Fragment shader gives you current interpolated xy position in screen space, eg x: 500, y: 100. That’s for the rendered quad.
Full size texture might be sized 1000 x 1000, being the same size as the window.
UV coordinate to read the full size texture then would be u: 500 / 1000, v: 100 / 1000 or [0.5, 0.1].
I think 