bevy_pbr: Fix incorrect and unnecessary normal-mapping code (#5766)

# Objective

- Fixes #4019 
- Fix lighting of double-sided materials when using a negative scale
- The FlightHelmet.gltf model's hose uses a double-sided material. Loading the model with a uniform scale of -1.0, and comparing against Blender, it was identified that negating the world-space tangent, bitangent, and interpolated normal produces incorrect lighting. Discussion with Morten Mikkelsen clarified that this is both incorrect and unnecessary.

## Solution

- Remove the code that negates the T, B, and N vectors (the interpolated world-space tangent, calculated world-space bitangent, and interpolated world-space normal) when seeing the back face of a double-sided material with negative scale.
- Negate the world normal for a double-sided back face only when not using normal mapping

### Before, on `main`, flipping T, B, and N

<img width="932" alt="Screenshot 2022-08-22 at 15 11 53" src="https://user-images.githubusercontent.com/302146/185965366-f776ff2c-cfa1-46d1-9c84-fdcb399c273c.png">

### After, on this PR

<img width="932" alt="Screenshot 2022-08-22 at 15 12 11" src="https://user-images.githubusercontent.com/302146/185965420-8be493e2-3b1a-4188-bd13-fd6b17a76fe7.png">

### Double-sided material without normal maps

https://user-images.githubusercontent.com/302146/185988113-44a384e7-0b55-4946-9b99-20f8c803ab7e.mp4

---

## Changelog

- Fixed: Lighting of normal-mapped, double-sided materials applied to models with negative scale
- Fixed: Lighting and shadowing of back faces with no normal-mapping and a double-sided material

## Migration Guide

`prepare_normal` from the `bevy_pbr::pbr_functions` shader import has been reworked.

Before:
```rust
    pbr_input.world_normal = in.world_normal;

    pbr_input.N = prepare_normal(
        pbr_input.material.flags,
        in.world_normal,
#ifdef VERTEX_TANGENTS
#ifdef STANDARDMATERIAL_NORMAL_MAP
        in.world_tangent,
#endif
#endif
        in.uv,
        in.is_front,
    );
```

After:
```rust
    pbr_input.world_normal = prepare_world_normal(
        in.world_normal,
        (material.flags & STANDARD_MATERIAL_FLAGS_DOUBLE_SIDED_BIT) != 0u,
        in.is_front,
    );

    pbr_input.N = apply_normal_mapping(
        pbr_input.material.flags,
        pbr_input.world_normal,
#ifdef VERTEX_TANGENTS
#ifdef STANDARDMATERIAL_NORMAL_MAP
        in.world_tangent,
#endif
#endif
        in.uv,
    );
```

assets/shaders/array_texture.wgsl

@@ -34,20 +34,23 @@ fn fragment(in: FragmentInput) -> @location(0) vec4<f32> {
 
     pbr_input.frag_coord = in.frag_coord;
     pbr_input.world_position = in.world_position;
-    pbr_input.world_normal = in.world_normal;
+    pbr_input.world_normal = prepare_world_normal(
+        in.world_normal,
+        (pbr_input.material.flags & STANDARD_MATERIAL_FLAGS_DOUBLE_SIDED_BIT) != 0u,
+        in.is_front,
+    );
 
     pbr_input.is_orthographic = view.projection[3].w == 1.0;
 
-    pbr_input.N = prepare_normal(
+    pbr_input.N = apply_normal_mapping(
         pbr_input.material.flags,
-        in.world_normal,
+        pbr_input.world_normal,
 #ifdef VERTEX_TANGENTS
 #ifdef STANDARDMATERIAL_NORMAL_MAP
         in.world_tangent,
 #endif
 #endif
         in.uv,
-        in.is_front,
     );
     pbr_input.V = calculate_view(in.world_position, pbr_input.is_orthographic);
 

crates/bevy_pbr/src/render/mesh.wgsl

@@ -70,7 +70,6 @@ fn vertex(vertex: Vertex) -> VertexOutput {
 }
 
 struct FragmentInput {
-    @builtin(front_facing) is_front: bool,
     #import bevy_pbr::mesh_vertex_output
 };
 

