title: In Unity3D, How to make the whole scene in dark, but some Area are not in dark
body: <p>I wan't make some teaching levels with UGUI, so need the whole to be in dark, but some rectangular area not in dark.</p>
  
  <p>I can make the whole scene in dark by adding a gameobject with a image component, and control it's alpha to dark.</p>
  
  <p>But I don't know how to make some rectangular area to be lighted, and the rectangular will change in different teaching levels. And different device with different resolution will also change the rec's bounder.</p>
  
  <p>Picutre3 is what I wanted, and just rectangular area is OK.</p>
  
  <p>Picture1
  <img src="https://i.stack.imgur.com/8cTjx.png" alt="P1"></p>
  
  <p>Picture2</p>
  
  <p><img src="https://i.stack.imgur.com/XaRv5.png" alt="P2"></p>
  
  <p>Picture3</p>
  
  <p><img src="https://i.stack.imgur.com/U9oEY.png" alt="P3"></p>
  
  <p>Picture4
  <img src="https://i.stack.imgur.com/IP06M.png" alt="P4"></p>