From 923765aff997c36a602d1523d5a605dceb233910 Mon Sep 17 00:00:00 2001
From: Chad Brokaw <cbrokaw@gmail.com>
Date: Mon, 28 Nov 2022 14:07:26 -0500
Subject: [PATCH] Address review feedback

* Add comment about avoiding NaNs to inverse alpha divisions
* Change inverse alpha computations to use max(a, epsilon) rather than +
* Emit alpha component in blit fragment shader (enables support for partially transparent windows)
---
 piet-wgsl/shader/fine.wgsl         | 5 +++--
 piet-wgsl/shader/shared/blend.wgsl | 6 +++---
 piet-wgsl/src/lib.rs               | 2 +-
 3 files changed, 7 insertions(+), 6 deletions(-)

diff --git a/piet-wgsl/shader/fine.wgsl b/piet-wgsl/shader/fine.wgsl
index cd6d940..74a783e 100644
--- a/piet-wgsl/shader/fine.wgsl
+++ b/piet-wgsl/shader/fine.wgsl
@@ -282,8 +282,9 @@ fn main(
         let coords = xy_uint + vec2(i, 0u);
         if coords.x < config.target_width && coords.y < config.target_height {
             let fg = rgba[i];
-            let a_inv = 1.0 / (fg.a + 1e-6);
-            let rgba_sep = vec4(fg.r * a_inv, fg.g * a_inv, fg.b * a_inv, fg.a);            
+            // Max with a small epsilon to avoid NaNs
+            let a_inv = 1.0 / max(fg.a, 1e-6);
+            let rgba_sep = vec4(fg.rgb * a_inv, fg.a);            
             textureStore(output, vec2<i32>(coords), rgba_sep);
         }
     } 
diff --git a/piet-wgsl/shader/shared/blend.wgsl b/piet-wgsl/shader/shared/blend.wgsl
index 7b72f2e..eef670d 100644
--- a/piet-wgsl/shader/shared/blend.wgsl
+++ b/piet-wgsl/shader/shared/blend.wgsl
@@ -318,10 +318,10 @@ fn blend_mix_compose(backdrop: vec4<f32>, src: vec4<f32>, mode: u32) -> vec4<f32
         // Both normal+src_over blend and clip case
         return backdrop * (1.0 - src.a) + src;
     }
-    // Un-premultiply colors for blending
-    let inv_src_a = 1.0 / (src.a + EPSILON);
+    // Un-premultiply colors for blending. Max with a small epsilon to avoid NaNs.
+    let inv_src_a = 1.0 / max(src.a, EPSILON);
     var cs = src.rgb * inv_src_a;
-    let inv_backdrop_a = 1.0 / (backdrop.a + EPSILON);
+    let inv_backdrop_a = 1.0 / max(backdrop.a, EPSILON);
     let cb = backdrop.rgb * inv_backdrop_a;
     let mix_mode = mode >> 8u;
     let mixed = blend_mix(cb, cs, mix_mode);
diff --git a/piet-wgsl/src/lib.rs b/piet-wgsl/src/lib.rs
index 91262f2..dde1ad2 100644
--- a/piet-wgsl/src/lib.rs
+++ b/piet-wgsl/src/lib.rs
@@ -205,7 +205,7 @@ impl BlitPipeline {
             @fragment
             fn fs_main(@builtin(position) pos: vec4<f32>) -> @location(0) vec4<f32> {
                 let rgba_sep = textureLoad(fine_output, vec2<i32>(pos.xy), 0);
-                return vec4(rgba_sep.rgb * rgba_sep.a, 1.0);
+                return vec4(rgba_sep.rgb * rgba_sep.a, rgba_sep.a);
             }
         "#;