medfilt2 (Image Processing Toolbox User's Guide)

Image Processing Toolbox User's Guide

Perform two-dimensional median filtering

Syntax

B = medfilt2(A,[m n])
B = medfilt2(A)
B = medfilt2(A,'indexed',...)

Description

Median filtering is a nonlinear operation often used in image processing to reduce "salt and pepper" noise. Median filtering is more effective than convolution when the goal is to simultaneously reduce noise and preserve edges.

B = medfilt2(A,[m n]) performs median filtering of the matrix A in two dimensions. Each output pixel contains the median value in the m-by-n neighborhood around the corresponding pixel in the input image. medfilt2 pads the image with 0's on the edges, so the median values for the points within [m n]/2 of the edges might appear distorted.

B = medfilt2(A) performs median filtering of the matrix A using the default 3-by-3 neighborhood.

B = medfilt2(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 of class logical, uint8, uint16, or double (unless the 'indexed' syntax is used, in which case A cannot be of class uint16). The output image B is of the same class as A.

Note For information about performance considerations, see ordfilt2.

Remarks

If the input image A is of an integer class, all the output values are returned as integers. If the number of pixels in the neighborhood (i.e., m*n) is even, some of the median values might not be integers. In these cases, the fractional parts are discarded. Logical input is treated similarly.

For example, suppose you call medfilt2 using 2-by-2 neighborhoods, and the input image is a uint8 array that includes this neighborhood.

```
1 5
4 8
```

medfilt2 returns an output value of 4 for this neighborhood, although the true median is 4.5.

Example

This example adds salt and pepper noise to an image, then restores the image using medfilt2.

I = imread('eight.tif');
J = imnoise(I,'salt & pepper',0.02);
K = medfilt2(J);
imshow(J), figure, imshow(K)

Algorithm

medfilt2 uses ordfilt2 to perform the filtering.

See Also

filter2, ordfilt2, wiener2

Reference

[1] Lim, Jae S., Two-Dimensional Signal and Image Processing, Englewood Cliffs, NJ, Prentice Hall, 1990, pp. 469-476.

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