Dispersed Dot or "FM" Halftoning
Digital printing (as well as traditional printing) requires that "continuous tone" images be represented by a printing process that is either "on or off" at any given point. Somehow, "dots" must be arranged and printed so that the illusion of a continuous tone is presented to the viewer.
Dispersed dot or "FM" halftoning is the cornerstone of most printing with digital inkjet devices. The "smallest available" dots are scattered in a "pseudorandom" manner to print varying densities. This is commonly contrasted with "clustered dot", or "AM", halftoning, illustrated below, which is used in more traditional printing processes.
It is common to hear these methods referred to as "random dot" halftoning. This really fails to convey the sophistication required. Read more about it under "Fine points of dispersed dot halftoning".
Traditional printing is done with a grid of dots arranged in straight lines, whose size is varied to print different densities. The number of rows per inch is commonly called the screen LPI, or "Lines Per Inch", and the larger dots are important for many printing processes. Because the "amplitude" of the dots is varied, rather than the "frequency" of the dots, this is sometimes called "AM" halftoning.
Because these dots are aligned in rows, care must be taken to avoid "moire" or "screen door effect" when combining multiple screens for color printing. Careful adjustments to screen rotation and LPI are made for this purpose.
Traditional screens can not only be rotated, they can also have dots of different shapes, such as the elliptical dots shown here. Different shapes are useful for different printing processes, for example, elliptical dots are preferred for most screen printing, while round dots are preferred for flexography.
Clustered dots don't necessarily need to be aligned in rows. "Semi random" halftones such this one can provide the larger dots needed for processes such as screen printing, while the fact that the dots are not lined up in rows eliminates "moire" when screens are combined to print color.
Such screens can also make color inkjet printing more predictable, by drastically reducing the effect of dot gain relative to the usual "pure FM" method.
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