1. A computer-implemented method for searching an image database comprising: detecting keypoints in a dictionary image composed of a plurality of pixels, wherein a density of the pixels distributed around a plurality of keypoints form a Gaussian distribution using coordinates of a keypoint as an origin point; selecting random pairs of pixels among the detected keypoints, the selection based on candidate coordinates 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; and 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.