bevy_pbr: Support flipping tangent space normal map y for DirectX normal maps (#4433)

# Objective

- Normal maps authored for DirectX use a left-handed convention and have their tangent space normal in the texture inverted from what we need. Support this.
- Details here: https://doc.babylonjs.com/divingDeeper/materials/advanced/normalMaps

## Solution

- Add a `StandardMaterial` `flip_normal_map_y` boolean field
- Add a `STANDARDMATERIAL_FLIP_NORMAL_MAP_Y` flag to `StandardMaterialFlags` and in the PBR shader
- Flip the y-component of the tangent space normal just after sampling it from the normal map texture

## Screenshots

### Before

<img width="1392" alt="Screenshot 2022-04-06 at 21 04 44" src="https://user-images.githubusercontent.com/302146/162050314-e7bfaaf6-9ee1-4756-9821-f6f5ff78f508.png">

### After

<img width="1392" alt="Screenshot 2022-04-06 at 21 03 39" src="https://user-images.githubusercontent.com/302146/162050255-36ee0745-1d79-4fd2-9a1c-18085376b643.png">

---

## Changelog

- Added: Support for flipping the normal map texture y component for normal maps authored for use with DirectX

Author: superdump

crates/bevy_pbr/src/pbr_material.rs

@@ -47,6 +47,9 @@ pub struct StandardMaterial {
     /// defaults to 0.5 which is mapped to 4% reflectance in the shader
     pub reflectance: f32,
     pub normal_map_texture: Option<Handle<Image>>,
+    /// Normal map textures authored for DirectX have their y-component flipped. Set this to flip
+    /// it to right-handed conventions.
+    pub flip_normal_map_y: bool,
     pub occlusion_texture: Option<Handle<Image>>,
     /// Support two-sided lighting by automatically flipping the normals for "back" faces
     /// within the PBR lighting shader.
@@ -84,6 +87,7 @@ impl Default for StandardMaterial {
             reflectance: 0.5,
             occlusion_texture: None,
             normal_map_texture: None,
+            flip_normal_map_y: false,
             double_sided: false,
             cull_mode: Some(Face::Back),
             unlit: false,
@@ -124,6 +128,7 @@ bitflags::bitflags! {
         const ALPHA_MODE_MASK            = (1 << 7);
         const ALPHA_MODE_BLEND           = (1 << 8);
         const TWO_COMPONENT_NORMAL_MAP   = (1 << 9);
+        const FLIP_NORMAL_MAP_Y          = (1 << 10);
         const NONE                       = 0;
         const UNINITIALIZED              = 0xFFFF;
     }
@@ -262,6 +267,9 @@ impl RenderAsset for StandardMaterial {
                 }
                 _ => {}
             }
+            if material.flip_normal_map_y {
+                flags |= StandardMaterialFlags::FLIP_NORMAL_MAP_Y;
+            }
         }
         // NOTE: 0.5 is from the glTF default - do we want this?
         let mut alpha_cutoff = 0.5;

crates/bevy_pbr/src/render/pbr.wgsl

@@ -59,6 +59,7 @@ let STANDARD_MATERIAL_FLAGS_ALPHA_MODE_OPAQUE: u32              = 64u;
 let STANDARD_MATERIAL_FLAGS_ALPHA_MODE_MASK: u32                = 128u;
 let STANDARD_MATERIAL_FLAGS_ALPHA_MODE_BLEND: u32               = 256u;
 let STANDARD_MATERIAL_FLAGS_TWO_COMPONENT_NORMAL_MAP: u32       = 512u;
+let STANDARD_MATERIAL_FLAGS_FLIP_NORMAL_MAP_Y: u32              = 1024u;
 
 [[group(1), binding(0)]]
 var<uniform> material: StandardMaterial;
@@ -525,6 +526,10 @@ fn fragment(in: FragmentInput) -> [[location(0)]] vec4<f32> {
         } else {
             Nt = textureSample(normal_map_texture, normal_map_sampler, in.uv).rgb * 2.0 - 1.0;
         }
+        // Normal maps authored for DirectX require flipping the y component
+        if ((material.flags & STANDARD_MATERIAL_FLAGS_FLIP_NORMAL_MAP_Y) != 0u) {
+            Nt.y = -Nt.y;
+        }
         N = normalize(TBN * Nt);
 #endif
 #endif