问题
I am constructing an adjacency list based on intensity difference of the pixels in an image. The code snippet in Matlab is as follows:
m=1;
len = size(cur_label, 1);
for j=1:len
for k=1:len
if(k~=j) % avoiding diagonal elements
intensity_diff = abs(indx_intensity(j)-indx_intensity(k)); %intensity defference of two pixels.
if intensity_diff<=10 % difference thresholded by 10
adj_list(m, 1) = j; % storing the vertices of the edge
adj_list(m, 2) = k;
m = m+1;
end
end
end
end
y = sparse(adj_list(:,1),adj_list(:,2),1); % creating a sparse matrix from the adjacency list
How can I avoid these nasty nested for loops? If the image size is big, then its working just as disaster. If anyone have any solution, it would be a great help for me. Regards Ratna
回答1:
I am assuming the input indx_intensity
as a 1D
array here. With that assumption, here's a vectorized approach with broadcasting/bsxfun -
%// Threshold parameter
thresh = 10;
%// Get elementwise differentiation between elements in indx_intensity
diffs = abs(bsxfun(@minus,indx_intensity(:),indx_intensity(:).')) %//'
%// Threshold the differentiations against the threshold, thus giving us a
%// 2D square matrix. Then, set the diagonal elements to zero to avoid them.
mask = diffs <= thresh;
mask(1:len+1:end) = 0;
%// Get the indices of the TRUE elements in the valid mask as final output.
[R,C] = find(mask);
adj_list_out = [C R];
来源:https://stackoverflow.com/questions/36417710/how-to-avoid-nested-for-loops-in-matlab