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Linear indices from row and column indices for a rectangular region of interest.

Asked by bayesianguy on 14 Jun 2019
Latest activity Commented on by Star Strider
on 17 Jun 2019
I have the row and column indices of a rectangular region of interest in a rectangular image. How can i get the linear indices from these row and column indices?
%this works
a = rand(5,5); %changed.
row_indices = 1:5;
col_indices = 1:5;
[X,Y] = meshgrid(row_indices,col_indices);
indices = sub2ind(size(a), Y, X)
%i want the indices when rows and cols are not necessarily of same length.
a = rand(5,3); %changed
row_indices = 1:5;
col_indices = 1:3;
[X,Y] = meshgrid(row_indices,col_indices);
indices = sub2ind(size(a), Y, X) %this would give out of range subscript.

  1 Comment

I had made a mistake in the code snippet. Now it is edited.
When the size of matrix is 5 x 5, the row and column indices going from 1:5 can create a nice grid which works with sub2ind.
However, when i have a rectangular matrix of size 5 x 3, the grid elements are going to have elements like (5,5) which is out of range.

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2 Answers

Answer by Star Strider
on 16 Jun 2019
 Accepted Answer

You need to reverse the order of the second and third arguments to sub2ind:
indices = sub2ind(size(a), X(:), Y(:)) %this would give out of range subscript.
and ideally make them column vectors, using the (:) subscript notation.
Note that ndgrid may be preferable to meshgrid.

  2 Comments

It works neatly with ndgrid than meshgrid.
The vectorized input was the key. Thanks a lot.

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Answer by KSSV
on 14 Jun 2019
Edited by KSSV
on 14 Jun 2019

YOu need to consider the size of matrix a.
%this works
a = rand(5,5);
row_indices = 1:5;
col_indices = 1:5;
[X,Y] = meshgrid(row_indices,col_indices);
indices = sub2ind(size(a), Y, X)
%i want the indices when rows and cols are not necessarily of same length.
row_indices = 1:5;
col_indices = 1:3;
a = rand(3,5) ;
[X,Y] = meshgrid(row_indices,col_indices);
indices = sub2ind(size(a), Y, X) %this would give out of range subscript.

  1 Comment

I have made a mistake in the code snippet.
When the size of matrix is 5 x 5, the row and column indices going from 1:5 can create a nice grid which works with sub2ind.
However, when i have a rectangular matrix of size 5 x 3, the grid elements are going to have elements like (5,5) which is out of range.

Sign in to comment.