In order to reduce the effects of discontinuities at the edge of a picture and reduce erosion of the usable picture area caused by a filtering or interpolation operation, the edges of the picture are extrapolated prior to such an operation. This may be achieved by a circuit which receives an input line of active pixel data; forms from the input line a delayed line of the active pixel data; produces at least one additional start pixel data derived from a pixel data at or adjacent the start of the input line; produces at least one additional end pixel data derived from a pixel data at or adjacent the end of the input line; and forms an output sequence of pixel data by selecting for the output sequence the additional start pixel data, then at least part of the delayed line, and then the additional end pixel data. This circuit therefore performs zero-order (repeat) extrapolation at the beginning and end of each line. Other circuits are also described for extrapolating lines at or adjacent the start and end of each frame, and for performing a first- or higher-order extrapolation.