gpui: Shadows look different on Blade and Metal

[iamnbutler]

Check for existing issues

• Completed

Describe the feature

While working on #22441 I noticed that the shadow rendering between blade and metal are pretty different. We should try to bring these a bit closer into alignment.

We generally design things from the perspective of the metal renderer on macOS, so we should try to bring the blade rendering closer into alignment to that of the metal.

Reference:
#22441 (comment)

Zed Version and System Specs

N/A

If applicable, add mockups / screenshots to help present your vision of the feature

N/A

[jansol]

My first guess would be that blade and metal do blending in different colorspaces (nonlinear sRGB vs linear sRGB). Specifically the big question is whether alpha is treated as linear or nonlinear. This was briefly a very noticeable problem in KDE Plasma when HDR was enabled -- alpha was being treated as linear, but most applications were written with the expectation of it being nonlinear (which is awful mathematically but visually ends up being more in line with what people expect), see https://bugs.kde.org/show_bug.cgi?id=476868

[jansol]

Oh, it occurred to me that if this is an issue with the blending gamma / colorspace, resolving it may also go a long way towards improving the muddy appearance of fonts on Linux, since they would be affected in that case too. cf #7992

[apricotbucket28]

Tried this with and without #11534

Linear (now)	sRGB (before)

Diff:

diff --git a/crates/gpui/src/platform/blade/blade_atlas.rs b/crates/gpui/src/platform/blade/blade_atlas.rs
index fb703f2a41..b005cf42b0 100644
--- a/crates/gpui/src/platform/blade/blade_atlas.rs
+++ b/crates/gpui/src/platform/blade/blade_atlas.rs
@@ -193,7 +193,7 @@ impl BladeAtlasState {
                 usage = gpu::TextureUsage::COPY | gpu::TextureUsage::RESOURCE;
             }
             AtlasTextureKind::Polychrome => {
-                format = gpu::TextureFormat::Bgra8UnormSrgb;
+                format = gpu::TextureFormat::Bgra8Unorm;
                 usage = gpu::TextureUsage::COPY | gpu::TextureUsage::RESOURCE;
             }
             AtlasTextureKind::Path => {
diff --git a/crates/gpui/src/platform/blade/blade_renderer.rs b/crates/gpui/src/platform/blade/blade_renderer.rs
index ee8ffdfda7..532ddcaaf4 100644
--- a/crates/gpui/src/platform/blade/blade_renderer.rs
+++ b/crates/gpui/src/platform/blade/blade_renderer.rs
@@ -329,7 +329,7 @@ impl BladeRenderer {
             size: config.size,
             usage: gpu::TextureUsage::TARGET,
             display_sync: gpu::DisplaySync::Recent,
-            color_space: gpu::ColorSpace::Linear,
+            color_space: gpu::ColorSpace::Srgb,
             allow_exclusive_full_screen: false,
             transparent: config.transparent,
         };
diff --git a/crates/gpui/src/platform/blade/shaders.wgsl b/crates/gpui/src/platform/blade/shaders.wgsl
index d497c40d7a..f4c4fd0709 100644
--- a/crates/gpui/src/platform/blade/shaders.wgsl
+++ b/crates/gpui/src/platform/blade/shaders.wgsl
@@ -145,7 +145,7 @@ fn hsla_to_rgba(hsla: Hsla) -> vec4<f32> {
     let c = (1.0 - abs(2.0 * l - 1.0)) * s;
     let x = c * (1.0 - abs(h % 2.0 - 1.0));
     let m = l - c / 2.0;
-    var color = vec3<f32>(m);
+    var color = vec4<f32>(m, m, m, a);
 
     if (h >= 0.0 && h < 1.0) {
         color.r += c;
@@ -167,12 +167,7 @@ fn hsla_to_rgba(hsla: Hsla) -> vec4<f32> {
         color.b += x;
     }
 
-    // Input colors are assumed to be in sRGB space,
-    // but blending and rendering needs to happen in linear space.
-    // The output will be converted to sRGB by either the target
-    // texture format or the swapchain color space.
-    let linear = srgb_to_linear(color);
-    return vec4<f32>(linear, a);
+    return color;
 }
 
 /// Convert a linear sRGB to Oklab space.