title: How to do the opposite of scratch off image effect
body: <p>I copied the code found here:</p>
  
  <p><a href="http://www.cocos2d-iphone.org/forum/topic/12557" rel="nofollow">http://www.cocos2d-iphone.org/forum/topic/12557</a></p>
  
  <p>To create a successful "scratch off" effect, i.e. when my burnsprite visits the top layer image it makes it transparent in the spot the burnsprite is in, revealing the bottom layer under it.</p>
  
  <p>I want to do the opposite.  I want the top layer to start off transparent and have it become visible in the spot the burnsprite visits.</p>
  
  <p>Below I list what I've tried to do:</p>
  
  <p>I changed the burnsprite blend function to <code>{GL_ZERO, GL_ONE}</code> .  I think this is the correct thing to do.  I don't know if it works or not though because my top layer does not start off transparent.  </p>
  
  <p>I changed  <code>[topLayer clear:0.0f g:0.0f b:0.0f a:1.0f]</code> to  <code>[topLayer clear:0.0f g:0.0f b:0.0f a:0.0f]</code> and nothing happened.  It was not transparent.</p>
  
  <p>I then did <code>topLayer.sprite.opacity = 0.0;</code>
  Then is seemed to be only partially transparent.  Still not what I wanted.</p>
  
  <p>Please help.</p>