bevy_pbr: Add support for 2-component normal map textures

In another branch there is work on adding calculation of vertex tangents using
the mikktspace algorithm. This algorithm works in a precise way and does not
normalize the tangent-space normals in order to exactly invert the calculations
done when creating the vertex tangent. If this level of precision is needed,
then a 3-component normal map must be used. Otherwise, a 2-component normal map
may be used, or the vertex tangents must be calculated in such a way as to
account for 2-component normal maps and the subsequent derivation of the third
component assuming the tangent-space normal is a unit normal.

crates/bevy_pbr/src/pbr_material.rs

@@ -113,6 +113,7 @@ bitflags::bitflags! {
         const ALPHA_MODE_OPAQUE          = (1 << 6);
         const ALPHA_MODE_MASK            = (1 << 7);
         const ALPHA_MODE_BLEND           = (1 << 8);
+        const TWO_COMPONENT_NORMAL_MAP   = (1 << 9);
         const NONE                       = 0;
         const UNINITIALIZED              = 0xFFFF;
     }
@@ -235,6 +236,22 @@ impl RenderAsset for StandardMaterial {
             flags |= StandardMaterialFlags::UNLIT;
         }
         let has_normal_map = material.normal_map_texture.is_some();
+        if has_normal_map {
+            match gpu_images
+                .get(material.normal_map_texture.as_ref().unwrap())
+                .unwrap()
+                .texture_format
+            {
+                // All 2-component unorm formats
+                TextureFormat::Rg8Unorm
+                | TextureFormat::Rg16Unorm
+                | TextureFormat::Bc5RgUnorm
+                | TextureFormat::EacRg11Unorm => {
+                    flags |= StandardMaterialFlags::TWO_COMPONENT_NORMAL_MAP
+                }
+                _ => {}
+            }
+        }
         // NOTE: 0.5 is from the glTF default - do we want this?
         let mut alpha_cutoff = 0.5;
         match material.alpha_mode {

crates/bevy_pbr/src/render/mesh.rs

@@ -329,6 +329,7 @@ impl FromWorld for MeshPipeline {
             GpuImage {
                 texture,
                 texture_view,
+                texture_format: image.texture_descriptor.format,
                 sampler,
                 size: Size::new(
                     image.texture_descriptor.size.width as f32,

crates/bevy_pbr/src/render/pbr.wgsl

@@ -58,6 +58,7 @@ let STANDARD_MATERIAL_FLAGS_UNLIT_BIT: u32                      = 32u;
 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;
 
 [[group(1), binding(0)]]
 var<uniform> material: StandardMaterial;
@@ -513,7 +514,16 @@ fn fragment(in: FragmentInput) -> [[location(0)]] vec4<f32> {
 #ifdef VERTEX_TANGENTS
 #ifdef STANDARDMATERIAL_NORMAL_MAP
         let TBN = mat3x3<f32>(T, B, N);
-        N = TBN * normalize(textureSample(normal_map_texture, normal_map_sampler, in.uv).rgb * 2.0 - 1.0);
+        // Nt is the tangent-space normal.
+        var Nt: vec3<f32>;
+        if ((material.flags & STANDARD_MATERIAL_FLAGS_TWO_COMPONENT_NORMAL_MAP) != 0u) {
+            // Only use the xy components and derive z for 2-component normal maps.
+            Nt = vec3<f32>(textureSample(normal_map_texture, normal_map_sampler, in.uv).rg * 2.0 - 1.0, 0.0);
+            Nt.z = sqrt(1.0 - Nt.x * Nt.x - Nt.y * Nt.y);
+        } else {
+            Nt = textureSample(normal_map_texture, normal_map_sampler, in.uv).rgb * 2.0 - 1.0;
+        }
+        N = TBN * Nt;
 #endif
 #endif
 

crates/bevy_render/src/texture/image.rs

@@ -538,6 +538,7 @@ impl TextureFormatPixelInfo for TextureFormat {
 pub struct GpuImage {
     pub texture: Texture,
     pub texture_view: TextureView,
+    pub texture_format: TextureFormat,
     pub sampler: Sampler,
     pub size: Size,
 }
@@ -602,6 +603,7 @@ impl RenderAsset for Image {
         Ok(GpuImage {
             texture,
             texture_view,
+            texture_format: image.texture_descriptor.format,
             sampler,
             size,
         })

crates/bevy_sprite/src/mesh2d/mesh.rs

@@ -194,6 +194,7 @@ impl FromWorld for Mesh2dPipeline {
             GpuImage {
                 texture,
                 texture_view,
+                texture_format: image.texture_descriptor.format,
                 sampler,
                 size: Size::new(
                     image.texture_descriptor.size.width as f32,