Merge pull request #418 from peabrainiac/develop

pen transparency fix

Author: cwillisf

src/PenSkin.js

@@ -154,6 +154,13 @@ class PenSkin extends Skin {
         return true;
     }
 
+    /**
+     * @returns {boolean} true if alpha is premultiplied, false otherwise
+     */
+    get hasPremultipliedAlpha () {
+        return true;
+    }
+
     /**
      * @return {Array<number>} the "native" size, in texels, of this skin. [width, height]
      */
@@ -181,8 +188,9 @@ class PenSkin extends Skin {
     clear () {
         const gl = this._renderer.gl;
         twgl.bindFramebufferInfo(gl, this._framebuffer);
-
-        gl.clearColor(1, 1, 1, 0);
+        
+        /* Reset framebuffer to transparent black */
+        gl.clearColor(0, 0, 0, 0);
         gl.clear(gl.COLOR_BUFFER_BIT);
 
         const ctx = this._canvas.getContext('2d');
@@ -598,7 +606,7 @@ class PenSkin extends Skin {
             this._silhouetteBuffer = twgl.createFramebufferInfo(gl, [{format: gl.RGBA}], width, height);
         }
 
-        gl.clearColor(1, 1, 1, 0);
+        gl.clearColor(0, 0, 0, 0);
         gl.clear(gl.COLOR_BUFFER_BIT);
 
         this._silhouetteDirty = true;

src/RenderWebGL.js

@@ -1620,7 +1620,14 @@ class RenderWebGL extends EventEmitter {
             }
 
             twgl.setUniforms(currentShader, uniforms);
-
+            
+            /* adjust blend function for this skin */
+            if (drawable.skin.hasPremultipliedAlpha){
+                gl.blendFuncSeparate(gl.ONE, gl.ONE_MINUS_SRC_ALPHA, gl.ONE, gl.ONE_MINUS_SRC_ALPHA);
+            } else {
+                gl.blendFuncSeparate(gl.SRC_ALPHA, gl.ONE_MINUS_SRC_ALPHA, gl.ONE, gl.ONE_MINUS_SRC_ALPHA);
+            }
+            
             twgl.drawBufferInfo(gl, this._bufferInfo, gl.TRIANGLES);
         }
 

src/Skin.js

@@ -76,6 +76,13 @@ class Skin extends EventEmitter {
         return false;
     }
 
+    /**
+     * @returns {boolean} true if alpha is premultiplied, false otherwise
+     */
+    get hasPremultipliedAlpha () {
+        return false;
+    }
+
     /**
      * @return {int} the unique ID for this Skin.
      */

src/shaders/sprite.frag

@@ -194,12 +194,6 @@ void main()
 		discard;
 	}
 	#endif // DRAW_MODE_colorMask
-
-	// WebGL defaults to premultiplied alpha
-	#ifndef DRAW_MODE_stamp
-	gl_FragColor.rgb *= gl_FragColor.a;
-	#endif // DRAW_MODE_stamp
-
 	#endif // DRAW_MODE_silhouette
 
 	#else // DRAW_MODE_lineSample