The masking can also create sudden jumps in shade where none appear in the original image. A region with a gray-scale value of 131 might appear noticeably lighter than an adjoining region with a value of 132, even though they differ imperceptibly in the original. Increasing the size of the cell creates more degrees of gray, Stanich notes, but at a cost. This allows more gray-scale levels without the image becoming dotty. But now, because the multicell units are large enough to be clearly noticeable, another problem appears.
Some grays look good, others are very ugly.
Sometimes you seem to see small insectlike artifacts everywhere. It's just extremely unpleasing to the eye. That problem arises, Wu explains, because laser printers work poorly when printing isolated dots, meaning that the masks must be designed to cluster the dots as much as possible. But certain patterns of clusters will catch the eye and mar the image. Multicell Masks Challenged to find a better mask, Thompson, Tresser and Wu set out to find a multicell pattern that would produce none of these annoying artifacts.