usual_efficiency_410 The shader outputs proper depth so it's always drawn "on the ground". If you want it to be drawn over everything just replace the line that assigns to DEPTH
with:
DEPTH = 0.0;
Note that you'll also need to keep your camera facing quad or sprite closer to the camera than any other object.
Btw don't assign the shader to a horizontal ground quad or plane because that's not how it's really intended to be used.