The Image Processing Corner

The median filter is a non-linear filter.

Its principle is simple: each pixel is replaced by the median value in a defined neighborhood. For instance, for a 3x3 pixels neighborhood, each color channel of each pixel is replaced by the fifth value among the 9 of the neighborhood sorted in increasing (or decreasing) order

The result of the median filter on a 3x3 neighborhood is illustrated below. The first row gives the input noisy images, the second row gives the results and the third and fourth rows show a detail with colors which enhance contrast.

Execution time on a AMD 1800+ for the 384x288 image: 0.04 seconds.

First remark: the filter gives excellent results on the "salt and pepper" noise. However, other results are poor. The periodic noise is still present.

Second remark: this filter keeps contours. But is has a defect. It rounds corners (or any angle if less than 180 degrees), as shown on a detail below on the white labels.