Handle pre-mult alpha properly

Author: photonstorm

src/renderer/webgl/shaders/src/ColorMatrix.frag

@@ -4,15 +4,36 @@ precision mediump float;
 
 uniform sampler2D uMainSampler;
 uniform float uColorMatrix[20];
+uniform float uAlpha;
 
 varying vec2 outTexCoord;
 
 void main ()
 {
     vec4 c = texture2D(uMainSampler, outTexCoord);
 
-    gl_FragColor.r = uColorMatrix[0] * c.r + uColorMatrix[1] * c.g + uColorMatrix[2] * c.b + uColorMatrix[4];
-    gl_FragColor.g = uColorMatrix[5] * c.r + uColorMatrix[6] * c.g + uColorMatrix[7] * c.b + uColorMatrix[9];
-    gl_FragColor.b = uColorMatrix[10] * c.r + uColorMatrix[11] * c.g + uColorMatrix[12] * c.b + uColorMatrix[14];
-    gl_FragColor.a = c.a;
+    if (uAlpha == 0.0)
+    {
+        gl_FragColor = c;
+
+        return;
+    }
+
+    if (c.a > 0.0)
+    {
+        c.rgb /= c.a;
+    }
+
+    vec4 result;
+
+    result.r = (uColorMatrix[0] * c.r) + (uColorMatrix[1] * c.g) + (uColorMatrix[2] * c.b) + (uColorMatrix[3] * c.a) + uColorMatrix[4];
+    result.g = (uColorMatrix[5] * c.r) + (uColorMatrix[6] * c.g) + (uColorMatrix[7] * c.b) + (uColorMatrix[8] * c.a) + uColorMatrix[9];
+    result.b = (uColorMatrix[10] * c.r) + (uColorMatrix[11] * c.g) + (uColorMatrix[12] * c.b) + (uColorMatrix[13] * c.a) + uColorMatrix[14];
+    result.a = (uColorMatrix[15] * c.r) + (uColorMatrix[16] * c.g) + (uColorMatrix[17] * c.b) + (uColorMatrix[18] * c.a) + uColorMatrix[19];
+
+    vec3 rgb = mix(c.rgb, result.rgb, uAlpha);
+
+    rgb *= result.a;
+
+    gl_FragColor = vec4(rgb, result.a);
 }