by employing an embodiment of the current invention , finding the median of n values is more efficient than the prior known process of full sorting . finding the median of n values is divided into a group of problems of finding the median of m values , where m is significantly smaller than n . the total number of comparisons required to solve the group of problems is smaller than that required to sort the original n values . more specifically , to find the median of n values , we divide the n values , in this example , into three groups , namely , g 1 , g 2 and g 3 . assume group g 1 has n 1 values , group g 2 has n 2 values and group g 3 has n 3 values . therefore , the three groups contain all n values , namely , n = n 1 + n 2 + n 3 . now assume that a median has been determined for each of the groups g 1 , g 2 and g 3 , and have been labeled as a 1 , a 2 and a 3 , respectively . a 1 is the median of group g 1 , a 2 is the median of group g 2 and a3 is the median of group g 3 . furthermore , it is assumed that all values have been determined in group g 1 that are less than or equal to median a 1 . these values are designated as being in sub - group g 11 . similarly , all values that are greater than or equal to median a 1 in group g 1 are designated as being in sub - group g 12 . all values in group g 2 that are less than or equal to median a 2 are designated as being in sub - group g 21 . similarly , all values in group g 2 that are greater than or equal to median a 2 are designated as being in sub - group g 22 . all values in group g 3 that are less than or equal to median a 3 are designated as being in sub - group g 31 . similarly , all values in group g 3 that are greater than or equal to median a 3 are designated as being in sub - group g 32 . how the median a 1 and the first set of sub - groups including g 11 and g 12 of group g 1 are determined is not being specified at this moment . it is sufficient to say that they may be found by using the current process : further dividing group g 1 into a second set of three sub - groups , and applying the current process recursively . when eventually , a sub - group in the second set contains 9 or fewer values , the median of the particular sub - group can be found by using the median determining process of the 9 - value , i . e ., point , spatial median filter to be described in more detail below in relationship to the 9 - value spatial median filter implementation . this process also applies to the remaining groups which , in this example , are groups g 2 and g 3 . medians a 2 and a 3 and the corresponding sub - groups g 21 , and g 22 , and g 31 and g 32 of groups g 2 and g 3 , respectively , are found in the same manner as median a 1 and sub - groups g 11 and g 12 of group g 1 . it is noted that the current process , in addition to determining the median of each group of values , also yields a sub - group including all values that are less than or equal to the median of the group and a sub - group including all values that are greater than or equal to the median of the group . this is realized without any additional processing . for now , it is assumed that the three medians and six sub - groups have been determined , namely , median a 1 , and sub - groups including g 11 and g 12 of group g 1 , median a 2 , and sub - groups g 21 and g 22 of group g 2 , and median a 3 , and sub - groups g 31 and g 32 of group g 3 . then , the three medians a 1 , a 2 and a 3 are fully sorted . without loss of generality , we can assume a 1 ≦ a 2 ≦ a 3 . if they are not , the smallest value of a 1 , a 2 and a 3 is re - labeled to be a 1 , and it &# 39 ; s associated group as g 1 , and the associated sub - groups as g 11 and g 12 etc . at this stage , some values can be eliminated from the n values . that is , the median of the n values is guaranteed to be among the surviving values . specifically , median a 1 and all values in sub - group g 11 can be eliminated . additionally , median a 3 and all values in sub - group g 32 can also be eliminated . let m 1 = floor ( n 1 / 2 ), i . e ., the largest integer that is less than or equal to n 1 / 2 . note m 1 =( n 1 − 1 )/ 2 , if n 1 is odd . m 1 includes all the values in sub - group g 11 because they are less than or equal to a 1 . similarly , let m 3 = floor ( n 3 / 2 ), i . e ., the smallest integer that is greater than or equal to n 3 / 2 . m 3 includes all values in sub - group g 32 because they are greater than or equal to a 3 . the total number of values eliminated is therefore equal to m 1 + m 3 + 2 . after the elimination of these values , the surviving values from the original n values are : a ) all values in sub - group g 12 from group g 1 that are greater than or equal to a 1 , excluding a 1 , there are m 1 − 1 of them ; b ) all values in group g 2 ( n 2 of them ); and c ) all values in sub - group g 31 from group g 3 that are less than or equal to a 3 , excluding