Only include distance fog in the PBR shader if the view uses it. (#17495)

Right now, we always include distance fog in the shader, which is
unfortunate as it's complex code and is rare. This commit changes it to
be a `#define` instead. I haven't confirmed that removing distance fog
meaningfully reduces VGPR usage, but it can't hurt.

crates/bevy_pbr/src/deferred/mod.rs

@@ -6,7 +6,8 @@ use crate::{
     TONEMAPPING_LUT_TEXTURE_BINDING_INDEX,
 };
 use crate::{
-    MeshPipelineKey, ShadowFilteringMethod, ViewFogUniformOffset, ViewLightsUniformOffset,
+    DistanceFog, MeshPipelineKey, ShadowFilteringMethod, ViewFogUniformOffset,
+    ViewLightsUniformOffset,
 };
 use bevy_app::prelude::*;
 use bevy_asset::{load_internal_asset, Handle};
@@ -328,6 +329,10 @@ impl SpecializedRenderPipeline for DeferredLightingLayout {
             shader_defs.push("HAS_PREVIOUS_MORPH".into());
         }
 
+        if key.contains(MeshPipelineKey::DISTANCE_FOG) {
+            shader_defs.push("DISTANCE_FOG".into());
+        }
+
         // Always true, since we're in the deferred lighting pipeline
         shader_defs.push("DEFERRED_PREPASS".into());
 
@@ -437,6 +442,7 @@ pub fn prepare_deferred_lighting_pipelines(
             (
                 Has<ScreenSpaceAmbientOcclusion>,
                 Has<ScreenSpaceReflectionsUniform>,
+                Has<DistanceFog>,
             ),
             (
                 Has<NormalPrepass>,
@@ -455,7 +461,7 @@ pub fn prepare_deferred_lighting_pipelines(
         tonemapping,
         dither,
         shadow_filter_method,
-        (ssao, ssr),
+        (ssao, ssr, distance_fog),
         (normal_prepass, depth_prepass, motion_vector_prepass),
         has_environment_maps,
         has_irradiance_volumes,
@@ -507,6 +513,9 @@ pub fn prepare_deferred_lighting_pipelines(
         if ssr {
             view_key |= MeshPipelineKey::SCREEN_SPACE_REFLECTIONS;
         }
+        if distance_fog {
+            view_key |= MeshPipelineKey::DISTANCE_FOG;
+        }
 
         // We don't need to check to see whether the environment map is loaded
         // because [`gather_light_probes`] already checked that for us before

crates/bevy_pbr/src/material.rs

@@ -702,6 +702,7 @@ pub fn queue_material_meshes<M: Material>(
         Option<&Camera3d>,
         Has<TemporalJitter>,
         Option<&Projection>,
+        Has<DistanceFog>,
         (
             Has<RenderViewLightProbes<EnvironmentMapLight>>,
             Has<RenderViewLightProbes<IrradianceVolume>>,
@@ -723,6 +724,7 @@ pub fn queue_material_meshes<M: Material>(
         camera_3d,
         temporal_jitter,
         projection,
+        distance_fog,
         (has_environment_maps, has_irradiance_volumes),
         has_oit,
     ) in &views
@@ -814,6 +816,9 @@ pub fn queue_material_meshes<M: Material>(
         if ssao {
             view_key |= MeshPipelineKey::SCREEN_SPACE_AMBIENT_OCCLUSION;
         }
+        if distance_fog {
+            view_key |= MeshPipelineKey::DISTANCE_FOG;
+        }
         if let Some(camera_3d) = camera_3d {
             view_key |= screen_space_specular_transmission_pipeline_key(
                 camera_3d.screen_space_specular_transmission_quality,

