Add support mjcf emissive layer materials

Mujoco supports emission as a constant coefficient, and emissive, an rgb texture. This cl supports the rgb emissive texture for usd.

PiperOrigin-RevId: 775236677
Change-Id: I0e2ce1d9f68d79b6e9d2c28eecab22b2692f061c

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

@@ -91,6 +91,7 @@ TF_DEFINE_PRIVATE_TOKENS(kTokens,
                          ((inputsWrapS, "inputs:wrapS"))
                          ((inputsWrapT, "inputs:wrapT"))
                          ((inputsDiffuseColor, "inputs:diffuseColor"))
+                         ((inputsEmissiveColor, "inputs:emissiveColor"))
                          ((outputsRgb, "outputs:rgb"))
                          ((outputsR, "outputs:r"))
                          ((outputsG, "outputs:g"))
@@ -794,6 +795,26 @@ class ModelWriter {
       }
     }
 
+    // Connect an emissive texture if specified.
+    if (mjTEXROLE_EMISSIVE < textures.size()) {
+      std::string emissive_texture_name = textures[mjTEXROLE_EMISSIVE];
+      mjsTexture *emissive_texture = mjs_asTexture(
+          mjs_findElement(spec_, mjOBJ_TEXTURE, emissive_texture_name.c_str()));
+      if (emissive_texture) {
+        pxr::SdfPath emissive_attr = CreateAttributeSpec(
+            data_, preview_surface_shader_path, kTokens->inputsEmissiveColor,
+            pxr::SdfValueTypeNames->Color3f);
+        const std::vector<pxr::SdfPath> emissive_map_output_attrs =
+            AddTextureShader(material_path, emissive_texture->file->c_str(),
+                             pxr::TfToken("emissive"), uvmap_st_output_attr,
+                             {kTokens->outputsRgb});
+        if (emissive_map_output_attrs.size() == 1) {
+          AddAttributeConnection(data_, emissive_attr,
+                                 emissive_map_output_attrs[0]);
+        }
+      }
+    }
+
     pxr::SdfPath diffuse_color_attr = CreateAttributeSpec(
         data_, preview_surface_shader_path, kTokens->inputsDiffuseColor,
         pxr::SdfValueTypeNames->Color3f);

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

@@ -192,6 +192,16 @@ TEST_F(MjcfSdfFileFormatPluginTest, TestMaterials) {
     stage, "/mesh_test/Materials/material_layered/orm_packed.inputs:file",
     pxr::SdfAssetPath("textures/orm.png"));
 
+  EXPECT_PRIM_VALID(stage, "/mesh_test/Materials/material_layered/emissive");
+  ExpectAttributeHasConnection(
+    stage,
+    "/mesh_test/Materials/material_layered/"
+    "PreviewSurface.inputs:emissiveColor",
+    "/mesh_test/Materials/material_layered/emissive.outputs:rgb");
+  ExpectAttributeEqual(
+    stage, "/mesh_test/Materials/material_layered/emissive.inputs:file",
+    pxr::SdfAssetPath("textures/emissive.png"));
+
   EXPECT_PRIM_VALID(stage, "/mesh_test/Materials/material_metallic");
   EXPECT_PRIM_VALID(stage,
                     "/mesh_test/Materials/material_metallic/PreviewSurface");

test/experimental/usd/plugins/mjcf/testdata/materials.xml

@@ -3,6 +3,7 @@
     <texture name="diffuse_tex" file="textures/cube.png" type="2d" />
     <texture name="normal_tex" file="textures/normal.png" type="2d" />
     <texture name="orm_tex" file="textures/orm.png" type="2d" />
+    <texture name="emissive_tex" file="textures/emissive.png" type="2d" />
     <material name="material_red" rgba="0.8 0 0 1" />
     <material name="material_texture" texture="diffuse_tex" />
     <material name="material_metallic" rgba="0.8 0 0 1" metallic="0.6"/>
@@ -10,6 +11,7 @@
       <layer texture="diffuse_tex" role="rgb"/>
       <layer texture="normal_tex" role="normal"/>
       <layer texture="orm_tex" role="orm"/>
+      <layer texture="emissive_tex" role="emissive"/>
     </material>
     2d texture="diffuse" normal="normal" />
     <mesh name="tetrahedron" vertex="0 0 0  1 0 0  0 1 0  0 0 1"/>