From f24f601e0505c3525e13aed5c65fc57b41f38de2 Mon Sep 17 00:00:00 2001
From: Son <leson.phung@gmail.com>
Date: Wed, 7 Aug 2024 22:12:26 +1000
Subject: [PATCH] Adjust erf estimation function (#15423)

Release Notes:

- Fixed a (potential) small error in erf estimation. Technically, the
error is negligible.

I am not sure where the current calculation for erf come from and if it
is intended or a simple mistake. However it looks slightly different
from the official calculation, notably
[this](https://en.wikipedia.org/wiki/Error_function#Approximation_with_elementary_functions)
from Wikipedia.

I will add a comment if it is intended.
---
 crates/gpui/Cargo.toml                      |  4 +++
 crates/gpui/examples/shadow.rs              | 29 +++++++++++++++++++++
 crates/gpui/src/platform/blade/shaders.wgsl |  2 +-
 crates/gpui/src/platform/mac/shaders.metal  |  6 ++---
 4 files changed, 37 insertions(+), 4 deletions(-)
 create mode 100644 crates/gpui/examples/shadow.rs

diff --git a/crates/gpui/Cargo.toml b/crates/gpui/Cargo.toml
index cc8ec90d67f34eb26813d7c9bf3fa718d7e498fd..10f85a01dccafd2777322b8b6a512330b76e18eb 100644
--- a/crates/gpui/Cargo.toml
+++ b/crates/gpui/Cargo.toml
@@ -172,3 +172,7 @@ path = "examples/window_shadow.rs"
 [[example]]
 name = "input"
 path = "examples/input.rs"
+
+[[example]]
+name = "shadow"
+path = "examples/shadow.rs"
diff --git a/crates/gpui/examples/shadow.rs b/crates/gpui/examples/shadow.rs
new file mode 100644
index 0000000000000000000000000000000000000000..cdf3ba42e9e2b0222055d77a50676b34b6b35fc1
--- /dev/null
+++ b/crates/gpui/examples/shadow.rs
@@ -0,0 +1,29 @@
+use gpui::*;
+
+struct Shadow {}
+
+impl Render for Shadow {
+    fn render(&mut self, _cx: &mut ViewContext<Self>) -> impl IntoElement {
+        div()
+            .flex()
+            .bg(rgb(0xffffff))
+            .size_full()
+            .justify_center()
+            .items_center()
+            .child(div().size_8().shadow_sm())
+    }
+}
+
+fn main() {
+    App::new().run(|cx: &mut AppContext| {
+        let bounds = Bounds::centered(None, size(px(300.0), px(300.0)), cx);
+        cx.open_window(
+            WindowOptions {
+                window_bounds: Some(WindowBounds::Windowed(bounds)),
+                ..Default::default()
+            },
+            |cx| cx.new_view(|_cx| Shadow {}),
+        )
+        .unwrap();
+    });
+}
diff --git a/crates/gpui/src/platform/blade/shaders.wgsl b/crates/gpui/src/platform/blade/shaders.wgsl
index 0eba972b6902bbb9f252e757a30dac1ad63fb8cc..77f14e8264b84d3766db29696c0c0b1969409077 100644
--- a/crates/gpui/src/platform/blade/shaders.wgsl
+++ b/crates/gpui/src/platform/blade/shaders.wgsl
@@ -150,7 +150,7 @@ fn gaussian(x: f32, sigma: f32) -> f32{
 fn erf(v: vec2<f32>) -> vec2<f32> {
     let s = sign(v);
     let a = abs(v);
-    let r1 = 1.0 + (0.278393 + (0.230389 + 0.078108 * (a * a)) * a) * a;
+    let r1 = 1.0 + (0.278393 + (0.230389 + (0.000972 + 0.078108 * a) * a) * a) * a;
     let r2 = r1 * r1;
     return s - s / (r2 * r2);
 }
diff --git a/crates/gpui/src/platform/mac/shaders.metal b/crates/gpui/src/platform/mac/shaders.metal
index c1089dbbec6f4f074e3c9e40c173c40463bdc948..e7afe2c11fe691ace58525f8f8793bc1ec66e944 100644
--- a/crates/gpui/src/platform/mac/shaders.metal
+++ b/crates/gpui/src/platform/mac/shaders.metal
@@ -657,9 +657,9 @@ float gaussian(float x, float sigma) {
 float2 erf(float2 x) {
   float2 s = sign(x);
   float2 a = abs(x);
-  x = 1. + (0.278393 + (0.230389 + 0.078108 * (a * a)) * a) * a;
-  x *= x;
-  return s - s / (x * x);
+  float2 r1 = 1. + (0.278393 + (0.230389 + (0.000972 + 0.078108 * a) * a) * a) * a;
+  float2 r2 = r1 * r1;
+  return s - s / (r2 * r2);
 }
 
 float blur_along_x(float x, float y, float sigma, float corner,