title: Calculate sum of pixels written by shader in Unity3D
body: <p>For each pixel written by a shader in Unity3D I would like to add it to a global variable somewhere so that I can read it back later. So, if the shader program iterates over 1000 pixels writing the color (1,0.5,0) the result I want back to my program would be (1000,500,0).</p>
  
  <p>The actual calculation is much more complex of course, and very time consuming as it has to be done millions of times, doing calculations using multiple textures. So, I need to take advantage of the parallel computing ability of the GPU.</p>
  
  <p>I read somethings about compute shaders that allows shaders to write numbers that can be read back, but I have a hard time to find any relevant example. </p>
  
  <p>Any pointers would be useful.</p>