interest in the set for example p is of the order of 10 for a probability of 99 and a minimum contamination rate of 50 of course a random draw according to another procedure may be performed in step 7 the number p being related to this probability that at least one key image representing a prevalent shot has been drawn one therefore obtains p candidates elements represented in black in the rectangle b out of the p candidates to which p costs are allocated a selection is performed which consists in choosing the candidate k 1 corresponding to the lowest cost this is the object of step 9 given the assumptions this candidate corresponds to the key image this element is designated by an arrow in the rectangle c step 10 carries out a calculation of the standard deviation t of the distribution with respect to the candidate k 1 the next step 11 determines for the draw or candidate k 1 picked the subset ik of the elements of which the distance from the candidate to the signatures is less than a threshold t here this is the standard deviation of the distribution of distances from the candidate but this threshold could equally well be a value fixed a priori this subset is surrounded in the rectangle d the determination of the seed k 2 in the.