Find the Dots Your Printer Hid on the Page DOTS
Most colour laser printers and copiers print a pattern of pale yellow dots on every page, too faint to see, that encodes the printer's serial number and usually the date and time. Scan a page in colour at 600 dpi and this finds the dots, shows the pattern, reads it where the layout is one that has been published, and makes a copy of the scan with the dots taken out.
The scan
A colour scan of a printed page, as PNG or JPEG. 600 dpi keeps the dots; a black and white or greyscale scan drops them, and a photo from a phone is usually too soft. It is decoded here and never leaves this tab.
Lower finds fainter dots and more specks of noise; higher is stricter. The middle is right for most scans.
What is on the page
Every dot found is marked in blue on the scan, so the pattern that is invisible on paper is visible here.
The cleaned copy is a PNG with every dot, and a one-pixel ring round it, painted from the paper beside it. Check it before you rely on it: a dot printed over ink, rather than paper, may leave a mark where it was.
Questions people ask about DOTS