remove mandatory mesh attributes (bevyengine#6127)

# Objective

- It's possible to create a mesh without positions or normals, but currently bevy forces these attributes to be present on any mesh.

## Solution

- Don't assume these attributes are present and add a shader defs for each attributes
- I updated 2d and 3d meshes to use the same logic.

---

## Changelog

- Meshes don't require any attributes

# Notes
I didn't update the pbr.wgsl shader because I'm not sure how to handle it. It doesn't really make sense to use it without positions or normals.

crates/bevy_pbr/src/render/mesh.rs

@@ -566,12 +566,19 @@ impl SpecializedMeshPipeline for MeshPipeline {
         key: Self::Key,
         layout: &MeshVertexBufferLayout,
     ) -> Result<RenderPipelineDescriptor, SpecializedMeshPipelineError> {
-        let mut vertex_attributes = vec![
-            Mesh::ATTRIBUTE_POSITION.at_shader_location(0),
-            Mesh::ATTRIBUTE_NORMAL.at_shader_location(1),
-        ];
-
         let mut shader_defs = Vec::new();
+        let mut vertex_attributes = Vec::new();
+
+        if layout.contains(Mesh::ATTRIBUTE_POSITION) {
+            shader_defs.push(String::from("VERTEX_POSITIONS"));
+            vertex_attributes.push(Mesh::ATTRIBUTE_POSITION.at_shader_location(0));
+        }
+
+        if layout.contains(Mesh::ATTRIBUTE_NORMAL) {
+            shader_defs.push(String::from("VERTEX_NORMALS"));
+            vertex_attributes.push(Mesh::ATTRIBUTE_NORMAL.at_shader_location(1));
+        }
+
         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));

crates/bevy_pbr/src/render/mesh.wgsl

@@ -5,8 +5,12 @@
 #import bevy_pbr::mesh_functions
 
 struct Vertex {
+#ifdef VERTEX_POSITIONS
     @location(0) position: vec3<f32>,
+#endif
+#ifdef VERTEX_NORMALS
     @location(1) normal: vec3<f32>,
+#endif
 #ifdef VERTEX_UVS
     @location(2) uv: vec2<f32>,
 #endif
@@ -30,25 +34,34 @@ struct VertexOutput {
 @vertex
 fn vertex(vertex: Vertex) -> VertexOutput {
     var out: VertexOutput;
+
+#ifdef VERTEX_NORMALS
 #ifdef SKINNED
     var model = skin_model(vertex.joint_indices, vertex.joint_weights);
     out.world_normal = skin_normals(model, vertex.normal);
 #else
     var model = mesh.model;
     out.world_normal = mesh_normal_local_to_world(vertex.normal);
 #endif
+#endif
+
+#ifdef VERTEX_POSITIONS
     out.world_position = mesh_position_local_to_world(model, vec4<f32>(vertex.position, 1.0));
+    out.clip_position = mesh_position_world_to_clip(out.world_position);
+#endif
+
 #ifdef VERTEX_UVS
     out.uv = vertex.uv;
 #endif
+
 #ifdef VERTEX_TANGENTS
     out.world_tangent = mesh_tangent_local_to_world(model, vertex.tangent);
 #endif
+
 #ifdef VERTEX_COLORS
     out.color = vertex.color;
 #endif
 
-    out.clip_position = mesh_position_world_to_clip(out.world_position);
     return out;
 }
 

crates/bevy_sprite/src/mesh2d/mesh.rs

@@ -325,13 +325,24 @@ impl SpecializedMeshPipeline for Mesh2dPipeline {
         key: Self::Key,
         layout: &MeshVertexBufferLayout,
     ) -> Result<RenderPipelineDescriptor, SpecializedMeshPipelineError> {
-        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();
+        let mut vertex_attributes = Vec::new();
+
+        if layout.contains(Mesh::ATTRIBUTE_POSITION) {
+            shader_defs.push(String::from("VERTEX_POSITIONS"));
+            vertex_attributes.push(Mesh::ATTRIBUTE_POSITION.at_shader_location(0));
+        }
+
+        if layout.contains(Mesh::ATTRIBUTE_NORMAL) {
+            shader_defs.push(String::from("VERTEX_NORMALS"));
+            vertex_attributes.push(Mesh::ATTRIBUTE_NORMAL.at_shader_location(1));
+        }
+
+        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_sprite/src/mesh2d/mesh2d.wgsl

@@ -5,9 +5,15 @@
 #import bevy_sprite::mesh2d_functions
 
 struct Vertex {
+#ifdef VERTEX_POSITIONS
     @location(0) position: vec3<f32>,
+#endif
+#ifdef VERTEX_NORMALS
     @location(1) normal: vec3<f32>,
+#endif
+#ifdef VERTEX_UVS
     @location(2) uv: vec2<f32>,
+#endif
 #ifdef VERTEX_TANGENTS
     @location(3) tangent: vec4<f32>,
 #endif
@@ -24,13 +30,24 @@ struct VertexOutput {
 @vertex
 fn vertex(vertex: Vertex) -> VertexOutput {
     var out: VertexOutput;
+
+#ifdef VERTEX_UVS
     out.uv = vertex.uv;
+#endif
+
+#ifdef VERTEX_POSITIONS
     out.world_position = mesh2d_position_local_to_world(mesh.model, vec4<f32>(vertex.position, 1.0));
     out.clip_position = mesh2d_position_world_to_clip(out.world_position);
+#endif
+
+#ifdef VERTEX_NORMALS
     out.world_normal = mesh2d_normal_local_to_world(vertex.normal);
+#endif
+
 #ifdef VERTEX_TANGENTS
     out.world_tangent = mesh2d_tangent_local_to_world(mesh.model, vertex.tangent);
 #endif
+
 #ifdef VERTEX_COLORS
     out.color = vertex.color;
 #endif
@@ -44,5 +61,9 @@ struct FragmentInput {
 
 @fragment
 fn fragment(in: FragmentInput) -> @location(0) vec4<f32> {
+#ifdef VERTEX_COLORS
+    return in.color;
+#else
     return vec4<f32>(1.0, 0.0, 1.0, 1.0);
+#endif
 }