1. A computer program product comprising a computer-readable storage device having computer readable program code embodied thereon, which when executed implements a computer implemented method for searching an image database, the method comprising: detecting keypoints in a dictionary image composed of a plurality of pixels; selecting random pairs of pixels among the detected keypoints, the selection made among candidate coordinates in a Gaussian distribution for pixels distributed around the detected keypoints; comparing brightness between the selected pairs of pixels, including calculating a brightness difference between the selected pairs of pixels; converting the calculated brightness difference into a feature vector, wherein the conversion includes calculating the feature vector; and using the feature vector to search for the dictionary image corresponding to an image of a target object.