creating a matrix from an image

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

[Paul Hoffrichter]

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]

I need also the index related to data of MM

Answer 1

[Ameer Hamza]

You can use meshgrid() like this

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

[Shabnam M]

Thanks for your kind reply,

Actually I checked your proposal but seems sth is wrong and could not correctly create MM, could you recommend another way?

[Rik]

What error did you get? Or is the result not what you wanted? If so, how is it different?

[Shabnam M]

the results are not right, I compared them with raw data and the data are different

[Shabnam M]

The size of r and c are (n*m), but M is (m*n) and that may create the difference?

[Rik]

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)

[Ameer Hamza]

Yes, ndgrid is more appropriate here. I have edited the answer.