What does the 1/w coordinate stand for in gl_FragCoord?

Question

The gl_FragCoord variable stores four components: (x, y, z, 1/w)

What is the w coordinate and why is it stored as 1/w?

Answer 1

A homogeneous coordinate is given by: (x, y, z, w), which projects to: (x/w, y/w, z/w). gl_FragCoord stores this projection, but rather than storing the (useless) (w/w) = (1) for the last component, it stores (1/w), to preserve useful information.