Fix forward decal depth_fade_factor. (#18772)

Fixes #18758

crates/bevy_pbr/src/decal/forward.rs

@@ -7,6 +7,9 @@ use bevy_asset::{load_internal_asset, weak_handle, Asset, Assets, Handle};
 use bevy_ecs::component::Component;
 use bevy_math::{prelude::Rectangle, Quat, Vec2, Vec3};
 use bevy_reflect::{Reflect, TypePath};
+use bevy_render::render_asset::RenderAssets;
+use bevy_render::render_resource::{AsBindGroupShaderType, ShaderType};
+use bevy_render::texture::GpuImage;
 use bevy_render::{
     alpha::AlphaMode,
     mesh::{Mesh, Mesh3d, MeshBuilder, MeshVertexBufferLayoutRef, Meshable},
@@ -86,16 +89,36 @@ pub type ForwardDecalMaterial<B: Material> = ExtendedMaterial<B, ForwardDecalMat
 /// The `FORWARD_DECAL` shader define will be made available to your shader so that you can gate
 /// the forward decal code behind an ifdef.
 #[derive(Asset, AsBindGroup, TypePath, Clone, Debug)]
+#[uniform(200, ForwardDecalMaterialExtUniform)]
 pub struct ForwardDecalMaterialExt {
-    /// Controls how far away a surface must be before the decal will stop blending with it, and instead render as opaque.
+    /// Controls the distance threshold for decal blending with surfaces.
     ///
-    /// Decreasing this value will cause the decal to blend only to surfaces closer to it.
+    /// This parameter determines how far away a surface can be before the decal no longer blends
+    /// with it and instead renders with full opacity.
+    ///
+    /// Lower values cause the decal to only blend with close surfaces, while higher values allow
+    /// blending with more distant surfaces.
     ///
     /// Units are in meters.
-    #[uniform(200)]
     pub depth_fade_factor: f32,
 }
 
+#[derive(Clone, Default, ShaderType)]
+pub struct ForwardDecalMaterialExtUniform {
+    pub inv_depth_fade_factor: f32,
+}
+
+impl AsBindGroupShaderType<ForwardDecalMaterialExtUniform> for ForwardDecalMaterialExt {
+    fn as_bind_group_shader_type(
+        &self,
+        _images: &RenderAssets<GpuImage>,
+    ) -> ForwardDecalMaterialExtUniform {
+        ForwardDecalMaterialExtUniform {
+            inv_depth_fade_factor: 1.0 / self.depth_fade_factor.max(0.001),
+        }
+    }
+}
+
 impl MaterialExtension for ForwardDecalMaterialExt {
     fn alpha_mode() -> Option<AlphaMode> {
         Some(AlphaMode::Blend)

crates/bevy_pbr/src/decal/forward_decal.wgsl

@@ -11,7 +11,7 @@
 #import bevy_render::maths::project_onto
 
 @group(2) @binding(200)
-var<uniform> depth_fade_factor: f32;
+var<uniform> inv_depth_fade_factor: f32;
 
 struct ForwardDecalInformation {
     world_position: vec4<f32>,
@@ -46,7 +46,7 @@ fn get_forward_decal_info(in: VertexOutput) -> ForwardDecalInformation {
     let uv = in.uv + delta_uv;
 
     let world_position = vec4(in.world_position.xyz + V * diff_depth_abs, in.world_position.w);
-    let alpha = saturate(1.0 - normal_depth * depth_fade_factor);
+    let alpha = saturate(1.0 - (normal_depth * inv_depth_fade_factor));
 
     return ForwardDecalInformation(world_position, uv, alpha);
 }