|Many G'MIC commands decide how to write an output pixel by accessing a neighborhood of input pixels, usually centered around the pixel in question. (See , , ). In principle, the neighborhood may be arbitrarily large, in practice it is a ball or sphere just a few pixels across.|
All this is well and good so long as we deal with images of infinite extent, but computer manufacturers behave badly when asked to provide such capacity, so as a practical matter, images have both finite extent and, therefore, edges. Difficulties arise with edges. At some point, a part of a neigborhood won't exist because the pixels are off an edge. How does one access a pixel that is not there?
|0||dirichlet||Off-border pixels are considered a constant black. Named for Peter Gustav Lejeune Dirichlet. In particular, see Dirichlet Boundary Condition.|
|1||neumann||The values of border pixels are presumed to extend infinity. Named for Carl Gottfried Neumann. In particular, see Neumann Boundary Condition.|
|2||cyclic||The right hand border wraps to the left, the top to the bottom, so that pixels on the opposite edge neighbor each other. The image is a torus. See Periodic Boundary Conditions.|
|3||mirror||The value of virtual pixels past the border form a mirror image of the actual pixels approaching the border.|