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) arranging the color gradient histograms in a plurality of correlation families, each correlation family comprising a corresponding set of correlated color gradient histograms having a similar statistical behavior; c) subdividing the descriptor array in at least two sub-arrays, each sub-array comprising a respective number of elements of the descriptor array; d) 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; e) arranging the at least two sub-arrays into at least one group of sub-arrays; and f) using a same codebook for compressing the at least two sub-arrays of the at least one group of sub-arrays; and 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, and for each of said at least one group of sub-arrays, an array element in one sub-array of the group occupying a same position as an array element in another sub-array of the group correspond to color gradient histograms belonging to a same correlation family.