Use 16-bit float to store path windings

Previously, we were using a normalized 8-bit unsigned integer which forced us
to represent each increment of the winding number as a fraction of the max
value (1 / 255) which we would then add up using additive alpha blending.

This had three major drawbacks:

- The max winding number could not be greater than 255.
- Adding up (1 / 255) several times could result in a loss of precision.
- Due to also computing anti-aliasing as a fractional winding number, we had to
  reduce the max winding number to 32. This was still not good enough because
  we would multiply a fractional value with `1 / 32`, thus introducing more and
  more loss of precision.

This commit changes the texture type to an `f16` which doesn't require the
division by 255 and enables greater precision in the computation of the
anti-aliased parts of a curve. Note how this also removes the limitation of 255
windings at most per curve. The tradeoff is paying twice as much memory for
storing the texture, but that seems totally valid to achieve rendering accuracy.

Note that this kind of texture should be compatible with WebGL2 once we start
working on a web version of Zed.

crates/gpui/src/platform/mac/renderer.rs

@@ -107,7 +107,7 @@ impl Renderer {
             "path_atlas",
             "path_atlas_vertex",
             "path_atlas_fragment",
-            MTLPixelFormat::R8Unorm,
+            MTLPixelFormat::R16Float,
         );
         Self {
             sprite_cache,
@@ -827,7 +827,7 @@ fn build_path_atlas_texture_descriptor() -> metal::TextureDescriptor {
     let texture_descriptor = metal::TextureDescriptor::new();
     texture_descriptor.set_width(2048);
     texture_descriptor.set_height(2048);
-    texture_descriptor.set_pixel_format(MTLPixelFormat::R8Unorm);
+    texture_descriptor.set_pixel_format(MTLPixelFormat::R16Float);
     texture_descriptor
         .set_usage(metal::MTLTextureUsage::RenderTarget | metal::MTLTextureUsage::ShaderRead);
     texture_descriptor.set_storage_mode(metal::MTLStorageMode::Private);

crates/gpui/src/platform/mac/shaders/shaders.metal

@@ -205,8 +205,6 @@ vertex SpriteFragmentInput sprite_vertex(
     };
 }
 
-#define MAX_WINDINGS 32.
-
 fragment float4 sprite_fragment(
     SpriteFragmentInput input [[stage_in]],
     texture2d<float> atlas [[ texture(GPUISpriteFragmentInputIndexAtlas) ]]
@@ -216,7 +214,7 @@ fragment float4 sprite_fragment(
     float4 sample = atlas.sample(atlas_sampler, input.atlas_position);
     float mask;
     if (input.compute_winding) {
-        mask = 1. - abs(1. - fmod(sample.r * MAX_WINDINGS, 2.));
+        mask = 1. - abs(1. - fmod(sample.r, 2.));
     } else {
         mask = sample.a;
     }
@@ -303,6 +301,6 @@ fragment float4 path_atlas_fragment(
     );
     float f = (input.st_position.x * input.st_position.x) - input.st_position.y;
     float distance = f / length(gradient);
-    float alpha = saturate(0.5 - distance) / MAX_WINDINGS;
+    float alpha = saturate(0.5 - distance);
     return float4(alpha, 0., 0., 1.);
 }