf_file_format_test.cc
+++ b/test/experimental/usd/plugins/mjcf/mjcf_file_format_test.cc
@@ -45,7 +45,9 @@
 #include <pxr/usd/usdGeom/sphere.h>
 #include <pxr/usd/usdGeom/tokens.h>
 #include <pxr/usd/usdPhysics/collisionAPI.h>
+#include <pxr/usd/usdPhysics/meshCollisionAPI.h>
 #include <pxr/usd/usdPhysics/rigidBodyAPI.h>
+#include <pxr/usd/usdPhysics/tokens.h>
 
 PXR_NAMESPACE_OPEN_SCOPE
 // clang-format off
@@ -675,7 +677,7 @@ TEST_F(MjcfSdfFileFormatPluginTest, TestPhysicsColliders) {
   //
   // body_3/body_3 [rigidbody]
   //   body_3/body_3/body_3_col []  <-- Intermediate prim for mesh instancing
-  //     body_3/body_3/body_3_col/Mesh [collider]
+  //     body_3/body_3/body_3_col/Mesh [collider, mesh collider]
 
   // ground [collider] (Static collider)
   EXPECT_PRIM_VALID(stage, "/test/ground");
@@ -753,13 +755,17 @@ TEST_F(MjcfSdfFileFormatPluginTest, TestPhysicsColliders) {
                               pxr::UsdPhysicsRigidBodyAPI);
   EXPECT_PRIM_API_NOT_APPLIED(stage, "/test/body_3/body_3/body_3_col",
                               pxr::UsdPhysicsCollisionAPI);
-  //     body_3/body_3/body_3_col/Mesh [collider]
+  //     body_3/body_3/body_3_col/Mesh [collider, mesh collider]
   EXPECT_PRIM_VALID(stage, "/test/body_3/body_3/body_3_col/Mesh");
-  EXPECT_PRIM_API_NOT_APPLIED(
-      stage, "/test/body_3/body_3/body_3_col/Mesh",
-      pxr::UsdPhysicsRigidBodyAPI);
+  EXPECT_PRIM_API_NOT_APPLIED(stage, "/test/body_3/body_3/body_3_col/Mesh",
+                              pxr::UsdPhysicsRigidBodyAPI);
   EXPECT_PRIM_API_APPLIED(stage, "/test/body_3/body_3/body_3_col/Mesh",
                           pxr::UsdPhysicsCollisionAPI);
+  EXPECT_PRIM_API_APPLIED(stage, "/test/body_3/body_3/body_3_col/Mesh",
+                          pxr::UsdPhysicsMeshCollisionAPI);
+  ExpectAttributeEqual(
+      stage, "/test/body_3/body_3/body_3_col/Mesh.physics:approximation",
+      pxr::UsdPhysicsTokens->convexHull);
 }
 
 }  // namespace