Rank filter

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Rank filter is an image processing term. Typically, in digital filtering, pixels within a window are ranked by intensity values, and the center pixel is replaced with a new value. The new value is calculated as a function of the ranked pixels. Only the original pixel values are used in the ranking when determining the new pixel value.

Typical functions used in ranking are the median, mean and mode giving the median filter, the mean filter and the mode filter, respectively. Also gradient diffusion filters can be made by multiplying with a matrix for a horizontal gradient before averaging the pixel values: $\begin{bmatrix} 1 & 0 & -1 \\ 1 & 0 & -1 \\ 1 & 0 & -1 \end{bmatrix}$