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Combining Conventional and Stochastic Halftones to Overcome Moiré

(March 2003) posted on Mon Apr 21, 2003

Learn how to overcome naturally occurring moiré by combining conventional and stochastic halftones on the same printing screen.

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By Mark A. Coudray

Frequently, you face situations in which moiré is impossible to manage. One example is when an original image contains subject matter that generates moiré on its own. Some examples of moiré-generating images include mini-blind window coverings, wicker furniture, highly textured fabrics, screen doors, window screens, roof shingles or tiles, chain-link or picket fencing, and other images that include small, regular patterns. When a design includes such images, the bulk of the print might be moiré free, but in these image areas, the moiré is a disaster. This month, I would like to share a fix for specific moiré issues of this nature, a solution that you can easily accomplish in Photoshop.


Moiré is nothing more that a repetitive interference pattern that occurs over a specific period or distance. In other words, moiré occurs when regular patterns are disrupted in a repetitive manner. The human eye picks up the repetition and perceives the variation as lines, bands, waves, or checkerboard patterns. You can almost plan to have moiré problems with subject matter containing a regular pattern that will be reproduced as a halftone image. The typical remedy is to change halftone frequency, angles, or both. But too often, this solution is of no avail because the regularity of the pattern is just too great to overcome.


Another common fix is to change the halftone type from conventional to stochastic. This may be a good solution, but printers often become trapped with the added grain and surface disruption that results. For example, if you were trying to eliminate the moiré in an automobile grille area, you would be able to cancel the pattern, but at the expense of the tonal smoothness of the painted finish of the car. You simply trade one problem for another.


The solution presented here is the best of both worlds. The technique that I'll outline will allow you to combine conventional halftones with stochastic halftones in the same screen. Combining those two halftone types is relatively easy to do, and can be done by just about any competent prepress technician. Here are the steps you'll need to follow in order to make both halftone types work together Figures 1A through 1E show the effects of this process on a sample image:


1. Begin by making CMYK separations as normal.



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