colfilt (Image Processing Toolbox User's Guide)

Image Processing Toolbox User's Guide

Perform neighborhood operations using columnwise functions

Syntax

B = colfilt(A,[m n],block_type,fun)
B = colfilt(A,[m n],[mblock nblock],block_type,fun)
B = colfilt(A,'indexed',...)

Description

colfilt processes distinct or sliding blocks as columns. colfilt can perform operations similar to blkproc and nlfilter, but often executes much faster.

B = colfilt(A,[m n],block_type,fun) processes the image A by rearranging each m-by-n block of A into a column of a temporary matrix, and then applying the function fun to this matrix. fun must be a function handle. colfilt zero-pads A, if necessary.

Before calling fun, colfilt calls im2col to create the temporary matrix. After calling fun, colfilt rearranges the columns of the matrix back into m-by-n blocks using col2im.

block_type is a string that can have one of the values listed in this table.

Value
Description

'distinct'
Rearranges each m-by-n distinct block of A into a column in a temporary matrix, and then applies the function fun to this matrix. fun must return a matrix the same size as the temporary matrix. colfilt then rearranges the columns of the matrix returned by fun into m-by-n distinct blocks.

'sliding'
Rearranges each m-by-n sliding neighborhood of A into a column in a temporary matrix, and then applies the function fun to this matrix. fun must return a row vector containing a single value for each column in the temporary matrix. (Column compression functions such as sum return the appropriate type of output.) colfilt then rearranges the vector returned by fun into a matrix the same size as A.

Value	Description
`'distinct'`	Rearranges each `m`-by-`n` distinct block of `A` into a column in a temporary matrix, and then applies the function `fun` to this matrix. `fun` must return a matrix the same size as the temporary matrix. `colfilt` then rearranges the columns of the matrix returned by `fun` into `m`-by-`n` distinct blocks`.`
`'sliding'`	Rearranges each `m`-by-`n` sliding neighborhood of `A` into a column in a temporary matrix, and then applies the function `fun` to this matrix. `fun` must return a row vector containing a single value for each column in the temporary matrix. (Column compression functions such as `sum` return the appropriate type of output.) `colfilt` then rearranges the vector returned by `fun` into a matrix the same size as `A`.

B = colfilt(A,[m n],[mblock nblock],block_type,fun) processes the matrix A as above, but in blocks of size mblock-by-nblock to save memory. Note that using the [mblock nblock] argument does not change the result of the operation.

B = colfilt(A,'indexed',...) processes A as an indexed image, padding with 0's if the class of A is uint8 or uint16, or 1's if the class of A is double or single.

Note To save memory, the colfilt function might divide A into subimages and process one subimage at a time. This implies that fun may be called multiple times, and that the first argument to fun may have a different number of columns each time.

Class Support

The input image A can be of any class supported by fun. The class of B depends on the class of the output from fun.

Example

This example sets each output pixel to the mean value of the input pixel's 5-by-5 neighborhood.

I = imread('tire.tif');
imshow(I)
I2 = uint8(colfilt(I,[5 5],'sliding',@mean));
figure, imshow(I2)

col2im colorbar