crates/bevy_pbr/src/meshlet/material_pipeline_prepare.rs

@@ -48,7 +48,7 @@ pub fn prepare_material_meshlet_meshes_main_opaque_pass<M: Material>(
             Option<&Tonemapping>,
             Option<&DebandDither>,
             Option<&ShadowFilteringMethod>,
-            Has<ScreenSpaceAmbientOcclusion>,
+            (Has<ScreenSpaceAmbientOcclusion>, Has<DistanceFog>),
             (
                 Has<NormalPrepass>,
                 Has<DepthPrepass>,
@@ -73,7 +73,7 @@ pub fn prepare_material_meshlet_meshes_main_opaque_pass<M: Material>(
         tonemapping,
         dither,
         shadow_filter_method,
-        ssao,
+        (ssao, distance_fog),
         (normal_prepass, depth_prepass, motion_vector_prepass, deferred_prepass),
         temporal_jitter,
         projection,
@@ -142,6 +142,9 @@ pub fn prepare_material_meshlet_meshes_main_opaque_pass<M: Material>(
         if ssao {
             view_key |= MeshPipelineKey::SCREEN_SPACE_AMBIENT_OCCLUSION;
         }
+        if distance_fog {
+            view_key |= MeshPipelineKey::DISTANCE_FOG;
+        }
 
         view_key |= MeshPipelineKey::from_primitive_topology(PrimitiveTopology::TriangleList);
 

crates/bevy_pbr/src/render/mesh.rs

@@ -1809,7 +1809,8 @@ bitflags::bitflags! {
         const HAS_PREVIOUS_SKIN                 = 1 << 18;
         const HAS_PREVIOUS_MORPH                = 1 << 19;
         const OIT_ENABLED                       = 1 << 20;
-        const LAST_FLAG                         = Self::OIT_ENABLED.bits();
+        const DISTANCE_FOG                      = 1 << 21;
+        const LAST_FLAG                         = Self::DISTANCE_FOG.bits();
 
         // Bitfields
         const MSAA_RESERVED_BITS                = Self::MSAA_MASK_BITS << Self::MSAA_SHIFT_BITS;
@@ -2268,6 +2269,10 @@ impl SpecializedMeshPipeline for MeshPipeline {
             shader_defs.push("VISIBILITY_RANGE_DITHER".into());
         }
 
+        if key.contains(MeshPipelineKey::DISTANCE_FOG) {
+            shader_defs.push("DISTANCE_FOG".into());
+        }
+
         if self.binding_arrays_are_usable {
             shader_defs.push("MULTIPLE_LIGHT_PROBES_IN_ARRAY".into());
             shader_defs.push("MULTIPLE_LIGHTMAPS_IN_ARRAY".into());

crates/bevy_pbr/src/render/pbr_functions.wgsl

@@ -759,6 +759,7 @@ fn apply_pbr_lighting(
 }
 #endif // PREPASS_FRAGMENT
 
+#ifdef DISTANCE_FOG
 fn apply_fog(fog_params: mesh_view_types::Fog, input_color: vec4<f32>, fragment_world_position: vec3<f32>, view_world_position: vec3<f32>) -> vec4<f32> {
     let view_to_world = fragment_world_position.xyz - view_world_position.xyz;
 
@@ -796,6 +797,7 @@ fn apply_fog(fog_params: mesh_view_types::Fog, input_color: vec4<f32>, fragment_
         return input_color;
     }
 }
+#endif  // DISTANCE_FOG
 
 #ifdef PREMULTIPLY_ALPHA
 fn premultiply_alpha(standard_material_flags: u32, color: vec4<f32>) -> vec4<f32> {
@@ -857,10 +859,12 @@ fn main_pass_post_lighting_processing(
 ) -> vec4<f32> {
     var output_color = input_color;
 
+#ifdef DISTANCE_FOG
     // 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) {
+    if ((pbr_input.material.flags & pbr_types::STANDARD_MATERIAL_FLAGS_FOG_ENABLED_BIT) != 0u) {
         output_color = apply_fog(view_bindings::fog, output_color, pbr_input.world_position.xyz, view_bindings::view.world_position.xyz);
     }
+#endif  // DISTANCE_FOG
 
 #ifdef TONEMAP_IN_SHADER
     output_color = tone_mapping(output_color, view_bindings::view.color_grading);