1. A method for processing digital data comprising a quantisation step of calculating in a dimension space d at least one leader index I 1 for at least one part of the vectors 1, the vectors 1 constituting descriptors of input data, the leader index I 1 corresponding to the number of vectors that precede the leader 1 in reverse lexicographical order without the step of determining all the leaders, wherein the step of calculating the leader index I 1 comprises one step of calculating selected from the group consisting of: step of calculating the norm r δ,d P of the said leader 1 (x 1 , x 2 ,. .. x d ), where x 1 to x d are sorted in increasing order, and recursive counting steps on the coordinates x i with the value of i ranging from d to 1, the said counting steps consisting in counting the vectors in which the coordinate x i is comprised between x i +1 and MIN (x i+1 , F(r δ,i P )), the index I 1 being equal to the sum of the results of the counting steps plus the difference between min (F(T(x 1 )), x 2 ) and x 1 , wherein T(x i ) is a function returning the result of dividing the coordinate x i raised to the power p by a precision factor delta, the result of the said division being rounded to the nearest integer, and F(A) is a function that returns an integer value w in which the value T(w) is lower than or equal to the argument A of the said function F; step of calculating the norm r δ,d p of the said leader 1 (x 1 , x 2 ,. .. x d ), where x to x d are sorted in decreasing order, and recursive counting steps on the coordinates x i with the value of i ranging from 1 to d, the said counting steps consisting in counting the vectors in which the coordinate x i is comprised between x i +1 and MIN (x i−1 , F(r δ,i p )), the index I 1 being equal to the sum of the results of the counting steps plus the difference between min (F(T(x δ )), x d-1 ) and x d , wherein T(x i ) is a function returning the result of dividing the coordinate x i raised to the power p by a precision factor delta, the result of the said division being rounded to the nearest integer, and F(A) is a function that returns an integer value w in which the value T(w) is lower than or equal to the argument A of the said function F.