| 摘要 |
A threshold mask for converting a contone image to a halftone image defines halftone cells that define halftone dots that undergo a growth in population as tone levels increase. After some growth in population, the halftone cells define halftone dots that then grow in size from a first predetermined size as tone levels increase further. During the growth in population, the halftone dots having a second predetermined size are placed into halftone gaps. The second predetermined size is smaller than the first predetermined size. In the simplest implementation, FM screening is used up to a tone level where the FM dither pattern holes begin to become visible. At this point AM screening is applied only to the remaining dot centers (corresponding to those holes) until they reach the stable dot size. Then, the dots are further grown according to the method used outside of the FM regime (e.g. the conventional AM process). In the FM regime, two types of visual disturbances can arise. The "FM regime" is the range of tones for which some form of FM screening is being used, and applies to all tones lighter than the one in which all halftone dot centers have, at minimum, achieved the stable halftone dot. In this regime, disturbances can arise due to: 1) variations in printed halftone dots smaller than the minimum stable size; and 2) unpleasant patterns produced by the dithered stable dots. The present invention allows for the creation of a screen with the best trade-off between these disturbances at tone levels in the FM regime.
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