An image processing apparatus generating output image pixels from groups of one or more input image pixels, the groups selected in accordance with an image feature direction in the input image. An interpolator interpolates intermediate pixels between the input image pixels, and a block comparator detects correlation between sets of test blocks of pixels in the input image, which sets are 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. For at least some sets of test blocks having an orientation with respect to the current pixel position within a predetermined range around a vertical orientation, the test blocks include the input image pixels and intermediate pixels interpolated from the input image pixels. For other sets of test blocks, the test blocks include only the input image pixels.