The pages behind the controls
Every control in the lab is a real parameter with a real unit, which makes it honest and leaves it silent. A cell size of eight can tell you it means eight pixels; it cannot tell you why nobody screens at four, or why the number you want changes the moment the file is going to a printer rather than to a phone.
These three pages carry that missing half. Between them they cover the one distinction the whole site is built on, the century of printing practice that decided the halftone defaults, and the two rival ways of faking a colour you do not have. None of them is a tutorial — there are no numbered steps and nothing to follow along with — and none needs the others first. Take whichever one matches the thing that is currently puzzling you, and come back for the other two when something else does.
They are written to be read away from the tool, so each one states its own numbers rather than telling you to go and look at a slider.
Effects, and why they are not filters
A single question — did the units the picture is made of change? — sorted into fifteen cases people argue about, including grain, greyscale and duotone.
Read it if you came here for a warm vintage look and got printer's dots instead.
Why halftone dots have angles
Where 15, 75, 0 and 45 degrees come from, what a rosette actually is, and the arithmetic that decides how many pixels a single dot needs before a printer can hold it.
Read it before sending a screened picture anywhere near paper.
Error diffusion versus ordered dither
Two families that spend missing colours in opposite ways: one carries its mistakes forward and hides them, the other takes a fixed matrix and shows its working.
Read it when a sky has gone stripy and you need to know which knob caused it.
Or skip the reading
Nothing here is required to use anything. Open a picture in the lab and the six effects appear on your own photograph within a second or two, which answers most of what these pages answer, faster and with less arithmetic. The essays are for afterwards, when a result has surprised you and you want to know whether it was the picture or the method.
Rendered on your computer, not on a server.