Preventing moire patterns

Unless you spent thousands of dollars on your scanner, you probably want to forget about the scanner software's color and tonal correction capabilities — Photoshop gives you more control. However, here is one thing that scanner software does much better than Photoshop, and it's a capability that you should use when appropriate: moire (pronounced, roughly, mwah-RAY) reduction. A moire pattern is a visible rosette pattern created by the pattern of dots placed by the printing press to reproduce color.

When you need to scan a color image or artwork that comes from a book, magazine, or newspaper (or other material printed on an offset printing press, such as product packaging or signs), you want to use the scanner's software to reduce moire. The scanner's software compensates for the pattern (when you let the scanner know the pattern is there) and smoothes the scanned image (as you can see in Figure 4-3).

Figure 4-3: Scanning without (left) and with the scanner's moire reduction option.

The moire reduction feature in your scanner's software might not be immediately recognizable. It might be labeled Descreening, or it could be a choice between Color (Photo) and Color (Document). As always, refer to your hardware's User Guide for specific guidance.

If rescanning is out of the question and you have a moire pattern to reduce in Photoshop, blur the image enough to disguise the problem, and then paint with the History Brush to restore areas of critical detail in the image.

Here's an important announcement from The Department of An Ounce of Prevention is Worth a Pound of Cure: Before doing any scanning, make sure that the scanner's glass is clean and free of dust. Likewise, check the inside of the scanner's lid. (What good is it to clean the glass if dust from the lid is going to contaminate it again as soon as you close it?) Remember that a minute or two spent cleaning the scanner could save hours of touch-up in Photoshop. Use a can of compressed air to dust the surface. If necessary, eliminate fingerprints or smears with appropriate glass cleaner. (Check the scanner's User Guide for cleaning instructions — and be careful when using liquid glass cleaner with an electrical device! I like to use the same premoistened wipes I use for my eyeglasses.) And use a burst of compressed air on the original, too, before placing it on the glass. Just be careful — hold the can of air some distance away from the photo and spray at an angle so that you don't damage the original.

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