diff --git a/src/ol/renderer/webgl/PointsLayer.js b/src/ol/renderer/webgl/PointsLayer.js
index f1945af4f7..02a69798c3 100644
--- a/src/ol/renderer/webgl/PointsLayer.js
+++ b/src/ol/renderer/webgl/PointsLayer.js
@@ -126,14 +126,13 @@ const FRAGMENT_SHADER = `
  * * Fragment shader:
  *   ```
  *   precision mediump float;
- *   uniform float u_opacity;
  *
  *   varying vec2 v_texCoord;
  *   varying float v_opacity;
  *
  *   void main(void) {
  *     gl_FragColor.rgb = vec3(1.0, 1.0, 1.0);
- *     float alpha = u_opacity * v_opacity;
+ *     float alpha = v_opacity;
  *     if (alpha == 0.0) {
  *       discard;
  *     }
diff --git a/src/ol/webgl/PostProcessingPass.js b/src/ol/webgl/PostProcessingPass.js
index 429d391abd..1563079605 100644
--- a/src/ol/webgl/PostProcessingPass.js
+++ b/src/ol/webgl/PostProcessingPass.js
@@ -55,6 +55,10 @@ const DEFAULT_FRAGMENT_SHADER = `
  * This class is used to define Post Processing passes with custom shaders and uniforms.
  * This is used internally by {@link module:ol/webgl/Helper~WebGLHelper}.
  *
+ * Please note that the final output on the DOM canvas is expected to have premultiplied alpha, which means that
+ * 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.
+ *
  * Default shaders are shown hereafter:
  *
  * * Vertex shader:
@@ -87,6 +91,7 @@ const DEFAULT_FRAGMENT_SHADER = `
  *
  *   void main() {
  *     gl_FragColor = texture2D(u_image, v_texCoord);
+ *     gl_FragColor.rgb *= gl_FragColor.a;
  *   }
  *   ```
  *