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

bayesianguy (view profile)

on 14 Jun 2019
Latest activity Commented on by Star Strider

Star Strider (view profile)

on 17 Jun 2019
Accepted Answer by Star Strider

Star Strider (view profile)

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.

bayesianguy

bayesianguy (view profile)

on 16 Jun 2019
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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Answer by Star Strider

Star Strider (view profile)

on 16 Jun 2019

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.

bayesianguy

bayesianguy (view profile)

on 17 Jun 2019
It works neatly with ndgrid than meshgrid.
The vectorized input was the key. Thanks a lot.
Star Strider

Star Strider (view profile)

on 17 Jun 2019
As always, my pleasure.

on 14 Jun 2019
Edited by KSSV

KSSV (view profile)

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.

bayesianguy

bayesianguy (view profile)

on 16 Jun 2019
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.