crates/bevy_pbr/src/render/pbr.wgsl

@@ -69,13 +69,17 @@ fn fragment(in: FragmentInput) -> @location(0) vec4<f32> {
 
         pbr_input.frag_coord = in.frag_coord;
         pbr_input.world_position = in.world_position;
-        pbr_input.world_normal = in.world_normal;
+        pbr_input.world_normal = prepare_world_normal(
+            in.world_normal,
+            (material.flags & STANDARD_MATERIAL_FLAGS_DOUBLE_SIDED_BIT) != 0u,
+            in.is_front,
+        );
 
         pbr_input.is_orthographic = view.projection[3].w == 1.0;
 
-        pbr_input.N = prepare_normal(
+        pbr_input.N = apply_normal_mapping(
             material.flags,
-            in.world_normal,
+            pbr_input.world_normal,
 #ifdef VERTEX_TANGENTS
 #ifdef STANDARDMATERIAL_NORMAL_MAP
             in.world_tangent,
@@ -84,7 +88,6 @@ fn fragment(in: FragmentInput) -> @location(0) vec4<f32> {
 #ifdef VERTEX_UVS
             in.uv,
 #endif
-            in.is_front,
         );
         pbr_input.V = calculate_view(in.world_position, pbr_input.is_orthographic);
         output_color = pbr(pbr_input);

crates/bevy_pbr/src/render/pbr_functions.wgsl

@@ -23,9 +23,23 @@ fn alpha_discard(material: StandardMaterial, output_color: vec4<f32>) -> vec4<f3
     return color;
 }
 
-// NOTE: This ensures that the world_normal is normalized and if
-// vertex tangents and normal maps then normal mapping may be applied.
-fn prepare_normal(
+fn prepare_world_normal(
+    world_normal: vec3<f32>,
+    double_sided: bool,
+    is_front: bool,
+) -> vec3<f32> {
+    var output: vec3<f32> = world_normal;
+#ifndef VERTEX_TANGENTS
+#ifndef STANDARDMATERIAL_NORMAL_MAP
+    // NOTE: When NOT using normal-mapping, if looking at the back face of a double-sided
+    // material, the normal needs to be inverted. This is a branchless version of that.
+    output = (f32(!double_sided || is_front) * 2.0 - 1.0) * output;
+#endif
+#endif
+    return output;
+}
+
+fn apply_normal_mapping(
     standard_material_flags: u32,
     world_normal: vec3<f32>,
 #ifdef VERTEX_TANGENTS
@@ -36,7 +50,6 @@ fn prepare_normal(
 #ifdef VERTEX_UVS
     uv: vec2<f32>,
 #endif
-    is_front: bool,
 ) -> vec3<f32> {
     // NOTE: The mikktspace method of normal mapping explicitly requires that the world normal NOT
     // be re-normalized in the fragment shader. This is primarily to match the way mikktspace
@@ -57,18 +70,6 @@ fn prepare_normal(
 #endif
 #endif
 
-    if ((standard_material_flags & STANDARD_MATERIAL_FLAGS_DOUBLE_SIDED_BIT) != 0u) {
-        if (!is_front) {
-            N = -N;
-#ifdef VERTEX_TANGENTS
-#ifdef STANDARDMATERIAL_NORMAL_MAP
-            T = -T;
-            B = -B;
-#endif
-#endif
-        }
-    }
-
 #ifdef VERTEX_TANGENTS
 #ifdef VERTEX_UVS
 #ifdef STANDARDMATERIAL_NORMAL_MAP

crates/bevy_sprite/src/mesh2d/color_material.wgsl

@@ -19,7 +19,6 @@ var texture_sampler: sampler;
 var<uniform> mesh: Mesh2d;
 
 struct FragmentInput {
-    @builtin(front_facing) is_front: bool,
     #import bevy_sprite::mesh2d_vertex_output
 };
 

crates/bevy_sprite/src/mesh2d/mesh2d.wgsl

@@ -55,7 +55,6 @@ fn vertex(vertex: Vertex) -> VertexOutput {
 }
 
 struct FragmentInput {
-    @builtin(front_facing) is_front: bool,
     #import bevy_sprite::mesh2d_vertex_output
 };