Image processing apparatus in which output pixel values are generated with respect to pixels of an input image selected in accordance with an image feature direction in the input image comprises comparing means for comparing blocks of pixels of the input image, the blocks being disposed with respect to a pixel position under test so that a correlation between the blocks would indicate an image feature direction applicable to that pixel position; and validating means, for detecting whether an image feature direction may be used to interpolate an output pixel value at the pixel position under test, the validating means being arranged to perform at least: (i) a direction test to detect whether a detected image feature direction for the pixel position under test meets one or more predetermined directional criteria; and (ii) in at least cases where the direction test is failed, a pixel value test to detect whether pixel values in image regions disposed with respect to the pixel position under test in accordance with possible image feature directions meet one or more predetermined pixel value criteria; the arrangement being such that if the direction test is failed, an interpolation direction is selected for the pixel position under test as that image feature direction having the greatest block correlation which passes the pixel value test.