Size function regarding image

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Shyam Joshi
Shyam Joshi 2019년 4월 4일
편집: DGM 2022년 11월 12일
What does the value of cc indicate about the image?


Steven Lord
Steven Lord 2019년 4월 4일
From the description of the "sz1,...szN" output arguments on the size documentation page:
"If fewer than ndims(A) output arguments are specified, then all remaining dimension lengths are collapsed into the last argument in the list. For example, if A is a 3-D array with size [3 4 5], then [sz1,sz2] = size(A) returns sz1 = 3 and sz2 = 20."
If you want the size of the array in all its dimensions (leaving out the trailing singleton dimensions) call size with one input and one output.
A = ones(2, 3, 4);
sz = size(A) % Returns [2 3 4]
If you want the size of the array in a specific dimension, the easiest way to do this is to call size with two inputs (not two outputs) where the second is the dimension whose size you want to compute.This will work even when the second input is a number past the length of the vector returned by the one input and one output call to size.
A = ones(2, 3, 4);
sz3 = size(A, 3) % Returns 4
sz42 = size(A, 42) % Returns 1

KSSV 2019년 4월 4일
I = imread('peppers.png') ;
[nx,ny,nt] = size(I)
If your image is RGB, in the baove nx,ny,nz gives number of rows, columns and matrices (here 3) respectively. If binary it will be number of rows, columns with nz = 1.
[nx,ny] = size(I)
If your image is RGB, in the baove nx,ny gives number of rows, and number of columns*number of matrices. If binary it will be number of rows, columns.
It is a simple can check on your own.
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Shyam Joshi
Shyam Joshi 2019년 4월 4일
Can you please reply based on the code provided below?
Screenshot (6).png

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DGM 2022년 11월 12일
편집: DGM 2022년 11월 12일
Another way to describe the scalar output syntax is simply that the last output argument is always the product of the sizes of any dimensions other than those previously requested.
[~,cc] = size(A);
returns the product of the sizes of all dimensions other than dim1.
  • If A is 10x20x1, cc is 20
  • If A is 10x20x3, cc is 60
  • If A is 10x20x3x2, cc is 120
In other words, the last output argument does not strictly refer to a single dimension. If you want to ensure that the nth output argument always returns the size of the nth dimension of your array, you need to request and discard an extra output.
[~,cc,~] = size(A);
In this case, cc always returns the size of dim2 of A. This is the same as
cc = size(A,2);
For images, getting the number of rows, columns would be done by:
[rows,cols,~] = size(A); % discard the last output
Omitting the third output would cause the number of columns to be wrong for any RGB input.
If you can't control the dimensionality of your inputs, and you're using size() with scalar outputs, and you want the outputs to always refer to specific dimensions, then you need to discard the last output.


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