Porter duff premul

src/babylon.engine.ts

@@ -245,6 +245,7 @@
         private static _ALPHA_MAXIMIZED = 5;
         private static _ALPHA_ONEONE = 6;
         private static _ALPHA_PREMULTIPLIED = 7;
+        private static _ALPHA_PREMULTIPLIED_PORTERDUFF = 8;
 
         private static _DELAYLOADSTATE_NONE = 0;
         private static _DELAYLOADSTATE_LOADED = 1;
@@ -376,6 +377,10 @@
             return Engine._ALPHA_PREMULTIPLIED;
         }
 
+        public static get ALPHA_PREMULTIPLIED_PORTERDUFF(): number {
+            return Engine._ALPHA_PREMULTIPLIED_PORTERDUFF;
+        }
+
         public static get DELAYLOADSTATE_NONE(): number {
             return Engine._DELAYLOADSTATE_NONE;
         }
@@ -2167,6 +2172,10 @@
                     this._alphaState.setAlphaBlendFunctionParameters(this._gl.ONE, this._gl.ONE_MINUS_SRC_ALPHA, this._gl.ONE, this._gl.ONE);
                     this._alphaState.alphaBlend = true;
                     break;
+                case Engine.ALPHA_PREMULTIPLIED_PORTERDUFF:
+                    this._alphaState.setAlphaBlendFunctionParameters(this._gl.ONE, this._gl.ONE_MINUS_SRC_ALPHA, this._gl.ONE, this._gl.ONE_MINUS_SRC_ALPHA);
+                    this._alphaState.alphaBlend = true;
+                    break;
                 case Engine.ALPHA_COMBINE:
                     this._alphaState.setAlphaBlendFunctionParameters(this._gl.SRC_ALPHA, this._gl.ONE_MINUS_SRC_ALPHA, this._gl.ONE, this._gl.ONE);
                     this._alphaState.alphaBlend = true;