Alternative Dithering Methods

It includes many methods of simulating tonal values with more or less random patterns as opposed to the traditional patterned halftones.

• These go by several names including stochastic screening, frequency-modulated screening, error diffusion dithering, diffusion dither and random pattern halftones.

• Such methods avoid the problem of Moiré patterns when colors are overprinted.

• The designation frequency-modulated (FM) screening results from observing that the random dithering methods use constant sized spots with variable spacing as opposed to variable sized dots with constant spacing (amplitude-modulation) for patterned halftones.

• With these methods, spots are widely spaced in highlights and closely spaced in shadows.

• The spacing of the spots can be determined by any one of several algorithms based on the tonal value and the presence of nearby spots.

• These are often included in desktop printer drivers and device RIPs (Raster Image Processing).

• They break the link between sampling resolution and halftone screen ruling. This makes possible reduction in sampling frequencies without loss of gray levels and better use of high sampling frequencies.

• These methods may break down for very small spot sizes which are smaller than can be reproduced by the printing process.

• A variation of FM screening (2nd-order FM) varies both the spot size and the spacing.