1. A method, the method comprising: using one or more processors to perform generating a set of image patch pairs for each of a set of images, wherein each of the image patch pairs contains a natural image and a corresponding downscaled lower-resolution image; defining a regression model based at least in part on the set of image patch pairs, wherein the regression model represents a gradient of a function of the downscaled lower-resolution image; and upscaling an image based at least in part on the regression model.