Filter

Filtering is a raster operation in which each pixel value in a raster map is replaced with a new value.

The new value is obtained by applying a certain function to each input pixel and its direct neighbours. These neighbours are usually the 8 adjacent pixels (in a 3 x 3 filter) or the 24 surrounding pixels (in a 5 x 5 filter). When you create or define your own filters, any odd sized matrix is allowed (5 x 1, 11 x 23, 25 x 25).

Filtering is for instance used to sharpen a satellite image, to detect line features, etc.