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Halftones and Dithers

On most printers (dye-sub is an exception), each printed dot has the same density of color. If the printer only combined these solid colors, it would be limited to the eight primary colors describe above. To get the millions of colors in a photograph, the printer has to "fake" it by generating a pattern of small dots that the eye blends to form the desired shade. This process is called halftoning or dithering and designing printer software that does it well is as much art as it is science. As a result, printers vary widely in the methods used and the results obtained. One sign of halftoning being well done is when a smooth gradation of color in the original looks smooth in the print. If the process isn't well done, these smooth transitions will be made up of distinct bands of color and may also include moiré or doily patterns. Halftoning is done by arranging printable dots into grid-like groups, called cells, and then using these larger dots as a single unit to print pixels with. Each cell may be 5 by 5 or 8 by 8 dots in size. The three or four primary colors are printed combined in a pattern of dots in these cells, and the eye perceives them as intermediate hues.

For example, to print purple the printer uses a combination of magenta and cyan dots. For less saturated hues, the printer leaves some dots unprinted and hence white in color.