** Convolution**

The * convolution* of two picture functions * g * and * f *,
denoted * g * f *, is defined as:

* g * f (x, y) = &int &int _{- &infin}^{&infin}
g(u, v) · f(x - u, y - v) du dv *

For example, the image recorded by a camera is the convolution of the original image with the point spread function of the camera optics.

If the function decays rapidly to zero outside a local area,
convolution can be approximated by applying a grid-like * operator*
to the image:

1 | 1 | 1 |

0 | 0 | 0 |

-1 | -1 | -1 |

Such an operator can rapidly be applied to a whole image by special hardware, either operating on a stored image or on a raster scan.