1. A method of pixel interpolation in which a two-dimensional digital filter is arranged so as to interpolate an output pixel from a two-dimensional array of colour input pixels by aligning a filtering operation with a detected direction of an image feature at a pixel position to be interpolated; the method comprising: detecting image activity within respective colour components of a group of input pixels around the pixel position to be interpolated; generating monochromatic test pixels by combining the colour components of input pixels in relative proportions dependent upon the detected image activities; detecting the direction of an image feature in a group of the monochromatic test pixels around the pixel position to be interpolated; and interpolating an output pixel using the two-dimensional filter, according to the detected image feature direction.