A few scanning tips


Scanning Images in

Scanning images from printed material like magazines has a very special problem (not to mention copyrights, which is a serious consideration too). You will discover a murky herringbone or crosshatched or dotted pattern, or maybe concentric arcs, in your scanned images from printed material. There are different moire patterns, one example is shown below, but moire is unwanted artificial detail shown in the image, added false detail not present in the scene.


Moire patterns

This interference is called a moiré pattern. In a scanned image, Moiré patterns are caused by interference between two sets of fine pattern grids, the scanner samples and the halftone screen in the original image. Any scanner will do this, it's a simple fact of life. Greater scanning resolution can eliminate it (at least 300 dpi, or better 600 dpi).

Moire is unwanted interference of overlapping patterns of greater resolution than the digital sensor. This case shown is the ink dots of the commercially printed magazine image and the scanner sensor pixels resolution. Scanning at least at 300 dpi is enough to normally eliminate that cause. Or digital cameras photographing rows of fine parallel lines at closer spacing than the camera sensor pixels can show moire (like maybe on Venetian blinds in a distant window). Such cases of scene detail at greater resolution then the scanner or camera sensor is technically named Aliasing, and digital cameras generally have anti-aliasing filters on the sensors to help filter out all but extreme cases.

Any image printed on a printing press (like a book, magazine, newspaper, postcard, calendar, etc.) is printed with halftone screen patterns. The printed image is composed of a pattern of dots. A strong magnifying glass will show them. The halftone dots are printed entirely in black if a B&W image, or there are four screens in each of the three primary colors plus black (CMYK) if a Color image. These fine dots cause optical problems in a scanned image because the scanned image is also composed of fine dots.

The two patterns of dots, the printed magazine's 133 or 150 lpi screened pattern, and the scanner CCD cells at less then 2x that resolution, combine into maximums or minimums every several pixels in the image, depending on the spacing of the dots. It affects the overall light intensity in periodic patterns that become very visible. The pattern is named Moiré.

The scan is possible however.

© 1998 Barrie Smith, Australian Digital Camera magazine.   Contact miffy@ozemail.com.au

A scan of an image printed in the review of the Agfa 1680 camera
in Issue 10 of Australian Digital Camera magazine.

The CCD in this camera is 1280x960 pixels, but the camera can output an interpolated 1600x1200 image. That 1600x1200 pixel image was scaled by the magazine to 300 dpi for creating the 150 lpi screen, which printed at about 5.6 x 4.1 inches in size. The upper right top border shows the edge of another inset photo.

There is a physics term Nyquist Limit, that says moire occurs if the sensor resolution is less than 2x the scene resolution. This is why 300 dpi scanning resolution normally eliminates moire from 150 dpi printing dots. That is however the minimum limit, and scanning at 600 can be better.

This image was scanned normally from the magazine, except due to Moiré, 300 dpi and the Scanwizard Descreen filter was used. Then it was resampled to 33% size (effectively 100 dpi size). The last operation was sharpening with the Unsharp Mask filter.


Copyright © 1997-2010 by Wayne Fulton - All rights are reserved.

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