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An extensive look at moir patterns, Part 1: 3 ways to reduce moir patterns in scanned images

Added on Tuesday 22nd of April 2008 07:18 am EST
Adobe Photoshop CS/CS2/CS3
Operating Systems:
Microsoft Windows, Macintosh

Line pattern interferences or scanning already-printed materials often causes unsightly moiré patterns. Don’t publish those images! Keep your professional image clean and clean those images up with these tips.

To help you eliminate moiré-pattern mishaps, we’ll:

  • Explain what a moiré pattern is and give you the run down on how one can sneak up on your images.
  • Give you some preventive scanning tips to reduce the possibility of moiré patterns appearing in your scanned images.
  • Show you a few different Photoshop techniques that will help you eliminate—or at least reduce—moiré patterns from scanned images.

As you strive to deliver nothing less than 100-percent quality to your clients, the introduction of a moiré pattern in your graphics sets your mind at nothing less than a panic.
            How did the moiré pattern get there? How do you get rid of it? Can you avoid it in the future? Moiré patterns have as many different causes as there are techniques to remove them. Sometimes preventive maintenance goes a long way, and sometimes you need a few moiré-removal tricks up your sleeve. In this two-part series, we’ll take an extensive look at moiré patterns and answer the above questions in regard to scanned and digital images. Our primary focus for this installment is scanned images.

What causes moiré patterns?
When two grids are overlaid at differing angles or when they have slightly different mesh sizes, this creates a moiré pattern, as shown in Figure A. While our examples look cool, moiré patterns are an undesired image artifact.

There are many ways that moiré patterns can attack your images; here are the most common ones:
  • Scanning a printed photo. During the printing process, inks print with different colored dots at different screen angles. All the inks together create a pattern that the human eye can’t detect easily. However, a scanner almost always picks it up, causing a moiré pattern on the scanned image, as shown in Figure B.

Note: The scanned image shown in Figure B is a printed PhotoSpin image from a previous issue of our Inside Photoshop publication.


  • Digital photos. Digital cameras can introduce moiré patterns when your photo’s subject lines interfere with the rows of pixels in the digital camera’s image sensor grid, as illustrated in Figure C.


  • Onscreen. A moiré pattern can appear as a display interference between lines in an image and the monitor pixel grid. This is why it’s important to preview your image in Photoshop at 100% magnification.

Tip: For more information on Photoshop image previews, read the companion article “Don’t trust anything more—or less—than 100% preview” in this issue of Inside Photoshop.

  • Printed materials. Moiré patterns can surface in printed materials, even when there is no moiré pattern visible in your Photoshop file. That’s why it’s very important to view high-quality, actual-size color proofs.

Tip: What’s even more frustrating, previewing a moiré pattern onscreen doesn’t guarantee that a moiré pattern will appear on your printed piece. Again, that’s why viewing accurate proofs is vital to your print job’s success. Before sending the entire job to press, discuss any particular image anomalies with your print vendor. They might agree to do a proof of just the affected image so you’ll know for sure how your image will print under their press conditions.

Use preventive scanning measures
While many designers will blame moiré patterns in scanned images on scanning an already-printed piece, that isn’t always the case. Any time you scan anything that contains a grid-like pattern, you run the risk of capturing a moiré pattern. This is because the sampling grid in the scanner interferes with the grid in your picture.
Preventive scanning measures are your best defense against capturing moiré patterns with your scans. Use these methods to prevent unsightly moiré patterns from ruining your scanned art:
  • Don’t scan an image from any printed material—scan only original photographs. Aside from potential copyright violations from scanning an already-printed piece, your scanner will detect the halftone pattern in the printed image.
  • Rotate the image on the scanner bed slightly and scan it on an angle.
  • Check to see if your scanning software has a descreening filter built in, like the one shown in Figure D. If so, select the option and scan your image at twice the resolution, then downsize and resample it in Photoshop using the Bicubic resampling method.


Reduce moiré patterns in scanned images
Unfortunately, you can’t always prevent the moiré patterns from popping up. So if you do capture one with your scan, you need to try to get rid if it.

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