Accessing 3-dimensional matrix by a 2-dimensional index in matlab/octave -
i have 3 dimensional matrix (3rd dimension represents multiple copies of m x n grayscale image).
i'm taking max value of each pixel across images, giving me max_val , max_ix matrix (2x2).
i want reference original test matrix max_ix values.
example: my_max_val = test(max_ix,:) should equal:
5 1 1 1 obviously can use max_val in simplified example, not in real use case. i'm altering max_ix, need reference original 3-dimensional matrix new index values create (not depicted in simplified example).
>> test test(:,:,1) = 1 1 1 1 test(:,:,2) = 1 1 1 1 test(:,:,3) = 5 1 1 1 test(:,:,4) = 1 1 1 1 >> [max_val, max_ix] = max(test, [], 3) max_val = 5 1 1 1 max_ix = 3 1 1 1 how recreate max_val test , max_ix?
one approach -
%// size of 3d input array [m,n,~] = size(test); %// calculate 2d starting, offset & actual indices array start_idx = bsxfun(@plus,[1:m]',[0:n-1]*m); %//' offset_idx = m*n*(max_ix-1); actual_idx = start_idx + offset_idx; %// index 3d input array extract specific elements, desired output max_val = test(actual_idx) thus, giving two-liner solution, if compact codes -
[m,n,~] = size(test); max_val = test( bsxfun(@plus,[1:m]',[0:n-1]*m) + m*n*(max_ix-1) ) please note bsxfun(@plus,[1:m]',[0:n-1]*m) replaced reshape(1:m*n,m,n).
sample run
inputs:
test(:,:,1) = 12 66 75 98 65 75 24 87 33 59 74 9 test(:,:,2) = 37 60 21 21 37 79 9 39 69 37 78 56 test(:,:,3) = 23 16 30 10 65 79 24 41 49 11 54 11 test(:,:,4) = 12 61 70 66 79 97 76 11 30 44 44 94 max_ix = 1 2 2 2 2 4 1 2 4 2 3 3 output:
max_val = 12 60 21 21 37 97 24 39 30 37 54 11
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