1. A method, comprising: training groups of training features derived from regions of training images to obtain a plurality of classifiers, wherein each classifier corresponds to each group of training features, utilizing the plurality of classifiers to classify groups of validation features derived from regions of validation images to obtain a plurality of weights, wherein each weight corresponds to each region of the validation images and indicates how important the each region of the validation images is; and discarding a weight from the plurality of weights based upon a certain criterion.