From c6a859d1ed594b09bc2cdf58f19e0f80a64839ba Mon Sep 17 00:00:00 2001
From: Olivier Guyot <olivier.guyot@camptocamp.com>
Date: Tue, 2 Apr 2019 22:12:45 +0200
Subject: [PATCH] Webgl / clarify premultiplied alpha handling

By default, alpha premultiplying should be done by the initial rendering
(eg quads) and not the final post processing pass.

The default post processing pass expects premultiplied color values and
will not do this operation itself.
---
 src/ol/layer/Heatmap.js              | 2 +-
 src/ol/renderer/webgl/PointsLayer.js | 3 +--
 src/ol/webgl/PostProcessingPass.js   | 5 +++--
 3 files changed, 5 insertions(+), 5 deletions(-)

diff --git a/src/ol/layer/Heatmap.js b/src/ol/layer/Heatmap.js
index dac335798e..eebebf84db 100644
--- a/src/ol/layer/Heatmap.js
+++ b/src/ol/layer/Heatmap.js
@@ -238,7 +238,7 @@ class Heatmap extends VectorLayer {
           float sqRadius = texCoord.x * texCoord.x + texCoord.y * texCoord.y;
           float value = (1.0 - sqrt(sqRadius)) * u_blurSlope;
           float alpha = smoothstep(0.0, 1.0, value) * v_opacity;
-          gl_FragColor = vec4(1.0, 1.0, 1.0, alpha);
+          gl_FragColor = vec4(alpha, alpha, alpha, alpha);
         }`,
       uniforms: {
         u_size: function() {
diff --git a/src/ol/renderer/webgl/PointsLayer.js b/src/ol/renderer/webgl/PointsLayer.js
index ce40fc4c3e..f486476a21 100644
--- a/src/ol/renderer/webgl/PointsLayer.js
+++ b/src/ol/renderer/webgl/PointsLayer.js
@@ -110,8 +110,7 @@ const FRAGMENT_SHADER = `
  *
  * The following uniform is used for the main texture: `u_texture`.
  *
- * Please note that the main shader output should have premultiplied alpha, otherwise the colors will be blended
- * additively.
+ * Please note that the main shader output should have premultiplied alpha, otherwise visual anomalies may occur.
  *
  * Points are rendered as quads with the following structure:
  *
diff --git a/src/ol/webgl/PostProcessingPass.js b/src/ol/webgl/PostProcessingPass.js
index 1563079605..1405398354 100644
--- a/src/ol/webgl/PostProcessingPass.js
+++ b/src/ol/webgl/PostProcessingPass.js
@@ -28,7 +28,6 @@ const DEFAULT_FRAGMENT_SHADER = `
    
   void main() {
     gl_FragColor = texture2D(u_image, v_texCoord);
-    gl_FragColor.rgb *= gl_FragColor.a;
   }
 `;
 
@@ -59,6 +58,9 @@ const DEFAULT_FRAGMENT_SHADER = `
  * a pixel which is 100% red with an opacity of 50% must have a color of (r=0.5, g=0, b=0, a=0.5).
  * Failing to provide pixel colors with premultiplied alpha will result in render anomalies.
  *
+ * The default post-processing pass does *not* multiply color values with alpha value, it expects color values to be
+ * premultiplied.
+ *
  * Default shaders are shown hereafter:
  *
  * * Vertex shader:
@@ -91,7 +93,6 @@ const DEFAULT_FRAGMENT_SHADER = `
  *
  *   void main() {
  *     gl_FragColor = texture2D(u_image, v_texCoord);
- *     gl_FragColor.rgb *= gl_FragColor.a;
  *   }
  *   ```
  *