Data processing apparatus arranged to receive successive images from a video camera; the apparatus comprising: means (50, 60) for detecting inter-image motion for selected points in an image region associated with a control function of the data processing apparatus; means (70, 80, 90) for executing the control function if inter-image motion is detected in the image region; and difference measuring means (30, 40) operable: to generate difference data, the difference data comprising a set of values proportional to the difference in pixel values between two successive images; to subsequently smooth the difference data; to apply a threshold to the difference data at a difference threshold level; and to output resulting binary difference data; in which the detecting means (50, 60) is arranged to: select points in an image region of a current image that are validated by selecting test points in a currently received image based upon their correspondence with points in a preceding received image, the points in the preceding received image themselves corresponding with the points originally selected in the currently received image; and deselect points in the current image whose corresponding test points do not substantially coincide; and in which the means for detecting inter-image motion is operable to discard, prior to any selection of points in a preceding image, points in the current image for which the equivalent position in the difference image shows a difference below the difference threshold level. means (50, 60) for detecting inter-image motion for selected points in an image region associated with a control function of the data processing apparatus; means (70, 80, 90) for executing the control function if inter-image motion is detected in the image region; and difference measuring means (30, 40) operable: to generate difference data, the difference data comprising a set of values proportional to the difference in pixel values between two successive images; to subsequently smooth the difference data; to apply a threshold to the difference data at a difference threshold level; and to output resulting binary difference data; to generate difference data, the difference data comprising a set of values proportional to the difference in pixel values between two successive images; to subsequently smooth the difference data; to apply a threshold to the difference data at a difference threshold level; and to output resulting binary difference data; in which the detecting means (50, 60) is arranged to: select points in an image region of a current image that are validated by selecting test points in a currently received image based upon their correspondence with points in a preceding received image, the points in the preceding received image themselves corresponding with the points originally selected in the currently received image; and deselect points in the current image whose corresponding test points do not substantially coincide; and in which the means for detecting inter-image motion is operable to discard, prior to any selection of points in a preceding image, points in the current image for which the equivalent position in the difference image shows a difference below the difference threshold level. select points in an image region of a current image that are validated by selecting test points in a currently received image based upon their correspondence with points in a preceding received image, the points in the preceding received image themselves corresponding with the points originally selected in the currently received image; and deselect points in the current image whose corresponding test points do not substantially coincide; and in which the means for detecting inter-image motion is operable to discard, prior to any selection of points in a preceding image, points in the current image for which the equivalent position in the difference image shows a difference below the difference threshold level.