Flatting is a step in the process of coloring line art which is widely used for comic books. The line art is separated into flat, solid blocks of color which a colourist then uses to render the page. It is a time-consuming process that most professional colourists farm out to peons like me.
The above example from Strange & Darke 1 show (from left to right), the line art, the flats, the layered flats and line art, and finally the rendered page.
Words: John Smith. Pictures: Colin MacNeil. Final colours: Len O'Grady. © 2012 Rebellion A/S.