Allow rendering meshes without UV coordinate data. (bevyengine#5222)

# Objective

Bevy requires meshes to include UV coordinates, even if the material does not use any textures, and will fail with an error `ERROR bevy_pbr::material: Mesh is missing requested attribute: Vertex_Uv (MeshVertexAttributeId(2), pipeline type: Some("bevy_pbr::material::MaterialPipeline<bevy_pbr::pbr_material::StandardMaterial>"))` otherwise. The objective of this PR is to permit this.

## Solution

This PR follows the design of bevyengine#4528, which added support for per-vertex colours. It adds a shader define called VERTEX_UVS which indicates the presence of UV coordinates to the shader.

crates/bevy_gltf/src/loader.rs

@@ -260,11 +260,6 @@ async fn load_gltf<'a, 'b>(
                 .map(|v| VertexAttributeValues::Float32x2(v.into_f32().collect()))
             {
                 mesh.insert_attribute(Mesh::ATTRIBUTE_UV_0, vertex_attribute);
-            } else {
-                let len = mesh.count_vertices();
-                let uvs = vec![[0.0, 0.0]; len];
-                bevy_log::debug!("missing `TEXCOORD_0` vertex attribute, loading zeroed out UVs");
-                mesh.insert_attribute(Mesh::ATTRIBUTE_UV_0, uvs);
             }
 
             if let Some(vertex_attribute) = reader

crates/bevy_pbr/src/render/mesh.rs

@@ -540,10 +540,14 @@ impl SpecializedMeshPipeline for MeshPipeline {
         let mut vertex_attributes = vec![
             Mesh::ATTRIBUTE_POSITION.at_shader_location(0),
             Mesh::ATTRIBUTE_NORMAL.at_shader_location(1),
-            Mesh::ATTRIBUTE_UV_0.at_shader_location(2),
         ];
 
         let mut shader_defs = Vec::new();
+        if layout.contains(Mesh::ATTRIBUTE_UV_0) {
+            shader_defs.push(String::from("VERTEX_UVS"));
+            vertex_attributes.push(Mesh::ATTRIBUTE_UV_0.at_shader_location(2));
+        }
+
         if layout.contains(Mesh::ATTRIBUTE_TANGENT) {
             shader_defs.push(String::from("VERTEX_TANGENTS"));
             vertex_attributes.push(Mesh::ATTRIBUTE_TANGENT.at_shader_location(3));

crates/bevy_pbr/src/render/mesh.wgsl

@@ -7,7 +7,9 @@
 struct Vertex {
     [[location(0)]] position: vec3<f32>;
     [[location(1)]] normal: vec3<f32>;
+#ifdef VERTEX_UVS
     [[location(2)]] uv: vec2<f32>;
+#endif
 #ifdef VERTEX_TANGENTS
     [[location(3)]] tangent: vec4<f32>;
 #endif
@@ -24,7 +26,9 @@ struct VertexOutput {
     [[builtin(position)]] clip_position: vec4<f32>;
     [[location(0)]] world_position: vec4<f32>;
     [[location(1)]] world_normal: vec3<f32>;
+#ifdef VERTEX_UVS
     [[location(2)]] uv: vec2<f32>;
+#endif
 #ifdef VERTEX_TANGENTS
     [[location(3)]] world_tangent: vec4<f32>;
 #endif
@@ -44,7 +48,9 @@ fn vertex(vertex: Vertex) -> VertexOutput {
     out.world_normal = mesh_normal_local_to_world(vertex.normal);
 #endif
     out.world_position = mesh_position_local_to_world(model, vec4<f32>(vertex.position, 1.0));
+#ifdef VERTEX_UVS
     out.uv = vertex.uv;
+#endif
 #ifdef VERTEX_TANGENTS
     out.world_tangent = mesh_tangent_local_to_world(model, vertex.tangent);
 #endif
@@ -60,7 +66,9 @@ struct FragmentInput {
     [[builtin(front_facing)]] is_front: bool;
     [[location(0)]] world_position: vec4<f32>;
     [[location(1)]] world_normal: vec3<f32>;
+#ifdef VERTEX_UVS
     [[location(2)]] uv: vec2<f32>;
+#endif
 #ifdef VERTEX_TANGENTS
     [[location(3)]] world_tangent: vec4<f32>;
 #endif

