Halftone Generator

Printing cannot produce grey

A press puts ink on paper or it does not; there is no half amount. Halftoning solves this by varying the size of dots at a fixed spacing, so an area of large dots reads as dark and small dots as light, and the eye blends them at normal viewing distance into continuous tone. Every printed photograph before digital and almost all of them since works this way, and looking at a newspaper photograph closely shows the dots directly.

The screen angle prevents moiré

Colour printing overlays four halftone screens, one per ink. If they share an angle the patterns interfere and produce a strong visible moiré across the whole image. Rotating each screen to a different angle, conventionally 15, 45, 75 and 0 degrees for cyan, magenta, black and yellow, breaks the interference into a much finer rosette pattern that the eye reads as texture rather than as a fault. Those specific angles were arrived at empirically and are still the standard.

Dot size and viewing distance

A newspaper prints at around 85 lines per inch because coarse paper cannot hold finer detail, which is why newspaper photographs have visible dots. A magazine manages 150 or more on coated stock. The pattern must be fine enough that the eye cannot resolve individual dots at the intended viewing distance, and the same logic applies here: a halftone that looks like texture on screen becomes obvious dots when enlarged.

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