Make FOG_ENABLED a shader_def instead of material flag (#13783)

# Objective

- If the fog is disabled it still generates a useless branch which can
hurt performance

## Solution

- Make the flag a shader_def instead

## Testing

- I tested enabling/disabling fog works as expected per-material in the
fog example
- I also tested that scenes that don't add the FogSettings resource
still work correctly

## Review notes

I'm not sure how to handle the removed material flag. Right now I just
commented it out and added a not to reuse it instead of creating a new
one.

crates/bevy_pbr/src/deferred/pbr_deferred_types.wgsl

@@ -2,18 +2,16 @@
 
 #import bevy_pbr::{
     mesh_types::MESH_FLAGS_SHADOW_RECEIVER_BIT,
-    pbr_types::{STANDARD_MATERIAL_FLAGS_FOG_ENABLED_BIT, STANDARD_MATERIAL_FLAGS_UNLIT_BIT},
+    pbr_types::{STANDARD_MATERIAL_FLAGS_UNLIT_BIT},
 }
 
 // Maximum of 8 bits available
 const DEFERRED_FLAGS_UNLIT_BIT: u32                 = 1u;
-const DEFERRED_FLAGS_FOG_ENABLED_BIT: u32           = 2u;
-const DEFERRED_MESH_FLAGS_SHADOW_RECEIVER_BIT: u32  = 4u;
+const DEFERRED_MESH_FLAGS_SHADOW_RECEIVER_BIT: u32  = 2u;
 
 fn deferred_flags_from_mesh_material_flags(mesh_flags: u32, mat_flags: u32) -> u32 {
     var flags = 0u;
     flags |= u32((mesh_flags & MESH_FLAGS_SHADOW_RECEIVER_BIT) != 0u) * DEFERRED_MESH_FLAGS_SHADOW_RECEIVER_BIT;
-    flags |= u32((mat_flags & STANDARD_MATERIAL_FLAGS_FOG_ENABLED_BIT) != 0u) * DEFERRED_FLAGS_FOG_ENABLED_BIT;
     flags |= u32((mat_flags & STANDARD_MATERIAL_FLAGS_UNLIT_BIT) != 0u) * DEFERRED_FLAGS_UNLIT_BIT;
     return flags;
 }
@@ -22,7 +20,6 @@ fn mesh_material_flags_from_deferred_flags(deferred_flags: u32) -> vec2<u32> {
     var mat_flags = 0u;
     var mesh_flags = 0u;
     mesh_flags |= u32((deferred_flags & DEFERRED_MESH_FLAGS_SHADOW_RECEIVER_BIT) != 0u) * MESH_FLAGS_SHADOW_RECEIVER_BIT;
-    mat_flags |= u32((deferred_flags & DEFERRED_FLAGS_FOG_ENABLED_BIT) != 0u) * STANDARD_MATERIAL_FLAGS_FOG_ENABLED_BIT;
     mat_flags |= u32((deferred_flags & DEFERRED_FLAGS_UNLIT_BIT) != 0u) * STANDARD_MATERIAL_FLAGS_UNLIT_BIT;
     return vec2(mesh_flags, mat_flags);
 }

crates/bevy_pbr/src/pbr_material.rs

@@ -867,7 +867,7 @@ bitflags::bitflags! {
         const UNLIT                      = 1 << 5;
         const TWO_COMPONENT_NORMAL_MAP   = 1 << 6;
         const FLIP_NORMAL_MAP_Y          = 1 << 7;
-        const FOG_ENABLED                = 1 << 8;
+        // const UNUSED                  = 1 << 8; // USE THIS IF YOU ADD A NEW FLAG
         const DEPTH_MAP                  = 1 << 9; // Used for parallax mapping
         const SPECULAR_TRANSMISSION_TEXTURE = 1 << 10;
         const THICKNESS_TEXTURE          = 1 << 11;
@@ -975,9 +975,6 @@ impl AsBindGroupShaderType<StandardMaterialUniform> for StandardMaterial {
         if self.unlit {
             flags |= StandardMaterialFlags::UNLIT;
         }
-        if self.fog_enabled {
-            flags |= StandardMaterialFlags::FOG_ENABLED;
-        }
         if self.depth_map.is_some() {
             flags |= StandardMaterialFlags::DEPTH_MAP;
         }
@@ -1115,6 +1112,7 @@ bitflags! {
         const CLEARCOAT_UV             = 0x040000;
         const CLEARCOAT_ROUGHNESS_UV   = 0x080000;
         const CLEARCOAT_NORMAL_UV      = 0x100000;
+        const FOG_ENABLED              = 0x200000;
         const DEPTH_BIAS               = 0xffffffff_00000000;
     }
 }
@@ -1222,6 +1220,7 @@ impl From<&StandardMaterial> for StandardMaterialKey {
                 material.clearcoat_normal_channel != UvChannel::Uv0,
             );
         }
+        key.set(StandardMaterialKey::FOG_ENABLED, material.fog_enabled);
 
         key.insert(StandardMaterialKey::from_bits_retain(
             (material.depth_bias as u64) << STANDARD_MATERIAL_KEY_DEPTH_BIAS_SHIFT,
@@ -1378,6 +1377,7 @@ impl Material for StandardMaterial {
                     StandardMaterialKey::ANISOTROPY_UV,
                     "STANDARD_MATERIAL_ANISOTROPY_UV",
                 ),
+                (StandardMaterialKey::FOG_ENABLED, "FOG_ENABLED"),
             ] {
                 if key.bind_group_data.intersects(flags) {
                     shader_defs.push(shader_def.into());

crates/bevy_pbr/src/render/pbr_functions.wgsl

@@ -809,9 +809,11 @@ fn main_pass_post_lighting_processing(
     var output_color = input_color;
 
     // fog
-    if (view_bindings::fog.mode != mesh_view_types::FOG_MODE_OFF && (pbr_input.material.flags & pbr_types::STANDARD_MATERIAL_FLAGS_FOG_ENABLED_BIT) != 0u) {
+#ifdef FOG_ENABLED
+    if (view_bindings::fog.mode != mesh_view_types::FOG_MODE_OFF) {
         output_color = apply_fog(view_bindings::fog, output_color, pbr_input.world_position.xyz, view_bindings::view.world_position.xyz);
     }
+#endif // FOG_ENABLED
 
 #ifdef TONEMAP_IN_SHADER
     output_color = tone_mapping(output_color, view_bindings::view.color_grading);

crates/bevy_pbr/src/render/pbr_types.wgsl

@@ -42,7 +42,7 @@ const STANDARD_MATERIAL_FLAGS_DOUBLE_SIDED_BIT: u32               = 16u;
 const STANDARD_MATERIAL_FLAGS_UNLIT_BIT: u32                      = 32u;
 const STANDARD_MATERIAL_FLAGS_TWO_COMPONENT_NORMAL_MAP: u32       = 64u;
 const STANDARD_MATERIAL_FLAGS_FLIP_NORMAL_MAP_Y: u32              = 128u;
-const STANDARD_MATERIAL_FLAGS_FOG_ENABLED_BIT: u32                = 256u;
+// const STANDARD_MATERIAL_FLAGS_UNUSED_BIT: u32                  = 256u;
 const STANDARD_MATERIAL_FLAGS_DEPTH_MAP_BIT: u32                  = 512u;
 const STANDARD_MATERIAL_FLAGS_SPECULAR_TRANSMISSION_TEXTURE_BIT: u32 = 1024u;
 const STANDARD_MATERIAL_FLAGS_THICKNESS_TEXTURE_BIT: u32          = 2048u;