Photoshop Tips : Moiré Pattern Removal

We’ve all probably scanned in pictures from magazines or newspapers and marveled at how crummy it looks. There are small dots all over the place and moiré patterns caused by the offset printing process used in the publishing industry which is called screening. How do you get rid of them so that your image looks half-way decent? Well, there is a method covered in the book Adobe Photoshop Restoration & Retouching by Katrin Eisenmann that does a fairly decent job.

If possible remove the moiré pattern from the image during the scanning process. Many of today’s software packages have a descreening function which works much better than any method in Photoshop.

The theory behind this method for removing moiré or screening patterns in Photoshop is to run the dots together by lowering the image’s resolution until they can’t be seen and then resizing the image. The resulting image may appear softer and blurrier but it will be decidedly better than all the dots. Sharpening the image using the Unsharp Mask may help make the image look better. Remember, this is a last resort method. Like Katrin Eismann says, ideally it would be best to process the original non-screened image.

First of all, make sure the photo is in either Grayscale or RGB mode not Indexed Color. Then view the image at 100%. Zoom out on the image by hitting Ctrl -. The moiré pattern will change as it interfaces with the monitor's resolution. Eventually the pattern will no longer be visible. In this example, the image had to be zoomed out to 25% until the pattern became neglible. Click on the image above (representing the image zoomed to 25%) to view the original screened image. Notice all the dots.

It's time to lower the image's resolution. Select Image Size from the Image menu. Check the Constrain Proportions and Resample Image checkboxes. In the Pixel Dimensions dropdown box, choose percent and type in the percent to which you zoomed. In this example, we zoomed to 25%. Click OK.

Now resize the image back to it's original size. Reselect Image Size from the Image menu. And type in the original pixel dimensions or choose the reciprocal percentage, which in this example would be 400%. Check the Constrain Proportions and Resample Image checkboxes. Choose the Bicubic Smoother option from the Resample Image option dropdown box. Click OK. The dots should be gone from the image. You can also try the Bicubic Sharper option which makes the image slighty sharper.

You may want to use the Unsharp Mask to sharpen up the image. Play with the amount, radius, and threshold to get the best result. I chose 90, 3.0 and 3, respectively. Click on the image above to view the final processed image. Compare with the original; it looks much better. Now you can use Photoshop's wide array of features like the Spot Healing Brush, Clone Stamp, Masking, Level and Curve Adjustment, and more Sharpening to touch up the photo even further.