1. Image processing apparatus in which pixels of an output image are generated from groups of one or more pixels of an input image, the groups being selected in accordance with an image feature direction in the input image, the apparatus comprising: an interpolator configured to interpolate intermediate pixels between pixels of the input image; and a block comparator configured to detect correlation between sets of test blocks of pixels in the input image, the sets of test blocks being oriented with respect to a current pixel position so that the orientation of a set having the greatest correlation indicates a probable image feature direction applicable to the current pixel position, wherein, for a set of test blocks associated with an image feature angle with respect to the current pixel position within a predetermined range around a vertical orientation, the test blocks include pixels of the input image and intermediate pixels interpolated from pixels of the input image; and for a set of test blocks associated with an image feature angle with respect to the current pixel position outside the predetermined range around the vertical orientation, the test blocks include only pixels of the input image.