1. An 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, the apparatus comprising: a processor programmed to compare 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 indicates an image feature direction applicable to that pixel position; and detect whether the image feature direction may be used to interpolate an output pixel value at the pixel position under test by performing at least: (i) a monotonicity test to determine whether at least one block of the block of pixels exhibits a monotonic variation in a pixel property in a horizontal direction; (ii) a direction test to detect whether a detected image feature direction for the pixel position under test meets predetermined directional criteria including at least a first directional criterion; and (iii) in at least cases where the direction test fails, 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, wherein if the direction test fails, 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.