a 3 , there are m 3 − 1 of them . therefore , the total number of surviving values is p = n − floor ( n 1 / 2 )− floor ( n 3 / 2 )− 2 . now the remaining task is to find the median of the remaining p =( n − floor ( n 1 / 2 )− floor ( n 3 / 2 )− 2 ) values . thus , the original problem of finding the median of n values is reduced to the following : 1 ) find three medians of the three groups of size n 1 , n 2 , and n 3 , and obtain sub - groups g 11 , g 12 , g 21 , g 22 , g 31 and g 32 ; 3 ) find the median of the remaining p =( n − floor ( n 1 / 2 )− floor ( n 3 / 2 )− 2 ) values , and find the two sub - groups , a first sub - group including all values that are less than or equal to the median of the p values and a second sub - group including all values that are greater than or equal to the median of the p values . this process can be applied recursively to finding the medians in the steps 1 ) and 3 ) above . for example , the problem of finding the median a 1 of group g 1 can be further divided into the above three steps . the recursion stops when the size of median filter is small , for example , 9 . at that point , the process of finding the median in a 9 - value spatial median filter can be used . if one starts with n & lt ; 9 , the above process still can be applied . the size is recursively reduced . the recursion stops when the size becomes three ( 3 ). then , the median of 3 - points can be found by sorting the three values . the optimal 3 - value spatial median filter is the same as the full sort of the 3 values . fig1 shows , in simplified form , details of a so - called 9 - value spatial median filter that is advantageously used in practicing the invention . the spatial median filter can be advantageously employed to measure the overall effect of all values that make up an object . additionally , since each individual value may be prone to random noise , use of spatial median filter can also reduce the effects of the noise . referring to fig1 it is seen that the 9 - points , i . e ., values , are arranged into three groups of three values each , namely , a first group including values a , b and c , a second group including values d , e and f , and a third group including values g , h and j . the first group is supplied to sorter 101 , the second group to sorter 102 and the third group to sorter 103 . sorters 101 , 102 and 103 each perform a complete sort of their respective supplied groups , i . e ., arrange the supplied values in either ascending or descending order . in the spatial median filter shown in fig1 it is assumed that the values are arranged in ascending order . that is , a 3 ≧ a 2 ≧ a 1 and so on for the other values . note that a sorter of three values requires three comparisons . thus , the three sorters 101 , 102 and 103 perform nine comparisons . the median of each group is determined to be the middle value in the sorted group . the three medians from sorters 101 , 102 and 103 , in this example , are a 2 , b 2 and c 2 , respectively , and are supplied to sorter 104 . in turn , sorter 104 sons the three medians a 2 , b 2 and c 2 . this requires another three comparisons . after sorting , the three medians a 2 , b 2 and c 2 , are assumed to be arranged in ascending order and are designated λ , β and γ , respectively , where λ ≦ β ≦ γ . now the nine values of the spatial median filter are reduced to five values by removing four values . the remaining five values include the median of the nine values . this reduction is realized by first identifying the group of three values who &# 39 ; s median is λ . these values are labeled in ascending order as d 1 ≦ d 2 ≦ d 3 . it is noted that these three values had been sorted in the prior sorting operations . additionally , since d 2 is the median of the group , it has the same value as λ . it can be shown that both d 1 and d 2 are each less than or equal to the median of the nine values and , hence , can be removed from the nine values . now label the three values having γ as its median in ascending order as f 1 ≦ f 2 ≦ f 3 . again , it is noted that f 2 has the same value as γ . it can be shown that the values f 2 and f 3 are each greater than or equal to the median of the nine values and , hence , can also be removed from the nine values . thus , leaving five values including d 3 , f 1 and a group of three values having β as its median that is labeled in ascending order as e 1 ≦ e 2 ≦ e 3 . these remaining five values arc divided into two groups and further sorted . one group includes d 3 and e 1 that after sorting via sorter 105 are labeled in ascending order as g 1 ≦ g 2 . this sorting requires only one comparison . the second group includes e 2 , e 3 and f 1 that after sorting via sorter 106 are labeled in ascending order as h 1 ≦ h 2 ≦ h 3 . this sorting only requires two comparisons because e 2 and e 3 have already been sorted . of the remaining five values g 1 , g 2 , h 1 , h 