Add support for flat shading with indiced meshes

Use the partial derivative (dFdx, dFdy) for computing facet normals
in the fragment shader.

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

Cargo.toml

@@ -181,6 +181,10 @@ path = "examples/3d/wireframe.rs"
 name = "z_sort_debug"
 path = "examples/3d/z_sort_debug.rs"
 
+[[example]]
+name = "flat_shading"
+path = "examples/3d/flat_shading.rs"
+
 # Application
 [[example]]
 name = "custom_loop"

crates/bevy_pbr/src/material.rs

@@ -42,6 +42,10 @@ pub struct StandardMaterial {
     #[render_resources(ignore)]
     #[shader_def]
     pub unlit: bool,
+    /// This allows for flat shading even with indiced meshes.
+    #[render_resources(ignore)]
+    #[shader_def]
+    pub flat_shading: bool,
 }
 
 impl Default for StandardMaterial {
@@ -69,6 +73,7 @@ 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

@@ -43,14 +43,17 @@ struct PointLight {
     vec4 color;
     vec4 lightParams;
 };
- 
+
 struct DirectionalLight {
     vec4 direction;
     vec4 color;
 };
 
 layout(location = 0) in vec3 v_WorldPosition;
+
+#ifndef STANDARDMATERIAL_FLAT_SHADING
 layout(location = 1) in vec3 v_WorldNormal;
+#endif
 layout(location = 2) in vec2 v_Uv;
 
 #ifdef STANDARDMATERIAL_NORMAL_MAP
@@ -368,7 +371,12 @@ void main() {
 
     float roughness = perceptualRoughnessToRoughness(perceptual_roughness);
 
-    vec3 N = normalize(v_WorldNormal);
+#    ifdef STANDARDMATERIAL_FLAT_SHADING
+    vec3 N = normalize(cross(dFdy(v_WorldPosition), dFdx(v_WorldPosition)));
+#    endif
+#    ifndef STANDARDMATERIAL_FLAT_SHADING
+    vec3 N = v_WorldNormal;
+#    endif
 
 #    ifdef STANDARDMATERIAL_NORMAL_MAP
     vec3 T = normalize(v_WorldTangent.xyz);

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

@@ -9,7 +9,9 @@ layout(location = 3) in vec4 Vertex_Tangent;
 #endif
 
 layout(location = 0) out vec3 v_WorldPosition;
+#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 {
@@ -27,7 +29,9 @@ layout(set = 2, binding = 0) uniform Transform {
 void main() {
     vec4 world_position = Model * vec4(Vertex_Position, 1.0);
     v_WorldPosition = world_position.xyz;
+#ifndef STANDARDMATERIAL_FLAT_SHADING
     v_WorldNormal = mat3(Model) * Vertex_Normal;
+#endif
     v_Uv = Vertex_Uv;
 #ifdef STANDARDMATERIAL_NORMAL_MAP
     v_WorldTangent = vec4(mat3(Model) * Vertex_Tangent.xyz, Vertex_Tangent.w);

examples/3d/flat_shading.rs

@@ -0,0 +1,63 @@
+use bevy::{
+    prelude::*,
+    render::{
+        mesh::{Indices, VertexAttributeValues},
+        pipeline::PrimitiveTopology,
+    },
+};
+
+fn main() {
+    App::new()
+        .insert_resource(Msaa { samples: 4 })
+        .add_plugins(DefaultPlugins)
+        .add_startup_system(setup)
+        .run();
+}
+
+/// set up a simple 3D scene
+fn setup(
+    mut commands: Commands,
+    mut meshes: ResMut<Assets<Mesh>>,
+    mut materials: ResMut<Assets<StandardMaterial>>,
+) {
+    // flat
+    commands.spawn_bundle(PbrBundle {
+        mesh: meshes.add(Mesh::from(shape::Icosphere {
+            radius: 0.5,
+            subdivisions: 4,
+        })),
+        material: materials.add(StandardMaterial {
+            base_color: Color::rgb(0.8, 0.7, 0.6),
+            flat_shading: true,
+            ..Default::default()
+        }),
+        transform: Transform::from_xyz(-0.55, 0.5, 0.0),
+        ..Default::default()
+    });
+    // smooth
+    commands.spawn_bundle(PbrBundle {
+        mesh: meshes.add(Mesh::from(shape::Icosphere {
+            radius: 0.5,
+            subdivisions: 4,
+        })),
+        material: materials.add(Color::rgb(0.8, 0.7, 0.6).into()),
+        transform: Transform::from_xyz(0.55, 0.5, 0.0),
+        ..Default::default()
+    });
+    // plane
+    commands.spawn_bundle(PbrBundle {
+        mesh: meshes.add(Mesh::from(shape::Plane { size: 5.0 })),
+        material: materials.add(Color::rgb(0.3, 0.5, 0.3).into()),
+        ..Default::default()
+    });
+    // light
+    commands.spawn_bundle(PointLightBundle {
+        transform: Transform::from_xyz(5.0, 5.0, 5.0),
+        ..Default::default()
+    });
+    // camera
+    commands.spawn_bundle(PerspectiveCameraBundle {
+        transform: Transform::from_xyz(1.0, 3.0, 4.0).looking_at(Vec3::ZERO, Vec3::Y),
+        ..Default::default()
+    });
+}