Adds support for transfer mode in Paint and SpriteNode classes. Adds support...

Adds support for transfer mode in Paint and SpriteNode classes. Adds support for opacity in sprites, and modifies example game.

R=eseidel@chromium.org

Review URL: https://codereview.chromium.org/1164633004

examples/game/lib/game_world.dart

@@ -66,6 +66,8 @@ class GameWorld extends TransformNode {
     SpriteNode sprt = new SpriteNode.withImage(_image);
     sprt.width = radius*2;
     sprt.height = radius*2;
+    sprt.colorOverlay = new Color(0x33ff0000);
+    sprt.transferMode = TransferMode.plusMode;
     body.userData = sprt;
     this.children.add(sprt);
   }

examples/game/lib/sprite_node.dart

@@ -6,6 +6,9 @@ class SpriteNode extends TransformNode {
   
   Image _image;
   bool constrainProportions = false;
+  double _opacity = 1.0;
+  Color colorOverlay;
+  TransferMode transferMode;
   
   SpriteNode() {
     this.pivot = new Vector2(0.5, 0.5);
@@ -15,7 +18,14 @@ class SpriteNode extends TransformNode {
     this.pivot = new Vector2(0.5, 0.5);
     _image = image;
   }
-  
+
+  double get opacity => _opacity;
+
+  void set opacity(double opacity) {
+    assert(opacity >= 0.0 && opacity <= 1.0);
+    _opacity = opacity;
+  }
+
   void paint(PictureRecorder canvas) {
 
     if (_image != null && _image.width > 0 && _image.height > 0) {
@@ -37,7 +47,18 @@ class SpriteNode extends TransformNode {
       }
       
       canvas.scale(scaleX, scaleY);
-      canvas.drawImage(_image, 0.0, 0.0, new Paint()..setARGB(255, 255, 255, 255));
+
+      // Setup paint object for opacity and transfer mode
+      Paint paint = new Paint();
+      paint.setARGB((255.0*_opacity).toInt(), 255, 255, 255);
+      if (colorOverlay != null) {
+        paint.setColorFilter(new ColorFilter(colorOverlay, TransferMode.srcATopMode));
+      }
+      if (transferMode != null) {
+        paint.setTransferMode(transferMode);
+      }
+
+      canvas.drawImage(_image, 0.0, 0.0, paint);
       canvas.restore();
     }
     else {