1. Apparatus for processing image data, comprising storage means for storing instructions, memory means for storing said instructions during execution and image data including foreground image data, processing means for performing image processing in which said foreground image data may be analysed to generate matte values for controlling a keying process in which said foreground image is to be composited against background image data, and monitoring means for facilitating user interaction with said image processing, wherein said processing means is configured such that, in response to said instructions, said image data is processed by the steps of: receiving a user indication of a background color, said indication defining a color diamond in a plane of constant luminance, and a luminance range; defining a transformation in response to said user indication; and processing pixels of said foreground image with said transformation in order to identify a matte value.