The median filter works well and is straightforward to understand for monochrome images, but how should pixel values be ranked for a color image? One choice is to apply the median filter separately to each color channel, which works well for the case of shot noise that may be present in each channel separately. But if the median filter selects values in each channel that come from different pixels within the neighborhood, the resulting color shifts can produce problems, particularly near edges. The most common solution is to perform the ranking on the monochrome brightness value, and then copy the red, green and blue values from the selected neighborhood pixel. For multichannel and color images, there is also a vector median that plots each pixel’s values in color space, and defines as the median values the coordinates of the point whose distances to its neighbors is minimum. That method produces the best results for color images. This interactive tutorial illustrates the effects of median filtering on shot and speckle noise in color images.

The tutorial initializes with a randomly selected specimen appearing in the Specimen Image window. The Choose A Specimen pull-down menu provides a selection of specimen images, in addition to the initial randomly chosen one. The Noise Level slider controls the amount of noise that is added, while the Noise Type radio buttons select either Shot or Speckle noise, which can be added either to the individual color Channels or to the Monochrome brightness only. The Choose An Operation pull-down menu determines the size of the neighborhood used for the median filtering operation that will be performed, while the Median Processing Mode buttons select whether the median filter performed is applied to the individual RGB Channels, to the Brightness Values, or using an RGB Vector median. The result of the filtering operation appears in the Filtered Image window.

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