In producing optimal images for interpretation, spatial filtering is applied. Filtering is usually carried out for a single band. Filters - algorithms - can be used to enhance images by, for example, reducing noise (“smoothing an image”) or sharpening a blurred image. Filter operations are also used to extract features from images, e.g. edges and lines, and to automatically recognize patterns and detect objects. There are two broad categories of filters: linear and non-linear filters.
Linear filters calculate the new value of a pixel as a linear combination of the given values of the pixel and those of neighbouring pixels. A simple example of the use of a linear smoothing filter is when the average of the pixel values in a 3×3 pixel neighbourhood is computed and that average is used as the new value of the central pixel in the neighbourhood.