From 4ee56a1e75a043c6f317ea2f71c96b2061e85cf5 Mon Sep 17 00:00:00 2001
From: Joshua Ashton <joshua@froggi.es>
Date: Sat, 13 Nov 2021 23:34:48 +0000
Subject: [PATCH] rendervulkan: Do not premultiply color

The surface should already have premultiplied alpha.

Gets us to how we look with planes.
---
 src/composite.comp | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/src/composite.comp b/src/composite.comp
index 4f2f0d1..9e7fe15 100644
--- a/src/composite.comp
+++ b/src/composite.comp
@@ -102,8 +102,14 @@ void main() {
 
     for (int i = 1; i < c_layerCount; i++) {
         vec4 layerColor = sampleLayer(i, uv);
-        float layerAlpha = u_opacity[i] * layerColor.a;
-        outputValue = layerColor * layerAlpha + outputValue * (1.0f - layerAlpha);
+        // wl_surfaces come with premultiplied alpha, so that's them being
+        // premultiplied by layerColor.a.
+        // We need to then multiply that by the layer's opacity to get to our
+        // final premultiplied state.
+        // For the other side of things, we need to multiply by (1.0f - (layerColor.a * opacity))
+        float opacity = u_opacity[i];
+        float layerAlpha = opacity * layerColor.a;
+        outputValue = layerColor * opacity + outputValue * (1.0f - layerAlpha);
     }
 
     if (c_swapChannels)