IMPROVEMENTS IN IMAGE HALFTONING

摘要

A method of halftoning CC'MM'YK and CC'MM'YKK' ink sets is disclosed. For CC'MM'YK printing, the yellow and black colors are halftoned in the normal manner. For CC'MM'YKK printing the yellow color is halftoned in the normal manner. Cyan and magenta (and black for CC'MM'YK printing) are halftoned with four levels per pixel. The halftoning can be achieved by simply dividing the range of pixel intensities into three equal bands, which are halftoned as follows: 1) in the darkest band, the 2/3 intensity ink drop is always present, and the 1/3 intensity ink drop is halftoned; 2) the middle band is halftoned between the 1/3 intensity ink drop and the 2/3 intensity ink drop; 3) the lightest of these bands is halftoned between no ink drop and the 1/3 intensity ink drop. Halftoning using this method achieves an optimum image density with minimum noise as measured by optical instruments. However, the human eye is sensitive to optical texture as well as optical density. The reduction in image 'roughness' where the pixel intensity is very close to 1/3 and 2/3 of the maximum level is detected by the eye as an image feature. This is usually undesirable. The elimination of these smooth bands is a further aspect of this invention. These bands can be eliminated by halftoning using three pixel values. The band around 1/3 pixel intensity can be eliminated by maintaining the probability of a 1/3 intensity ink drop at the pixel location constant for the pixel intensities within the band, and halftoning the remaining pixel locations between no ink drop and a drop of 2/3 ink intensity. The band around 2/3 pixel intensity can be eliminated by maintaining the probability of a 2/3 intensity ink drop at the pixel location constant for the pixel intensities within the band, and halftoning the remaining pixel locations between a drop of 2/3 ink intensity and both 1/3 ink intensity and 2/3 ink intensity drops present simultaneously.