Video signal processing apparatus in which at least first and second input video signals are combined to generate an output compressed video signal, the first and second video signals each having respective associated motion vectors from a data compression process applied to the video signals; comprises: means for establishing a set of the motion vectors associated with the first and second video signals which can potentially be re-used in compressing the output compressed video signal; and means for testing the set of motion vectors, the testing means being operable: (c) to define, with respect to reference images referred to by the motion vectors under test, a first border region of a predetermined width surrounding those parts of the reference images derived in part from the first video signal, and a second border region of a predetermined width surrounding those parts of the reference images derived in part from the second video signal; and (d) to test whether motion vectors of the set which are associated with parts of each video signal refer either to image areas derived in part from the other video signal, to the first border region or to the second border region; in which any motion vectors of the set being found by the testing means to refer to images areas derived to at least the predetermined degree from the other input video signal, the first border region or the second border region are not used in the compression of the output compressed video signal.