title: Is it possible to use a circle (SKShapeNode) as a mask in Sprite Kit?
body: <p>I'm trying to create a circular mask in a Sprite Kit project. I create the circle like this (positioning it at the centre of the screen):</p>
  
  <pre><code>SKCropNode *cropNode = [[SKCropNode alloc] init];
  
  SKShapeNode *circleMask = [[SKShapeNode alloc ]init];
  CGMutablePathRef circle = CGPathCreateMutable();
  CGPathAddArc(circle, NULL, CGRectGetMidX(self.frame), CGRectGetMidY(self.frame), 50, 0, M_PI*2, YES);
  circleMask.path = circle;
  circleMask.lineWidth = 0;
  circleMask.fillColor = [SKColor blueColor];
  circleMask.name=@"circleMask";
  </code></pre>
  
  <p>and further down the code, I set it as the mask for the <code>cropNode</code>:</p>
  
  <pre><code>[cropNode setMaskNode:circleMask];
  </code></pre>
  
  <p>... but instead of the content showing inside a circle, the mask appears as a square.</p>
  
  <p>Is it possible to use a <code>SKShapeNode</code> as a mask, or do I need to use an image?</p>