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Moire pattern

Moire pattern in photo (Image by Alexandre Van de Sande via flickr.com; CC by SA) | Alpha Index to Full Glossary:

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Definition:

Images scanned from printed sources such as newspapers, magazines, and books may exhibit an odd, wavy, rippled, or herringbone patterned appearance known as moiré or moire pattern. The word itself comes from a French word for a type of silk fabric that has a wavy appearance.

What Causes Moire in Scanned Images

When two opposing grids or patterns are overlaid it can create a noticeable and usually unwelcome pattern in an image. Printed images are formed from dots of ink. This ink is laid down in various patterns as halftones using a specific LPI (varies by type of paper and printing method). When scanned, the samples taken by the scanner superimposed on the original halftone screen of the photo interfere with each other leaving behind a moire pattern on the resulting scanned image.
"Halftone images contain a series of dots in a specific pattern that simulate the look of a continuous tone image." Read more about Halftones
"The way printers reproduce images, simulating continuous tone images by printing lines of halftone spots is measured in LPI." Read more about LPI or lines per inch

This same effect can be seen outside of scanning by looking at two pieces of overlapping window screen or other mesh material. Scans of newspaper images generally exhibit a more pronounced moire pattern than images printed with higher LPI such as photos in glossy magazines.

Preventing or Removing Moire Patterns

The software you use to scan printed images often includes a descreening option that can reduce or eliminate moire. After the fact, you can manipulate images in photo editing software such as Adobe Photoshop to remove the appearance of moire patterns.

As noted in several of the tutorials, above, you may run into copyright issues when scanning images from books and magazines. Before worrying about moire, make sure you have the proper permissions to make and use scans from published sources.

More Moiré Beyond the Scan

Moire patterns are not just an issue with scanned images. Moire can occur at the time a photograph is taken (or a video filmed). As with scanned photos, it is an issue with competing patterns.

In this Wikipedia entry on Moiré pattern you'll find information that goes well beyond simply scanning images. It includes mathematical equations related to types of moire patterns as well as a look at where you might find moire patterns such as on TV (created by some types of clothing) and in currency (patterns that create moire when scanned as a sort of counterfeiting counter-measure).

Understanding Moire Patterns in Digital Photography explore color moire and a similar pattern-related issue of maze artifacts. These type of problems come when photographing certain highly detailed or pattern-heavy scenes using digital (as opposed to film) photography.

The Photoshop Book for Digital Photographers has a chapter on Removing Moiré Patterns from Coats, Shirts, Etc. that is reproduced at the publisher's site (Peachpit Press). It explains how to fix the moire pattern that shows up in some digital photographs because of the way certain fabrics photograph.

Pronunciation: more-ay
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