Add support for flat shading using indices

Will apply the flat interpolation qualifier for vertex_normal,
this uses the provoking vertex for the whole primitive in the fragment
shader, so mostly useful for custom meshes.

I made this as an alternative for bevyengine#1808 when for example making
procedually generated terrain.

Author: sweepline

crates/bevy_pbr/src/material.rs

@@ -42,6 +42,13 @@ pub struct StandardMaterial {
     #[render_resources(ignore)]
     #[shader_def]
     pub unlit: bool,
+    /// Use the 'flat' Type Qualifier for the normal vectors.
+    /// This allows for flat shading even with indiced meshes.
+    /// The normal that is used for the whole primitive in the fragment
+    /// shader will be the [Provoking Vertex](https://www.khronos.org/opengl/wiki/Primitive#Provoking_vertex)
+    #[render_resources(ignore)]
+    #[shader_def]
+    pub flat_shading: bool,
 }
 
 impl Default for StandardMaterial {
@@ -69,6 +76,7 @@ impl Default for StandardMaterial {
             emissive: Color::BLACK,
             emissive_texture: None,
             unlit: false,
+            flat_shading: false,
         }
     }
 }

crates/bevy_pbr/src/render_graph/pbr_pipeline/pbr.frag

@@ -50,7 +50,13 @@ struct DirectionalLight {
 };
 
 layout(location = 0) in vec3 v_WorldPosition;
+
+#ifdef STANDARDMATERIAL_FLAT_SHADING
+layout(location = 1) flat in vec3 v_WorldNormal;
+#endif
+#ifndef STANDARDMATERIAL_FLAT_SHADING
 layout(location = 1) in vec3 v_WorldNormal;
+#endif
 layout(location = 2) in vec2 v_Uv;
 
 #ifdef STANDARDMATERIAL_NORMAL_MAP

crates/bevy_pbr/src/render_graph/pbr_pipeline/pbr.vert

@@ -9,7 +9,12 @@ layout(location = 3) in vec4 Vertex_Tangent;
 #endif
 
 layout(location = 0) out vec3 v_WorldPosition;
+#ifdef STANDARDMATERIAL_FLAT_SHADING
+layout(location = 1) flat out vec3 v_WorldNormal;
+#endif
+#ifndef STANDARDMATERIAL_FLAT_SHADING
 layout(location = 1) out vec3 v_WorldNormal;
+#endif
 layout(location = 2) out vec2 v_Uv;
 
 layout(set = 0, binding = 0) uniform CameraViewProj {

examples/3d/3d_scene.rs

@@ -20,11 +20,26 @@ fn setup(
         material: materials.add(Color::rgb(0.3, 0.5, 0.3).into()),
         ..Default::default()
     });
+    // flat shaded Icosphere
+    commands.spawn_bundle(PbrBundle {
+        mesh: meshes.add(Mesh::from(shape::Icosphere {
+            radius: 0.45,
+            subdivisions: 2,
+        })),
+        material: materials.add(StandardMaterial {
+            base_color: Color::hex("ffd891").unwrap(),
+            // vary key PBR parameters on a grid of spheres to show the effect
+            flat_shading: true,
+            ..Default::default()
+        }),
+        transform: Transform::from_xyz(-0.5, 0.5, 0.0),
+        ..Default::default()
+    });
     // cube
     commands.spawn_bundle(PbrBundle {
         mesh: meshes.add(Mesh::from(shape::Cube { size: 1.0 })),
         material: materials.add(Color::rgb(0.8, 0.7, 0.6).into()),
-        transform: Transform::from_xyz(0.0, 0.5, 0.0),
+        transform: Transform::from_xyz(0.6, 0.5, 0.0),
         ..Default::default()
     });
     // light