1. A method for determining a feature in a subject image, comprising: receiving a subject image; determining a sparse transformed subject image that represents the subject image with a few significant coefficients compared to a number of values in the subject image; receiving a plurality of number I of patch functions that are based on a portion of a sparse transformed image for each of a training set of images and represent learned features in the training set; and sending data that indicates a feature in the subject image based on the transformed subject image and the plurality of patch functions.