MATLAB: A question about different image size generation using Matlab

paddarrayresizesize;

Hello everyone!
I need your help! Can anyone please tell me why, the Y1 becomes empty and its size changes when I input a true color image ?
X1 =  double(imread('true_color_image'));
Y1 = padarray(X1(end-1:end,end-1:end),[1 1],'replicate','pre')
size(X1)
size(Y1)
Y1 =
       0     0     0
       0     0     0
       0     0     0
ans =
     120   120     3
ans =
       3     3
While when I use input data, as shown below, Y1 does not change:
X2 =  [1 2 3; 4 5 6; 7 8 9]
Y2 = padarray(X2(end-1:end,end-1:end),[1 1],'replicate','pre')
size(X2)
size(Y2)
X2=
       1     2     3
       4     5     6
       7     8     9
Y2=
       5     5     6
       5     5     6
       8     8     9
ans =
       3     3
ans =
       3     3
What can I do so that the size of Y1 does not change (i.e. remains same as X1) ?
Thank you!

Best Answer

X1(end-1:end,end-1:end)
Since you're using 2D indexing to index a 3D matrix, this only return values from the 1st plane (the red colour plane). This is equivalent to
X1(end-1:end,end-1:end, 1)
If you want a 3D matrix as output, you must pass a 3D matrix as input, so:
Y1 = padarray(X1(end-1:end,end-1:end, :), [1 1], 'replicate', 'pre')
Note that the above will work on greyscale images as wel, since the : will match the only colour plane.
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