From 1e4e4f7043e4247e299b4affae740885df23d728 Mon Sep 17 00:00:00 2001
From: Marco Buono <thecoreh@gmail.com>
Date: Tue, 26 Sep 2023 03:01:18 -0300
Subject: [PATCH] Add a fast codepath for fetching background with zero
 roughness

---
 crates/bevy_pbr/src/render/pbr_lighting.wgsl | 16 +++++++++++++++-
 1 file changed, 15 insertions(+), 1 deletion(-)

diff --git a/crates/bevy_pbr/src/render/pbr_lighting.wgsl b/crates/bevy_pbr/src/render/pbr_lighting.wgsl
index 798b33baf2..424b8854fc 100644
--- a/crates/bevy_pbr/src/render/pbr_lighting.wgsl
+++ b/crates/bevy_pbr/src/render/pbr_lighting.wgsl
@@ -308,7 +308,13 @@ fn specular_transmissive_light(world_position: vec4<f32>, frag_coord: vec3<f32>,
     let offset_position = (clip_exit_position.xy / clip_exit_position.w) * vec2<f32>(0.5, -0.5) + 0.5;
 
     // Fetch background color
-    let background_color = fetch_transmissive_background(offset_position, frag_coord, view_z, perceptual_roughness);
+    var background_color: vec4<f32>;
+    if perceptual_roughness == 0.0 {
+        // If the material has zero roughness, we can use a faster approach without the blur
+        background_color = fetch_transmissive_background_non_rough(offset_position);
+    } else {
+        background_color = fetch_transmissive_background(offset_position, frag_coord, view_z, perceptual_roughness);
+    }
 
     // Calculate final color by applying specular transmissive color to a mix of background color and transmitted specular environment light
     return specular_transmissive_color * mix(transmitted_environment_light_specular, background_color.rgb, background_color.a);
@@ -326,6 +332,14 @@ fn interleaved_gradient_noise(pixel_coordinates: vec2<f32>) -> f32 {
     return fract(52.9829189 * fract(0.06711056 * xy.x + 0.00583715 * xy.y));
 }
 
+fn fetch_transmissive_background_non_rough(offset_position: vec2<f32>) -> vec4<f32> {
+    return textureSample(
+        view_bindings::view_transmission_texture,
+        view_bindings::view_transmission_sampler,
+        offset_position,
+    );
+}
+
 fn fetch_transmissive_background(offset_position: vec2<f32>, frag_coord: vec3<f32>, view_z: f32, perceptual_roughness: f32) -> vec4<f32> {
     // Calculate view aspect ratio, used to scale offset so that it's proportionate
     let aspect = view_bindings::view.viewport.z / view_bindings::view.viewport.w;