Halftone

A printing press cannot do half-grey: ink is either there or not. Halftone turns the image into a grid of dots of varying size, which the eye averages into tone. Put a magnifier to a newspaper or a cereal box and there it is. This effect prints the picture below that way, in one ink or four.

How it works

The picture is split into cells (“Screen ruling” counts them across the short side), and each cell's tone becomes a shape of matching area. This is amplitude modulation: the pitch is fixed, the dot size varies. A round dot's radius grows as √a; at a = π/4 ≈ 78.5% neighbours touch and the cell corners fill in. Diamonds meet in a checkerboard at exactly 50%.

“CMYK” splits colour into cyan, magenta, yellow and black, each on its own screen; the inks multiply like transparent films. The screens are rotated — black at the “Screen angle” (45° by default), cyan and magenta 30° either side — because of moiré. Two identical grids at a small angle Δθ beat with a period of d/(2·sin(Δθ/2)) — at 5° that is bands eleven cells wide. At 30° it shrinks to two cells: a fine rosette. But a square grid repeats every 90°, so only three screens fit 30° apart; yellow goes 15° from its neighbours, since yellow moiré is barely visible.

A bit of history

Fox Talbot first proposed breaking a photograph into dots, in an 1852 patent that put gauze in front of the plate. On 4 March 1880 New York's Daily Graphic ran the first halftone photograph in a newspaper, and in 1882 Georg Meisenbach of Munich patented a practical screen process. Roy Lichtenstein's dots are Ben-Day dots (1879), not halftone: equal dots for flat tints in comics. PostScript imagesetters hit the problem this effect meets in tile mode: on a pixel grid only angles with a rational tangent — 0°, 18.4°, 26.6°, 45° — come out exact.

What to tweak

• A “Screen ruling” of 30–50 is poster and pop art; 150 and up is magazine print, where dots merge into tone at reading distance.
• “Element” set to “line” gives an engraved look: stroke width follows the tone continuously along each line.
• “Softness” blurs the dot edges, like ink spreading into paper.
• In “CMYK” the “Screen angle” turns all four screens at once, keeping the rosettes.
• It looks best on the smooth tones of Fractal noise and Mesh gradient, where dots visibly grow and merge; CMYK over Mandelbrot and Julia looks like an old magazine plate.