title: Creating cleaning effect in unity3d
body: <p>I'm trying to build some simple game(learning) where you will have to clean objects. 
  That means for example you have a monitor with dust on it so you can't see the image. You then uses mouse or finger to move arround monitor to make image visible.</p>
  
  <p>So what I've got so far is 2 images first being the monitor image and second being the dust before it. I've managed to realize when user swipped all image down. But what would I like to do is when user tap the screen somewhere only in that radius r the front image disappears. </p>
  
  <p>I'm guessing I should be using layer mask, but I have no idea how to do it.</p>
  
  <p>I basicly managed to write my own shader that only shows layer with correct image but no idea how to do that you can accualy get some cool effect from it.</p>
  
  <p>Any ideas will be apriciated!</p>