crates/bevy_pbr/src/render/pbr.wgsl

@@ -13,7 +13,9 @@ struct FragmentInput {
     [[builtin(position)]] frag_coord: vec4<f32>;
     [[location(0)]] world_position: vec4<f32>;
     [[location(1)]] world_normal: vec3<f32>;
+#ifdef VERTEX_UVS
     [[location(2)]] uv: vec2<f32>;
+#endif
 #ifdef VERTEX_TANGENTS
     [[location(3)]] world_tangent: vec4<f32>;
 #endif
@@ -28,9 +30,11 @@ fn fragment(in: FragmentInput) -> [[location(0)]] vec4<f32> {
 #ifdef VERTEX_COLORS
     output_color = output_color * in.color;
 #endif
+#ifdef VERTEX_UVS
     if ((material.flags & STANDARD_MATERIAL_FLAGS_BASE_COLOR_TEXTURE_BIT) != 0u) {
         output_color = output_color * textureSample(base_color_texture, base_color_sampler, in.uv);
     }
+#endif
 
     // NOTE: Unlit bit not set means == 0 is true, so the true case is if lit
     if ((material.flags & STANDARD_MATERIAL_FLAGS_UNLIT_BIT) == 0u) {
@@ -45,26 +49,32 @@ fn fragment(in: FragmentInput) -> [[location(0)]] vec4<f32> {
 
         // TODO use .a for exposure compensation in HDR
         var emissive: vec4<f32> = material.emissive;
+#ifdef VERTEX_UVS
         if ((material.flags & STANDARD_MATERIAL_FLAGS_EMISSIVE_TEXTURE_BIT) != 0u) {
             emissive = vec4<f32>(emissive.rgb * textureSample(emissive_texture, emissive_sampler, in.uv).rgb, 1.0);
         }
+#endif
         pbr_input.material.emissive = emissive;
 
         var metallic: f32 = material.metallic;
         var perceptual_roughness: f32 = material.perceptual_roughness;
+#ifdef VERTEX_UVS
         if ((material.flags & STANDARD_MATERIAL_FLAGS_METALLIC_ROUGHNESS_TEXTURE_BIT) != 0u) {
             let metallic_roughness = textureSample(metallic_roughness_texture, metallic_roughness_sampler, in.uv);
             // Sampling from GLTF standard channels for now
             metallic = metallic * metallic_roughness.b;
             perceptual_roughness = perceptual_roughness * metallic_roughness.g;
         }
+#endif
         pbr_input.material.metallic = metallic;
         pbr_input.material.perceptual_roughness = perceptual_roughness;
 
         var occlusion: f32 = 1.0;
+#ifdef VERTEX_UVS
         if ((material.flags & STANDARD_MATERIAL_FLAGS_OCCLUSION_TEXTURE_BIT) != 0u) {
             occlusion = textureSample(occlusion_texture, occlusion_sampler, in.uv).r;
         }
+#endif
         pbr_input.occlusion = occlusion;
 
         pbr_input.frag_coord = in.frag_coord;
@@ -81,7 +91,9 @@ fn fragment(in: FragmentInput) -> [[location(0)]] vec4<f32> {
             in.world_tangent,
 #endif
 #endif
+#ifdef VERTEX_UVS
             in.uv,
+#endif
             in.is_front,
         );
         pbr_input.V = calculate_view(in.world_position, pbr_input.is_orthographic);

crates/bevy_pbr/src/render/pbr_functions.wgsl

@@ -10,7 +10,9 @@ fn prepare_normal(
     world_tangent: vec4<f32>,
 #endif
 #endif
+#ifdef VERTEX_UVS
     uv: vec2<f32>,
+#endif
     is_front: bool,
 ) -> vec3<f32> {
     var N: vec3<f32> = normalize(world_normal);
@@ -39,6 +41,7 @@ fn prepare_normal(
     }
 
 #ifdef VERTEX_TANGENTS
+#ifdef VERTEX_UVS
 #ifdef STANDARDMATERIAL_NORMAL_MAP
     // Nt is the tangent-space normal.
     var Nt = textureSample(normal_map_texture, normal_map_sampler, uv).rgb;
@@ -60,6 +63,7 @@ fn prepare_normal(
     // http://www.mikktspace.com/
     N = normalize(Nt.x * T + Nt.y * B + Nt.z * N);
 #endif
+#endif
 #endif
 
     return N;