Good morning, I'm new to shader coding and trying to implement some visual effects code in shaders using billboards. (Yes, I couldn't have picked anything harder to start with, but I'm lucky that way)

Setup: I have rendered the full screen z depth to an array of floats in a previous pass. In the fragment shader I need the scene depth where the rendered fragment is displayed (to see if it's occluded). I can use tex2d() to get the depth value if I have the screen coordinates of the point being rendered in the fragment shader.

Question: In the fragment shader how do you calculate the screen coordinates of the pixel (in the range 0-1.0)?

Is the position passed to the fragment shader a pixel offset? If so, I guess it would be: float2( position.x / screen-width, position.y / screen-height )

Thanks for any help/