hi I am working on gait analysis, frame based image processing. I am giving the code here someone please tell me how to do subtraction between the cell structure?

j=1; idx=1; k=1; for i=1100:1120 a1=imread(strcat('',num2str(i),'.png')); a = imsubtract(a1(:,:,1), rgb2gray(a1)); [r,c]=size(a);

b=im2bw(a,.19); % b=im2bw(a,40/256); se = strel('disk',2); closeBW = imclose(b,se); % figure,imshow(closeBW); s1 = regionprops(closeBW, 'Centroid'); % centroids= cat(1,s1.Centroid); % centroids=floor(centroids);

centroids_temp= floor(cat(1,s1.Centroid)); centroids_temp1=randn(10,2);

 %  Display original image and superimpose centroids.
%           hold(imgca,'on')
%     plot(imgca,centroids_temp(:,1), centroids_temp(:,2), 'r*');
%     impixelinfo; 
%    if length(centroids)==10

 centroids{idx,1}=sortrows(centroids_temp,2);
end

in my code i first read the frames, subtract red components(which are my markers), find the coordinates. Then I am storing them all in a cell strcuture. now i need to find out which are the frames having less than 20 elements(where markers are missing). once i find them i need to subtract them with the previous frame coordinates to check if they fall within the threshold. If not that marker is missing. Than i have to go for interpolation. Now someone please help to do the subtraction part