1. Apparatus for detecting a bounded region of an image, which apparatus comprises means for scanning the image via a raster scan to produce a signal indicative of image intensity at a plurality of picture points, means for comparing the image intensity at each of the said picture points with that of a neighbouring point in each of at least two directions to derive therefrom a difference value, means for labelling the picture points as corresponding to region boundaries whenever the difference values exceed a predetermined threshold value, means for expanding the region boundaries by labelling a number of further picture points as region boundaries such that boundaries between neighbouring picture elements along a scan line are unidirectionally expanded to successive neighbouring picture elements along the said scan line and boundaries between neighbouring picture elements of neighbouring scan lines are unidirectionally expanded to neighbouring successive scan lines, and means for time-shifting the signals associated with different types of expanded region boundaries by different amounts to superimpose the expanded boundaries for a region lying wholly within the bounded region of the image.