Linear (convolution) filters
Linear convolution filters consist of a matrix with values and a gain factor. When a linear filter of size 3x3 is used, the 9 matrix values are multiplied with 9 pixel values in the input raster map, this is summed and then multiplied with the gain factor. The result is assigned to the center pixel in the output map.
Standard linear filters are: AVG3x3, EDGESENH, LAPLACE, SHADOW. The gradient filters are also linear filters: DFDX, DFDY, DFDDN, DFDUP, D2FDX2, D2FDY2, D2FDXDY. For technical information on gradient filters, see Filters: gradient or derivatitve filters.
Furthermore, user-defined linear filters may be created and stored on disk: specify filter size, matrix values, and gain factor. You can also use user-defined average filters:specify the filter size.