A question about different image size generation using Matlab

Question

Gobert 2016-7-5

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/293801-a-question-about-different-image-size-generation-using-matlab

评论： Image Analyst 2016-7-5

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!

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Gobert 2016-7-5

编辑：Gobert 2016-7-5

在 MATLAB Online 中打开

@ Image Analyst . It simply says this:

X1 =  double(imread('true_color_image'));
[rows, columns, numberOfColorChannels] = size(X1)
rows =
     120
columns =
     120
numberOfColorChannels =
       3

Image Analyst 2016-7-5

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Answer 1

Guillaume 2016-7-5

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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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Guillaume 2016-7-5

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Well, end-1:end only select the last 2 columns/rows. So padding it by 1 is always going to give you 3 columns/rows.

Matlab is doing exactly what you're asking. What you're asking is obviously not what you really mean. You haven't say what it is though.

If you want to pad the whole array:

Y1 = padarray(X1, [1 1], 'replicate', 'pre');

Gobert 2016-7-5

编辑：Gobert 2016-7-5

在 MATLAB Online 中打开

Thank you very much, Guillaume .

I got it:

X1 =  double(imread('true_color_image'));
Y1 = padarray(X1(2:end,2:end,:), [1 1], 'replicate', 'pre');
size(X1)
size(Y1)
ans =
     120   120     3
ans =
     120   120     3