Import the second UV map if present in glTF files.

Conventionally, the second UV map (`TEXCOORD1`, `UV1`) is used for
lightmap UVs. This commit allows Bevy to import them, so that a custom
shader that applies lightmaps can use those UVs if desired.

Note that this doesn't actually apply lightmaps to Bevy meshes; that
will be a followup. It does, however, open the door to future Bevy
plugins that implement baked global illumination.

Author: pcwalton

crates/bevy_gltf/src/vertex_attributes.rs

@@ -263,6 +263,7 @@ pub(crate) fn convert_attribute(
         gltf::Semantic::Tangents => Some((Mesh::ATTRIBUTE_TANGENT, ConversionMode::Any)),
         gltf::Semantic::Colors(0) => Some((Mesh::ATTRIBUTE_COLOR, ConversionMode::Rgba)),
         gltf::Semantic::TexCoords(0) => Some((Mesh::ATTRIBUTE_UV_0, ConversionMode::TexCoord)),
+        gltf::Semantic::TexCoords(1) => Some((Mesh::ATTRIBUTE_UV_1, ConversionMode::TexCoord)),
         gltf::Semantic::Joints(0) => {
             Some((Mesh::ATTRIBUTE_JOINT_INDEX, ConversionMode::JointIndex))
         }

crates/bevy_pbr/src/render/mesh.rs

@@ -779,14 +779,19 @@ impl SpecializedMeshPipeline for MeshPipeline {
             vertex_attributes.push(Mesh::ATTRIBUTE_UV_0.at_shader_location(2));
         }
 
+        if layout.contains(Mesh::ATTRIBUTE_UV_1) {
+            shader_defs.push("VERTEX_UVS_1".into());
+            vertex_attributes.push(Mesh::ATTRIBUTE_UV_1.at_shader_location(3));
+        }
+
         if layout.contains(Mesh::ATTRIBUTE_TANGENT) {
             shader_defs.push("VERTEX_TANGENTS".into());
-            vertex_attributes.push(Mesh::ATTRIBUTE_TANGENT.at_shader_location(3));
+            vertex_attributes.push(Mesh::ATTRIBUTE_TANGENT.at_shader_location(4));
         }
 
         if layout.contains(Mesh::ATTRIBUTE_COLOR) {
             shader_defs.push("VERTEX_COLORS".into());
-            vertex_attributes.push(Mesh::ATTRIBUTE_COLOR.at_shader_location(4));
+            vertex_attributes.push(Mesh::ATTRIBUTE_COLOR.at_shader_location(5));
         }
 
         let mut bind_group_layout = match key.msaa_samples() {
@@ -800,7 +805,7 @@ impl SpecializedMeshPipeline for MeshPipeline {
         bind_group_layout.push(setup_morph_and_skinning_defs(
             &self.mesh_layouts,
             layout,
-            5,
+            6,
             &key,
             &mut shader_defs,
             &mut vertex_attributes,

crates/bevy_pbr/src/render/mesh.wgsl

@@ -16,15 +16,16 @@ struct Vertex {
 #ifdef VERTEX_UVS
     @location(2) uv: vec2<f32>,
 #endif
+// (Alternate UVs are at location 3, but they're currently unused here.)
 #ifdef VERTEX_TANGENTS
-    @location(3) tangent: vec4<f32>,
+    @location(4) tangent: vec4<f32>,
 #endif
 #ifdef VERTEX_COLORS
-    @location(4) color: vec4<f32>,
+    @location(5) color: vec4<f32>,
 #endif
 #ifdef SKINNED
-    @location(5) joint_indices: vec4<u32>,
-    @location(6) joint_weights: vec4<f32>,
+    @location(6) joint_indices: vec4<u32>,
+    @location(7) joint_weights: vec4<f32>,
 #endif
 #ifdef MORPH_TARGETS
     @builtin(vertex_index) index: u32,

crates/bevy_render/src/mesh/mesh/mod.rs

@@ -146,6 +146,14 @@ impl Mesh {
     pub const ATTRIBUTE_UV_0: MeshVertexAttribute =
         MeshVertexAttribute::new("Vertex_Uv", 2, VertexFormat::Float32x2);
 
+    /// Alternate texture coordinates for the vertex. Use in conjuction with
+    /// [`Mesh::insert_attribute`].
+    ///
+    /// Typically, these are used for lightmaps, textures that provide
+    /// precomputed illumination.
+    pub const ATTRIBUTE_UV_1: MeshVertexAttribute =
+        MeshVertexAttribute::new("Vertex_Uv_1", 2, VertexFormat::Float32x2);
+
     /// The direction of the vertex tangent. Used for normal mapping.
     /// Usually generated with [`generate_tangents`](Mesh::generate_tangents).
     pub const ATTRIBUTE_TANGENT: MeshVertexAttribute =