Add basic support for geom rgba.

Adds primvar:displayColor/primvar:displayOpacity to the geom prim if its rgba attribute differs from the default (0.5, 0.5, 0.5, 1.0).  This sets the color and opacity directly on the prim with no need for an external material, making it consistent with how rgba is used in mujoco.

PiperOrigin-RevId: 751013128
Change-Id: I059943ec5b266e62c3696d9a0ac4f2033db9e07f

src/experimental/usd/plugins/mjcf/mujoco_to_usd.cc

@@ -897,6 +897,29 @@ class ModelWriter {
       }
     }
 
+    // If geom rgba is not the default (0.5, 0.5, 0.5, 1), then set the
+    // displayColor attribute.
+    // No effort is made to properly handle the interaction between geom rgba
+    // and the material if both are specified.
+    if (geom->rgba[0] != 0.5f || geom->rgba[1] != 0.5f ||
+        geom->rgba[2] != 0.5f || geom->rgba[3] != 1.0f) {
+      // Set the displayColor attribute.
+      pxr::SdfPath display_color_attr = CreateAttributeSpec(
+          data_, geom_path, pxr::UsdGeomTokens->primvarsDisplayColor,
+          pxr::SdfValueTypeNames->Color3fArray);
+      SetAttributeDefault(data_, display_color_attr,
+                          pxr::VtArray<pxr::GfVec3f>{
+                              {geom->rgba[0], geom->rgba[1], geom->rgba[2]}});
+      // Set the displayOpacity attribute, only if the opacity is not 1.
+      if (geom->rgba[3] != 1.0f) {
+        pxr::SdfPath display_opacity_attr = CreateAttributeSpec(
+            data_, geom_path, pxr::UsdGeomTokens->primvarsDisplayOpacity,
+            pxr::SdfValueTypeNames->FloatArray);
+        SetAttributeDefault(data_, display_opacity_attr,
+                            pxr::VtArray<float>{geom->rgba[3]});
+      }
+    }
+
     if (body_id == kWorldIndex) {
       SetPrimKind(data_, geom_path, pxr::KindTokens->component);
     }

test/experimental/usd/plugins/mjcf/fixture.h

@@ -60,6 +60,13 @@
         .Get(&prim_purpose);                         \
     EXPECT_EQ(prim_purpose, purpose);                \
   }
+
+#define EXPECT_ATTRIBUTE_HAS_VALUE(stage, path) \
+  EXPECT_TRUE((stage)->GetAttributeAtPath(SdfPath(path)).HasValue());
+
+#define EXPECT_ATTRIBUTE_HAS_NO_VALUE(stage, path) \
+  EXPECT_FALSE((stage)->GetAttributeAtPath(SdfPath(path)).HasValue());
+
 namespace mujoco {
 
 pxr::SdfLayerRefPtr LoadLayer(

test/experimental/usd/plugins/mjcf/mjcf_file_format_test.cc

@@ -140,6 +140,53 @@ TEST_F(MjcfSdfFileFormatPluginTest, TestMaterials) {
       pxr::SdfAssetPath("textures/cube.png"));
 }
 
+TEST_F(MjcfSdfFileFormatPluginTest, TestGeomRgba) {
+  static constexpr char kXml[] = R"(
+    <mujoco model="test">
+      <worldbody>
+        <geom type="sphere" name="sphere_red" size="1" rgba="1 0 0 1"/>
+        <geom type="sphere" name="sphere_default" size="1"/>
+        <geom type="sphere" name="sphere_also_default" size="1" rgba="0.5 0.5 0.5 1"/>
+        <geom type="sphere" name="sphere_almost_default" size="1" rgba="0.5 0.5 0.5 0.9"/>
+      </worldbody>
+    </mujoco>
+  )";
+
+  pxr::SdfLayerRefPtr layer = LoadLayer(kXml);
+  auto stage = pxr::UsdStage::Open(layer);
+
+  EXPECT_PRIM_VALID(stage, "/test/sphere_red");
+  ExpectAttributeEqual(stage, "/test/sphere_red.primvars:displayColor",
+                       pxr::VtArray<pxr::GfVec3f>{{1, 0, 0}});
+  EXPECT_ATTRIBUTE_HAS_NO_VALUE(stage,
+                                "/test/sphere_red.primvars:displayOpacity");
+
+  // There's no mechanism in Mujoco to specify whether an attribute was set
+  // explicitly or not. We do the same as Mujoco does, which is to compare with
+  // the default value.
+  // Which explains why not setting rgba is the same as setting it to the
+  // default value of (0.5, 0.5, 0.5, 1).
+  EXPECT_PRIM_VALID(stage, "/test/sphere_default");
+  EXPECT_ATTRIBUTE_HAS_NO_VALUE(stage,
+                                "/test/sphere_default.primvars:displayColor");
+  EXPECT_ATTRIBUTE_HAS_NO_VALUE(stage,
+                                "/test/sphere_default.primvars:displayOpacity");
+
+  EXPECT_PRIM_VALID(stage, "/test/sphere_also_default");
+  EXPECT_ATTRIBUTE_HAS_NO_VALUE(
+      stage, "/test/sphere_also_default.primvars:displayColor");
+  EXPECT_ATTRIBUTE_HAS_NO_VALUE(
+      stage, "/test/sphere_also_default.primvars:displayOpacity");
+
+  EXPECT_PRIM_VALID(stage, "/test/sphere_almost_default");
+  ExpectAttributeEqual(stage,
+                       "/test/sphere_almost_default.primvars:displayColor",
+                       pxr::VtArray<pxr::GfVec3f>{{0.5, 0.5, 0.5}});
+  ExpectAttributeEqual(stage,
+                       "/test/sphere_almost_default.primvars:displayOpacity",
+                       pxr::VtArray<float>{0.9});
+}
+
 TEST_F(MjcfSdfFileFormatPluginTest, TestFaceVaryingMeshSourcesSimpleMjcfMesh) {
   static constexpr char kXml[] = R"(
     <mujoco model="mesh test">