From 04ea3266279b72beb01e3a5cee046be7d955cb39 Mon Sep 17 00:00:00 2001
From: Patrick Walton <pcwalton@mimiga.net>
Date: Fri, 4 Dec 2015 21:43:27 -0800
Subject: [PATCH] Fix incorrect composite blending.

We can't clear to transparent white because that will cause the white
color to be partially mixed into the result if the composited content
has alpha between 0 and 1.
---
 src/renderer.rs | 29 ++++++++++++++++++++++-------
 1 file changed, 22 insertions(+), 7 deletions(-)

diff --git a/src/renderer.rs b/src/renderer.rs
index 09dc4a76e7..3a5f018853 100644
--- a/src/renderer.rs
+++ b/src/renderer.rs
@@ -931,8 +931,14 @@ impl Renderer {
                             // TODO: probably worth sorting front to back to minimize overdraw (if profiling shows fragment / rop bound)
 
                             gl::enable(gl::BLEND);
-                            gl::blend_func_separate(gl::SRC_ALPHA, gl::ONE_MINUS_SRC_ALPHA,
-                                                    gl::ONE, gl::ONE);
+
+                            if layer.render_targets[0].texture.is_some() {
+                                gl::blend_func_separate(gl::SRC_ALPHA, gl::ONE_MINUS_SRC_ALPHA,
+                                                        gl::ONE, gl::ONE);
+                            } else {
+                                gl::blend_func(gl::SRC_ALPHA, gl::ONE_MINUS_SRC_ALPHA);
+                            }
+
                             gl::blend_equation(gl::FUNC_ADD);
 
                             self.device.bind_program(self.quad_program_id,
@@ -1236,11 +1242,20 @@ fn clear_framebuffer(device: &mut Device,
                      uv_rects: &[Rect<u32>],
                      program_id: ProgramId,
                      clear_to_transparent: bool) {
-    let clear_color = ColorF {
-        r: 1.0,
-        g: 1.0,
-        b: 1.0,
-        a: if clear_to_transparent { 0.0 } else { 1.0 },
+    let clear_color = if clear_to_transparent {
+        ColorF {
+            r: 0.0,
+            g: 0.0,
+            b: 0.0,
+            a: 0.0,
+        }
+    } else {
+        ColorF {
+            r: 1.0,
+            g: 1.0,
+            b: 1.0,
+            a: 1.0,
+        }
     };
 
     // Fast path if we only have one rect: just use glClear().