creating a matrix from an image

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Shabnam M 2020-12-11

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https://ww2.mathworks.cn/matlabcentral/answers/690490-creating-a-matrix-from-an-image

编辑： Rik 2020-12-11

采纳的回答： Ameer Hamza

Dear all,

I have an image and when it is read in matlab, it is [m*n] matrix, named M,

I need to create another matrix, named MM, which is exactly M, but the indexes are also listed in that.

In fact, MM has 3 columns and the third column is M (:), but the first two columns are i and j which

i=1:m, j=1:n

Thanks in advance

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Paul Hoffrichter 2020-12-11

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Trying to follow you. For this example, spell out what you want MM to be.

M =
     1     2     3
     4     5     6
     7     8     9
    10    11    12
>> M(:)
ans =
     1
     4
     7
    10
     2
     5
     8
    11
     3
     6
     9
    12

Shabnam M 2020-12-11

I need also the index related to data of MM

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Ameer Hamza 2020-12-11

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编辑：Ameer Hamza 2020-12-11

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You can use meshgrid() like this

[r,c] = ndgrid(1:size(M,1), 1:size(M,2));
MM = [r(:), c(:), M(:)];

The cause of this inconsistency is that Matlab stores arrays in a column-major form. So if you want to use (:) to transform an array into a vector, you need to make sure the result of your coordinate grid matches that order.

The solution is simple: replace meshgrid by ndgrid. Each of these functions has their use, but ndgrid will give the required order in this case. (you could also sort r and c, but that would be an unnecessary operation)