1. A method for processing an image, comprising: identifying a group of keypoints in the image; for each keypoint of the group: a) calculating a corresponding descriptor array including a plurality of array elements, each array element storing values taken by a corresponding color gradient histogram of a respective sub-region of the image in the neighborhood of the keypoint; b) subdividing the descriptor array in at least two sub-arrays, each sub-array comprising a respective number of elements of the descriptor array, and c) generating a compressed descriptor array comprising a corresponding compressed sub-array for each of said at least two sub-arrays, each compressed sub-array being obtained by compressing the corresponding sub-array of said at least two sub-arrays by means of vector quantization using a respective codebook; exploiting the compressed descriptor arrays of the keypoints of said group for analysing the image, wherein: for each keypoint of said group, the subdivision of the descriptor array in at least two sub-arrays is carried out based on correlation relationships among the color gradient histograms whose values are stored in the elements of the descriptor array of said each keypoint.