   Point Sprites and Pixel Coordinates
  I am currently working on a heat haze effect for my engine.  I am using Point Sprites as they seems to give better performance, but they have a few drawbacks.  Primarily, I can't get any screen space coordinates of each pixel when render (since all input data is simply copied per vertex).  I need the screen space coordinates to read the backbuffer and distort it properly.  As it is, I calculate the transformed position in the vertex shader, then let it get interpolated per pixel to my pixel shader.  With point sprites, since the input is a single vertex, this doesn't work (since there's nothing to interpolate).  Short of creating my own quads, does anyone know of any way to handle this?   