This filter is found in
This filter halftones the image using a clustered-dot dither. Halftoning
is the process of rendering an image with multiple levels of grey or
colour (i.e. a continuous tone image) on a device with fewer tones;
often a bi-level device such as a printer or typesetter.
The basic premise is to trade off resolution for greater apparent tone
depth (this is known as spatial dithering).
There are many approaches to this, the simplest of which is to throw
away the low-order bits of tone information; this is what the posterize
filter does. Unfortunately, the results don't look too good. However, no
spatial resolution is lost.
This filter uses a clustered-dot ordered dither, which reduces the
resolution of the image by converting cells into spots which grow or
shrink according to the intensity that cell needs to represent.
Imagine a grid super-imposed on the original image. The image is
divided into cells by the grid - each cell will ultimately hold a single
spot made up of multiple output pixels in order to approximate the
darkness of the original image in that cell.
Obviously, a large cell size results in a heavy loss in resolution! The
spots in the cells typically start off as circles, and grow to be
diamond shaped. This change in shape is controlled by a spot
function. By using different spot functions, the evolution
in the shape of the spots as the cell goes from fully black to fully
white may be controlled.