Resize independent fragment shader?

[dbalsom]

I am trying to implement a shader which is dependent on the horizontal resolution of the pixel buffer. It expects 640 pixels or it doesn't work right. If I set my window to 640x400 everything is fine, but the fragment shader gives incorrect results as soon as the window is resized, even if the window hasn't been resized far enough that the pixel buffer has been scaled up.

I suppose this is more of a question than a bug report. How can I run a fragment shader that operates just on the logical pixel buffer and not the viewport window size?