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ATPM 4.11
November 1998



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Art Department

by Jamal Ghandour,

Removing Moiré

This column is brought to you directly from Sharm El-Sheikh using a replacement 5300c (to me, the eagerness with which the dealer offered me the 5300c until my G3 PowerBook arrives reflects nothing but Apple’s improved relationship with its outlets.)

Sharm is a real heaven on earth. If, like me, you are a sea freak, you will find here everything your little sea-heart desires. Scuba diving, water-skiing, para-sailing, beautiful beaches and even those delicious seafood dinners (did I forget to mention the gorgeous ladies? :D ) Even the artist in me had lots to learn; like I found here a fish called “Om-El-Ba7ar” (mother of the sea) that supposedly can detect as many as 64 billion colors through its eyes. Amazing, huh? Now let’s see a video card beat that. If you are interested in going to Sharm, drop me an e-mail. I will be more than happy to act as a tour guide (of course, you will have to pay 5-star accommodation). Anyway, back to work!

Last month, we discussed some HTML editors. The column proved so popular that we decided to do a follow-up with more popular editors. If you have an HTML editor that you would like to be in there, please e-mail me the name (many thanks to those of you who have written already, we are considering each and every e-mail that came in.)

[dots graphic]

On the other hand, this month we are going to discuss a very useful technique usually called by graphic professionals “taking off those darn dots!” :D

[artbeforeafter graphic]

Removing Scanning Artifacts

Nowadays, scanners are so cheap that they even come bundled with software such as Painter. What does that mean? Well, if anything it means the over-saturated, low quality hardware equipment leaves behind a perfect breeding ground for all those bad-scanning artifacts. One such artifact is the infamous “moiré” pattern, an artifact that is generated due to wrong overlapping of halftones after scanning.

Halftones is a very long subject, which we can discuss later (if I get enough requests). As it is now, this column will just define halftones and focus on the best technique to get rid of bad halftones (moiré).


In the ATPM 4.02 article about color modes, we discussed three very important factors that are the basis for the halftone explanation: pigments, CMYK, and gamuts. To sum it up briefly, unlike the screen mode RGB (Red, Green, Blue) CMYK (Cyan, Magenta, Yellow, Black) uses an additive (color is added rather than subtracted when mixed) mode to ensure ranges of color (gamuts), that although more subtle and less bright than their RGB counterpart, are the only valid mode supported by today’s commercial printers. RGB color is projected on-screen using pixels (very small squares that make up the computer’s screen). CMYK however, makes the illusion of many colors by overlapping the four colors main (CMYK) in different sizes and angles (this very factor is called halftoning.)

[arthalftone graphic]

The above was a very simplified version of the real operation. There are always turn-arounds. An example is fluorescent color printing. Even though CMYK can’t make that color combination, spot (costly additional colors that are pre-made rather than mixed from the main (CMYK) colors are used to compensate.


The whole idea of halftoning lies in the very fact that neither the dots nor their angle should show in the final prints; halftones should be seamless to the naked eye. Now, imagine a picture captured with a digital camera, then directly imported to the computer. The picture is cleaned, made into a layout and finally printed. The process for commercial printing is making the digital artwork into films (transparent black & white sheets). The only way to do this from a computer is through a high-end RIP (Raster Image Processor). Assuming the screens between Photoshop and your service bureau’s RIP match, then the output will be a crisp, clear-printed image with optimal screen angles. Now, if the printed picture is scanned again, more often than not, the angles differ to produce moiré patterns (even more visible patterns show according to the experience of the scanner operator and the quality of the scanner).

Tip: It is always advisable to scan pictures at a 30-degree angle, then rotate them back in your image editor. This way you reduce the moiré pattern risk.

Taking Off Those Darn Dots!

Over the years, many techniques have been developed to overcome this irritating flaw. My personal experience proves the following to be the most effective solution (using Photoshop):

1) Convert the image to LAB mode.

2) Select and apply a “Median..” filter on the Lightness channel of the image (Filters:Noise:Median...)

[artmedian graphic]

3) Apply a slight blur to both the A & B channels of the image.

4) Convert the image back to RGB mode.

5) Apply a slight “Sharpen” (Filters:Noise:Sharpen...)

Please note, that the setting of the above steps (amount of pixels) depends highly on your image, however a subtle 1-2 pixels, normally does the trick.

[dots graphic]

Hope you enjoyed it much as I did. As always, do not hesitate to send any suggestions, comments, criticisms, or even topics you would like to see in this column to [apple graphic]

Copyright © 1998 Jamal Ghandour, Jamal Ghandour is currently the Executive Manager at SwiftAd international.

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