Fix bloom luma calc in sRGB (#22565)

# Objective

- Luminance is a physical value that should be calculated in linear, not
logarithmic (perceptual) space.

## Solution

- Don't convert from linear to srgb to do a luma calc.

Author: aevyrie

crates/bevy_post_process/src/bloom/bloom.wgsl

@@ -37,15 +37,11 @@ fn tonemapping_luminance(v: vec3<f32>) -> f32 {
     return dot(v, vec3<f32>(0.2126, 0.7152, 0.0722));
 }
 
-fn rgb_to_srgb_simple(color: vec3<f32>) -> vec3<f32> {
-    return pow(color, vec3<f32>(1.0 / 2.2));
-}
-
 // http://graphicrants.blogspot.com/2013/12/tone-mapping.html
 fn karis_average(color: vec3<f32>) -> f32 {
-    // Luminance calculated by gamma-correcting linear RGB to non-linear sRGB using pow(color, 1.0 / 2.2)
-    // and then calculating luminance based on Rec. 709 color primaries.
-    let luma = tonemapping_luminance(rgb_to_srgb_simple(color)) / 4.0;
+    // Luminance calculated based on Rec. 709 color primaries.
+    // This must be done in *linear* color space.
+    let luma = tonemapping_luminance(color) / 4.0;
     return 1.0 / (1.0 + luma);
 }
 

release-content/migration-guides/bloom-luma-fix.md

@@ -0,0 +1,14 @@
+---
+title: "Bloom luma calculation now in linear space"
+pull_requests: [22561]
+---
+
+The luma calculation for bloom's Karis average (used for downsampling) has been corrected to use linear color space instead of non-linear sRGB space.
+
+As a result, the intensity of the bloom effect may appear reduced, especially for colors with high saturation or those that were significantly affected by the previous non-linear calculation.
+
+If your scene's bloom now appears too dim, you can:
+
+- Increase the `intensity` field on the `Bloom` component.
+- Increase the `emissive` strength of your materials.
+- Adjust the `prefilter` settings in the `Bloom` component.