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Another way to screen an image for printing
is to break it into a pattern of dots that have the same tiny size but
are distributed according to image density. This is called stochastic or FM (frequency modulated) screening.
It is the opposite of conventional
or AM (amplitude modulated)
screening, in which dots of varying sizes are spaced at regular
intervals on an invisible grid. In an image that has been
stochastically screened, shadow areas contain more dots than highlight
areas. The "clumping" of the dots, not their size, determines the
density of the image in any given area. Compared with AM
screening, stochastic screening can produce smoother tone gradations,
sharper detail, and printed color that looks closer to the color of the
original. This makes it desirable for high-end product catalogs, art
reproductions, and other work with strict quality requirements.
However, stochastic screening is an expensive, technically challenging
technique that not every printer can provide. Stochastically screened
images also are more prone to dot gain than images that have been
screened conventionally.
In the black-and-white image below, stochastic screening is seen on the
right, and conventional screening is seen on the left. The color image
shows FM screening on the left and AM screening on the right.
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