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

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]

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.

Answer 1

[Star Strider]

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]

It works neatly with ndgrid than meshgrid.

The vectorized input was the key. Thanks a lot.

[Star Strider]

As always, my pleasure.

Answer 2

[KSSV]

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]

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.