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Fig. 3 Let's suppose we wish to automatically detect the strong peaks of contrasts 2 and 3 without deteeting the smooth peak 1. For that we compare the images given by the thresholds Tf ptA «. For each pak, we will retain the image given by the threshold fï? only if the image given by the threshold T. is not too large. So we will retain the images 2—2 and 3—2 for peaks 2 and 3 but not the image 1-2 because 1-1 was much larger than 2-1 and 3-1, Praetically we de¬ tect the size of a given pattern in a image by the morpho-logical transformation "opening" (see réf. 3 by J. SERRA). It is easy to see that 1/ If the size of the opening increases, the detected contrasts become sinoother. 2/ If the différence of grey levels between the 2 thresholds increases, the detected contrasts become higher : For example, if in fig. 3 , we had used the grey levels 1 and 3 the low contrast peak 3 would have been eliminated.