A chroma keying system 1 processes foreground image data representing a foreground subject in front of a matte, together with background data representing a background image, to produce composite image data representing the foreground subject and the background in place of the matte. A programmable look up table 8 is provided for receiving on a pixel-by-pixel basis the most significant bits (MSBs) of the foreground image data and outputting in response thereto range data representing limits of a keying range. An interpolator 9 is also provided for interpolating the range data depending on the least significant bits (LSBs) of the foreground image data and outputting keying data. A combiner 4 combines the foreground image data and the background image data on a pixel-by-pixel basis in response to the keying data to produce the composite image data. The system is operable in a calibration mode in which a generator 15 is connected to input the colour point data to an external keyer. The generator 15 generates colour point data representing all possible colours that could be represented by the most significant bits of the foreground image data, causing the external keyer to output keying data. The look up table 8 is also connected to the external keyer in this mode so that it can receive and store the keying data for subsequent use in the processing of foreground image data together with background image data to produce composite image data.