1. An apparatus which facilitates the use of a printed or scribed document as an interface with a computer and which comprises: b. a printed or scribed document bearing specialised calibration marks, the document being freely variably positioned on a work surface; c. a camera focused on the document for generating video signals representing the document in electronic form; and d. a processor linked to the camera for processing an image captured by the camera and configured to identify the calibration marks of the document in the captured image and then determine from the location of the calibration marks in the image a transformation between co-ordinates of features in the image and corresponding co-ordinates of features in the document that compensates for the freely variable positioning of the document on the work surface.