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General sliding-neighborhood operations



B = nlfilter(A,[m n],fun) applies the function fun to each m-by-n sliding block of the grayscale image A.

B = nlfilter(A,'indexed',___) processes A as an indexed image, padding with 0s if the class of A is uint8, uint16, or logical, and padding with 1s otherwise.


nlfilter can take a long time to process large images. In some cases, the colfilt function can perform the same operation much faster.


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This example shows how to apply a median filter to an image using nlfilter. This example produces the same result as calling medfilt2 with a 3-by-3 neighborhood.

Read an image into the workspace.

A = imread('cameraman.tif'); 

Convert the image to double.

A = im2double(A);

Create the function you want to apply to the image--a median filter.

fun = @(x) median(x(:));

Apply the filter to the image.

B = nlfilter(A,[3 3],fun); 

Display the original image and the filtered image, side-by-side.

title('Original Image (Left) and Median Filtered Image (Right)')

Figure contains an axes. The axes with title Original Image (Left) and Median Filtered Image (Right) contains an object of type image.

Input Arguments

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Image to be filtered, specified as a numeric array of any class supported by fun. When A is grayscale, it can be any numeric type or logical. When A is indexed, it can be logical, uint8, uint16, single, or double.

Data Types: single | double | int8 | int16 | int32 | int64 | uint8 | uint16 | uint32 | uint64 | logical

Block size, specified as a 2-element vector of positive integers. m is the number of rows and n is the number of columns in the block.

Example: B = nlfilter(A,[3 3],fun);

Data Types: single | double | logical

Function handle specified as a handle. The function must accept an m-by-n matrix as input and return a scalar result.

c = fun(x)

c is the output value for the center pixel in the m-by-n block x. nlfilter calls fun for each pixel in A. nlfilter zero-pads the m-by-n block at the edges, if necessary.

Data Types: function_handle

Output Arguments

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Filtered image, returned as numeric array. The class of B depends on the class of the output from fun.

Introduced before R2006a