title: Pixel local position within a quad
body: <p>I have this quad in the 3D scene:</p>
  <p><a href="https://i.stack.imgur.com/yBox7.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/yBox7.png" alt="enter image description here" /></a></p>
  <p>I need to get the local positions of all painted (non transparent) pixels of this quad. Already tried to use GetPixels() and filter the result by the alpha value to get only pixels with a valid color in it. But then I noticed that it isn't possible to get the pixels' local positions using this method, cause it returns a Color array, which doesn't offer a way to retrieve that information. Already tried to google and nothing came up, maybe the only way to get what I want is to build something at shader level, but I don't know much about this subject either. I can offer more context to my doubt if needed, but I'm trying to keep things short here. Also, there's no code to show except for the wrong one using GetPixels(), which doesn't work for my case as far as I know.</p>
  <p>Any help is appreciated!</p>