There is no
gray in
a printed
photograph.

Look closely at any photograph ever printed on paper — a newspaper, a magazine, a book. The grays are not there. What is there is dots: thousands of them, larger where the picture is dark, small enough that your eye gives up and averages them into tone.

This site takes apart the halftone shader running behind these words — where the technique came from, and how a rule invented for ruled glass in 1894 ended up as four floats in a fragment shader.