ForwardDecal: Allow custom alpha mode and fix using with OIT

crates/bevy_pbr/src/decal/forward.rs

@@ -11,7 +11,6 @@ use bevy_math::{prelude::Rectangle, Quat, Vec2, Vec3};
 use bevy_mesh::{Mesh, Mesh3d, MeshBuilder, MeshVertexBufferLayoutRef, Meshable};
 use bevy_reflect::{Reflect, TypePath};
 use bevy_render::{
-    alpha::AlphaMode,
     render_asset::RenderAssets,
     render_resource::{
         AsBindGroup, AsBindGroupShaderType, CompareFunction, RenderPipelineDescriptor, ShaderType,
@@ -111,8 +110,12 @@ impl AsBindGroupShaderType<ForwardDecalMaterialExtUniform> for ForwardDecalMater
 }
 
 impl MaterialExtension for ForwardDecalMaterialExt {
-    fn alpha_mode() -> Option<AlphaMode> {
-        Some(AlphaMode::Blend)
+    fn enable_shadows() -> bool {
+        false
+    }
+
+    fn enable_prepass() -> bool {
+        false
     }
 
     fn specialize(

crates/bevy_pbr/src/extended_material.rs

@@ -347,7 +347,7 @@ impl<B: Material, E: MaterialExtension> Material for ExtendedMaterial<B, E> {
     }
 
     fn enable_shadows() -> bool {
-        E::enable_prepass()
+        E::enable_shadows()
     }
 
     fn prepass_vertex_shader() -> ShaderRef {

crates/bevy_pbr/src/render/mesh.rs

@@ -2379,8 +2379,9 @@ impl SpecializedMeshPipeline for MeshPipeline {
         let (mut is_opaque, mut alpha_to_coverage_enabled) = (false, false);
         if key.contains(MeshPipelineKey::OIT_ENABLED) && pass == MeshPipelineKey::BLEND_ALPHA {
             label = "oit_mesh_pipeline".into();
-            // TODO tail blending would need alpha blending
-            blend = None;
+            // TODO tail blending would need to return color in shader to do alpha blending
+            // Alpha blending is also needed by forward decals.
+            blend = Some(BlendState::ALPHA_BLENDING);
             shader_defs.push("OIT_ENABLED".into());
             // TODO it should be possible to use this to combine MSAA and OIT
             // alpha_to_coverage_enabled = true;

crates/bevy_pbr/src/render/pbr.wgsl

@@ -90,6 +90,11 @@ fn fragment(
     out.color = main_pass_post_lighting_processing(pbr_input, out.color);
 #endif
 
+#ifdef FORWARD_DECAL
+        out.color.a = min(forward_decal_info.alpha, out.color.a);
+#else
+// Forward decal is incompatible with OIT as it needs to be rendered
+// even it's occluded by opaque objects, but OIT will exclude the fragments.
 #ifdef OIT_ENABLED
     let alpha_mode = pbr_input.material.flags & pbr_types::STANDARD_MATERIAL_FLAGS_ALPHA_MODE_RESERVED_BITS;
     if alpha_mode != pbr_types::STANDARD_MATERIAL_FLAGS_ALPHA_MODE_OPAQUE {
@@ -98,9 +103,6 @@ fn fragment(
         discard;
     }
 #endif // OIT_ENABLED
-
-#ifdef FORWARD_DECAL
-        out.color.a = min(forward_decal_info.alpha, out.color.a);
 #endif
 
         return out;

examples/3d/decal.rs

@@ -27,18 +27,60 @@ fn setup(
 ) {
     // Spawn the forward decal
     commands.spawn((
-        Name::new("Decal"),
+        Name::new("Decal Red"),
         ForwardDecal,
         MeshMaterial3d(decal_standard_materials.add(ForwardDecalMaterial {
             base: StandardMaterial {
                 base_color_texture: Some(asset_server.load("textures/uv_checker_bw.png")),
+                base_color: Color::srgba(1.0, 0.0, 0.0, 0.5),
+                // Use alpha blending for decals
+                alpha_mode: AlphaMode::Blend,
+                depth_bias: 0.0,
                 ..default()
             },
             extension: ForwardDecalMaterialExt {
                 depth_fade_factor: 1.0,
             },
         })),
-        Transform::from_scale(Vec3::splat(4.0)),
+        Transform::from_scale(Vec3::splat(2.0)).with_translation(Vec3::new(0.5, 0.0, -0.5)),
+    ));
+
+    commands.spawn((
+        Name::new("Decal Green"),
+        ForwardDecal,
+        MeshMaterial3d(decal_standard_materials.add(ForwardDecalMaterial {
+            base: StandardMaterial {
+                base_color_texture: Some(asset_server.load("textures/uv_checker_bw.png")),
+                base_color: Color::srgba(0.0, 1.0, 0.0, 0.5),
+                // Use alpha blending for decals
+                alpha_mode: AlphaMode::Blend,
+                depth_bias: 1.0,
+                ..default()
+            },
+            extension: ForwardDecalMaterialExt {
+                depth_fade_factor: 1.0,
+            },
+        })),
+        Transform::from_scale(Vec3::splat(2.0)).with_translation(Vec3::new(-0.5, 0.0, -0.5)),
+    ));
+
+    commands.spawn((
+        Name::new("Decal Blue"),
+        ForwardDecal,
+        MeshMaterial3d(decal_standard_materials.add(ForwardDecalMaterial {
+            base: StandardMaterial {
+                base_color_texture: Some(asset_server.load("textures/uv_checker_bw.png")),
+                base_color: Color::srgba(0.0, 0.0, 1.0, 0.5),
+                // Use alpha blending for decals
+                alpha_mode: AlphaMode::Blend,
+                depth_bias: 2.0,
+                ..default()
+            },
+            extension: ForwardDecalMaterialExt {
+                depth_fade_factor: 1.0,
+            },
+        })),
+        Transform::from_scale(Vec3::splat(2.0)).with_translation(Vec3::new(0.0, 0.0, 0.5)),
     ));
 
     commands.spawn((