In apparatus for spatially interpolating between lines of a digital video signal, to produce interpolated lines, a supersampler horizontally interpolates between samples of the signal to produce a supersampled signal consisting of the original samples and interpolated samples located between them. Block matching circuits each determine, for each sample of the supersampled signal, the extent of matching between two blocks of N.times.M samples (N=number of lines and M=number of samples), the blocks being vertically offset in opposite directions with respect to a line to be interpolated, and being horizontally offset in opposite directions with respect to a predetermined sample position. Each block matching circuit produces a matching value for a respective different horizontal offset. A selector responds to the matching values to select, for each sample of the line to be interpolated, from a set of gradient vectors associated with the different offsets, the vector associated with the offset that produces greatest matching between the blocks. A variable direction spatial interpolator spatially interpolates the video signal, its direction of interpolation being controlled, for each sample it generates, in accordance with the vector selected for the predetermined sample position corresponding to that generated sample.