Anamorphosis is a distorted projection or perspective requiring the viewer to use special devices or occupy a specific vantage point to reconstitute the image. The most famous example is Holbein's painting "The Ambassadors", with a memento mori in the foreground in the shape of a distorted skull:
Andrea Pozzo's 17th century ceiling for the Church of Sant'Ignazio in Rome combines anamorphosis with trompe l'oeil, to make the flat-roofed church look like it actually has a dome:
Some modern examples of anamorphosis by Julian Beever:
Thought we'd have a bit of art history today, as I'm planning to hit the galleries at the weekend. What was the last good exhibition/installation/other you saw? And can anyone recommend stuff to see in London at the moment?