Abstract: Image frames are stored as a plurality of pixel values. An image frame is viewed in which a perceived object (301) is to be distinguished from the remainder of the image frame. Input signals identify control positions in proximity to the boundary of said object. Pixel values are compared around each of the control positions to define a local color transition. Color transitions for the whole object are then produced by a process of interpolation. In this way, a process similar to chroma-keying may be performed without the need to record a foreground object against a specific type of background object. The chroma-keying process is performed locally with respect to a local color transition. This results in local control signals being produced and control signals for the whole object are then produced by a process of interpolation.