1. A computer-implemented method for detecting image impairments caused by interpolation in an image interpolated from two or more adjacent images thereto, each image comprising a plurality of pixels, the method comprising: applying a substantially shift invariant image transform to the interpolated image and to at least one of the adjacent images to derive a transformed image representation for each image, each transformed image representation comprising a plurality of values which are substantially independent of the spatial position of the pixels in the image; comparing corresponding ones of the values in the transformed image representations of the interpolated image and at least one of the adjacent images to produce comparison results; summing the comparison results; and detecting differences between the transformed image representations indicative of image impairments in the interpolated image caused by interpolation from the summed comparison results.