[Merged by Bors] - bevy_pbr: Fix tangent and normal normalization

crates/bevy_pbr/src/render/mesh.rs

@@ -9,7 +9,7 @@ use bevy_ecs::{
     prelude::*,
     system::{lifetimeless::*, SystemParamItem, SystemState},
 };
-use bevy_math::{Mat4, Vec2};
+use bevy_math::{Mat3A, Mat4, Vec2};
 use bevy_reflect::TypeUuid;
 use bevy_render::{
     extract_component::{ComponentUniforms, DynamicUniformIndex, UniformComponentPlugin},
@@ -117,6 +117,9 @@ bitflags::bitflags! {
     #[repr(transparent)]
     struct MeshFlags: u32 {
         const SHADOW_RECEIVER            = (1 << 0);
+        // Indicates the sign of the determinant of the 3x3 model matrix. If the sign is positive,
+        // then the flag should be set, else it should not be set.
+        const SIGN_DETERMINANT_MODEL_3X3 = (1 << 31);
         const NONE                       = 0;
         const UNINITIALIZED              = 0xFFFF;
     }
@@ -143,13 +146,16 @@ pub fn extract_meshes(
 
     for (entity, _, transform, handle, not_receiver, not_caster) in visible_meshes {
         let transform = transform.compute_matrix();
-        let shadow_receiver_flags = if not_receiver.is_some() {
-            MeshFlags::empty().bits
+        let mut flags = if not_receiver.is_some() {
+            MeshFlags::empty()
         } else {
-            MeshFlags::SHADOW_RECEIVER.bits
+            MeshFlags::SHADOW_RECEIVER
         };
+        if Mat3A::from_mat4(transform).determinant().is_sign_positive() {
+            flags |= MeshFlags::SIGN_DETERMINANT_MODEL_3X3;
+        }
         let uniform = MeshUniform {
-            flags: shadow_receiver_flags,
+            flags: flags.bits,
             transform,
             inverse_transpose_model: transform.inverse().transpose(),
         };

crates/bevy_pbr/src/render/mesh_functions.wgsl

@@ -17,20 +17,47 @@ fn mesh_position_local_to_clip(model: mat4x4<f32>, vertex_position: vec4<f32>) -
 }
 
 fn mesh_normal_local_to_world(vertex_normal: vec3<f32>) -> vec3<f32> {
-    return mat3x3<f32>(
-        mesh.inverse_transpose_model[0].xyz,
-        mesh.inverse_transpose_model[1].xyz,
-        mesh.inverse_transpose_model[2].xyz
-    ) * vertex_normal;
+    // NOTE: The mikktspace method of normal mapping requires that the world normal is
+    // re-normalized in the vertex shader to match the way mikktspace bakes vertex tangents
+    // and normal maps so that the exact inverse process is applied when shading. Blender, Unity,
+    // Unreal Engine, Godot, and more all use the mikktspace method. Do not change this code
+    // unless you really know what you are doing.
+    // http://www.mikktspace.com/
+    return normalize(
+        mat3x3<f32>(
+            mesh.inverse_transpose_model[0].xyz,
+            mesh.inverse_transpose_model[1].xyz,
+            mesh.inverse_transpose_model[2].xyz
+        ) * vertex_normal
+    );
+}
+
+// Calculates the sign of the determinant of the 3x3 model matrix based on a
+// mesh flag
+fn sign_determinant_model_3x3() -> f32 {
+    // bool(u32) is false if 0u else true
+    // f32(bool) is 1.0 if true else 0.0
+    // * 2.0 - 1.0 remaps 0.0 or 1.0 to -1.0 or 1.0 respectively
+    return f32(bool(mesh.flags & MESH_FLAGS_SIGN_DETERMINANT_MODEL_3X3_BIT)) * 2.0 - 1.0;
 }
 
 fn mesh_tangent_local_to_world(model: mat4x4<f32>, vertex_tangent: vec4<f32>) -> vec4<f32> {
+    // NOTE: The mikktspace method of normal mapping requires that the world tangent is
+    // re-normalized in the vertex shader to match the way mikktspace bakes vertex tangents
+    // and normal maps so that the exact inverse process is applied when shading. Blender, Unity,
+    // Unreal Engine, Godot, and more all use the mikktspace method. Do not change this code
+    // unless you really know what you are doing.
+    // http://www.mikktspace.com/
     return vec4<f32>(
-        mat3x3<f32>(
-            model[0].xyz,
-            model[1].xyz,
-            model[2].xyz
-        ) * vertex_tangent.xyz,
-        vertex_tangent.w
+        normalize(
+            mat3x3<f32>(
+                model[0].xyz,
+                model[1].xyz,
+                model[2].xyz
+            ) * vertex_tangent.xyz
+        ),
+        // NOTE: Multiplying by the sign of the determinant of the 3x3 model matrix accounts for
+        // situations such as negative scaling.
+        vertex_tangent.w * sign_determinant_model_3x3()
     );
 }

crates/bevy_pbr/src/render/mesh_types.wgsl

@@ -14,3 +14,5 @@ struct SkinnedMesh {
 #endif
 
 let MESH_FLAGS_SHADOW_RECEIVER_BIT: u32 = 1u;
+// 2^31 - if the flag is set, the sign is positive, else it is negative
+let MESH_FLAGS_SIGN_DETERMINANT_MODEL_3X3_BIT: u32 = 2147483648u;

crates/bevy_pbr/src/render/pbr_functions.wgsl

@@ -15,7 +15,13 @@ fn prepare_normal(
 #endif
     is_front: bool,
 ) -> vec3<f32> {
-    var N: vec3<f32> = normalize(world_normal);
+    // 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
+    // bakes vertex tangents and normal maps so that this is the exact inverse. Blender, Unity,
+    // Unreal Engine, Godot, and more all use the mikktspace method. Do not change this code
+    // unless you really know what you are doing.
+    // http://www.mikktspace.com/
+    var N: vec3<f32> = world_normal;
 
 #ifdef VERTEX_TANGENTS
 #ifdef STANDARDMATERIAL_NORMAL_MAP
@@ -236,7 +242,7 @@ fn pbr(
 fn tone_mapping(in: vec4<f32>) -> vec4<f32> {
     // tone_mapping
     return vec4<f32>(reinhard_luminance(in.rgb), in.a);
-    
+
     // Gamma correction.
     // Not needed with sRGB buffer
     // output_color.rgb = pow(output_color.rgb, vec3(1.0 / 2.2));