Fragment position in CG post-process shaders

I come from GLSL world and for some post process effects I need to retrieve fragment position x and y.In GLSL I do it using :

  gl_FragCoord.xy

Now , in CG , based on this article I should have access to VPOS.But I haven't found any Unity docs which say how to get it. Shouldn't vert_img supply it?What does vert_img.pos mean? Position of what and in what space?How do I get VPOS using CG ?

Thanks.