From 0ca6957d0c551967319dcfd100a7978b0a7520a9 Mon Sep 17 00:00:00 2001
From: Mitchell Hashimoto <mitchell.hashimoto@gmail.com>
Date: Fri, 18 Aug 2023 18:33:30 -0700
Subject: [PATCH] renderer/{metal,opengl}: need to premult our cell alpha to
 have effect

---
 src/renderer/shaders/cell.f.glsl | 3 ++-
 src/renderer/shaders/cell.metal  | 6 +++++-
 2 files changed, 7 insertions(+), 2 deletions(-)

diff --git a/src/renderer/shaders/cell.f.glsl b/src/renderer/shaders/cell.f.glsl
index 336fd073c..c261d03e5 100644
--- a/src/renderer/shaders/cell.f.glsl
+++ b/src/renderer/shaders/cell.f.glsl
@@ -39,7 +39,8 @@ void main() {
 
     case MODE_FG:
         a = texture(text, glyph_tex_coords).r;
-        out_FragColor = vec4(color.rgb*a, color.a*a);
+        vec3 premult = color.rgb * color.a;
+        out_FragColor = vec4(premult.rgb*a, a);
         break;
 
     case MODE_FG_COLOR:
diff --git a/src/renderer/shaders/cell.metal b/src/renderer/shaders/cell.metal
index 2e50f4938..4e76f264d 100644
--- a/src/renderer/shaders/cell.metal
+++ b/src/renderer/shaders/cell.metal
@@ -160,8 +160,12 @@ fragment float4 uber_fragment(
 
     // We premult the alpha to our whole color since our blend function
     // uses One/OneMinusSourceAlpha to avoid blurry edges.
+    // We first premult our given color.
+    float4 premult = float4(in.color.rgb * in.color.a, 1);
+    // Then premult the texture color
     float a = textureGreyscale.sample(textureSampler, coord).r;
-    return in.color * a;
+    premult = premult * a;
+    return premult;
   }
 
   case MODE_FG_COLOR: {