2 and h 3 , it can be shown that value g 1 is less than or equal to the median of the nine values and can be removed , and that value h 3 is greater than or equal to the median of the nine values and can be removed , leaving values g 2 , h 1 and h 2 . these remaining three values are sorted via sorter 107 and labeled in ascending order as j 1 ≦ j 2 ≦ j 3 . this sorting takes only two comparisons because values h 1 and h 2 have already been sorted . the median value of the group from sorter 107 is the median of the nine values and is value j 2 . it is noted that in this process four values have been identified that are less than or equal to the median of the nine values , namely , d 1 , d 2 , g 1 and j 1 . similarly , four values have been identified that are greater than or equal to the median of the nine values , namely , f 2 , f 3 , h 3 and j 3 . it should be noted that if so - called pipelining is used in the median filter 110 , only one three value sorter is required for sorters 101 , 102 , 103 and 104 because the prior sorted results are stored for use in the subsequent sorting . as indicated above , d 1 and d 2 are each less than or equal to the median of the nine values and , hence , can be removed from the nine values . this can be done because d 2 ( and hence d 1 which is less than d 2 ) is less than or equal to at least 5 of the 9 values ( for which the median is desired ). consider the following : d 2 ≦ d 3 ( because they are sorted ); d 2 ≦ e 2 ≦ e 3 ( because d 2 = λ , e 2 = β and λ ≦ β , and e 1 , e 2 and e 3 are sorted ); and d 2 ≦ f 2 ≦ f 3 ( because d 2 = λ , f 2 ≦ γ , λ ≦ β ≦ γ , and f 1 , f 2 and f 3 are sorted ). consequently , d 2 is less than or equal to 5 values , namely , d 3 , e 2 , e 3 , f 2 and f 3 . recall that the median of the 9 values is the value that is greater than or equal to 4 values , and less than or equal to 4 other values . since d 2 is less than or equal to 5 values , then either d 2 is not the median , in which case it can be removed , or d 2 is the median . however , if d 2 is the median , then d 2 must either be equal to d 3 or e 2 , in which case d 3 or e 2 is also a median , consequently d 2 can be removed as well ( note that our assertion is that the median is guaranteed to be in the surviving values ). how is it known that if d 2 is the median , then d 2 must be equal to d 3 or e 2 ? the proof is as follows : if d 2 is equal to d 3 then proof done . otherwise , d 2 must be less than d 3 , i . e ., d 2 & lt ; d 3 . in this case , d 2 must be equal to e 2 . because if otherwise , d 2 is not equal to e 2 , then d 2 & lt ; e 2 ≦ e 3 ≦ f 2 ≦ f 3 . that is , d 2 is strictly less than 5 values d 3 , e 2 , e 3 , f 2 , f 3 . this shows that d 2 could not be the median . this contradiction shows that if d 2 is the median , then d 2 must be equal to either d 3 or e 2 . since d 1 ≦ d 2 , the same analysis shows that d 1 can be removed . an analysis essentially identical to that above can be used to show that f 2 and f 3 , and that g 1 and h 3 can be removed . the main difference is that “ less than ” is replaced by “ greater than ” in the above analysis . fig2 shows in simplified form a so - called pseudo 9 - point spatial median filter . referring to fig2 it is seen that the 9 - points , i . e ., values , are arranged into three groups of three values each , namely , a first group including values a , b and c , a second group including values d , e and f , and a third group including values g , h and j . the first group is supplied to sorter 201 , the second group to sorter 202 and the third group to sorter 203 . sorters 201 , 202 and 203 each perform a complete sort of their respective supplied groups , i . e ., arrange the supplied values in either ascending or descending order . in the spatial median filter shown in fig2 it is assumed that the values are arranged in ascending order . that is , a 3 ≦ a 2 ≦ a 1 and so on for the other values . note that a sort of three values requires three comparisons . thus , the three sorters 201 , 202 and 203 perform nine comparisons . the median of each group is determined to be the middle value in the sorted group . the three medians from sorters 201 , 202 and 203 , in this example , are a 2 , b 2 and c 2 , respectively , and are supplied to sorter 204 . in turn , sorter 204 sorts the three medians a 2 , b 2 and c 2 . this requires another three comparisons . after sorting , the three medians a 2 , b 2 and c 2 , are assumed to be arranged in ascending order and are designated d 1 , d 2 , and d 3 , where d 1 ≦ d 2 ≦ d 3 . take the median of them , namely , d 2 , as the pseudo - median . total number of comparisons is 12 . the above - described embodiments are , of course , merely illustrative of the principles of the invention . indeed , numerous other methods or apparatus may be devised by those skilled in the art without departing from the spirit and scope of the invention .