On a raster, an analysis boundary or processing window within which cell values affect
calculations and outside which they do not. Filters are used mainly in cell-based analysis where the value
of a center cell is changed to the mean, the sum, or some other function of all cell values inside the
filter. A filter moves systematically across a raster until each cell has been processed. Filters can be of
various shapes and sizes, but three-cell by three-cell squares are common.