im2col (Image Processing Toolbox User's Guide)

Image Processing Toolbox User's Guide

Rearrange image blocks into columns

Syntax

B = im2col(A,[m n],block_type)
B = im2col(A,[m n])
B = im2col(A,'indexed',...)

Description

B = im2col(A,[m n],block_type) rearranges image blocks into columns. block_type is a string that can have one of these values. The default value is enclosed in braces ({}).

Value
Description

'distinct'
Rearranges each distinct m-by-n block in the image A into a column of B. im2col pads A with 0's, if necessary, so its size is an integer multiple of m-by-n. If A = [A11 A12; A21 A22], where each A_ij is m-by-n, then B = [A11(:) A12(:) A21(:) A22(:)].

{'sliding'}
Converts each sliding m-by-n block of A into a column of B, with no zero padding. B has m*n rows and contains as many columns as there are m-by-n neighborhoods of A. If the size of A is [mm nn], then the size of B is (m*n)-by-((mm-m+1)*(nn-n+1)).

Value	Description
`'distinct'`	Rearranges each distinct `m`-by-`n` block in the image `A` into a column of `B`. `im2col` pads `A` with 0's, if necessary, so its size is an integer multiple of `m`-by-`n`. If `A = [A11 A12; A21 A22]`, where each `A`_ij is `m`-by-`n`, then `B = [A11(:) A12(:) A21(:) A22(:)].`
`{'sliding'}`	Converts each sliding `m`-by-`n` block of `A` into a column of `B`, with no zero padding. `B` has `mn` rows and contains as many columns as there are `m`-by-`n` neighborhoods of `A`. If the size of `A` is `[mm nn]`, then the size of `B` is (`mn`)-by-`((mm-m+1)*(nn-n+1)).`

For the sliding block case, each column of B contains the neighborhoods of A reshaped as nhood(:) where nhood is a matrix containing an m-by-n neighborhood of A. im2col orders the columns of B so that they can be reshaped to form a matrix in the normal way. For example, suppose you use a function, such as sum(B), that returns a scalar for each column of B. You can directly store the result in a matrix of size (mm-m+1)-by-(nn-n+1), using these calls.

B = im2col(A,[m n],'sliding');
C = reshape(sum(B),mm-m+1,nn-n+1);

B = im2col(A,'indexed',...) processes A as an indexed image, padding with 0's if the class of A is uint8, or 1's if the class of A is double.

Class Support

The input image A can be numeric or logical. The output matrix B is of the same class as the input image.

See Also

blkproc, col2im, colfilt, nlfilter

im2bw im2double