Image editing applications such as Adobe Photoshop include a wide variety of different filters than can be applied to help improve the quality of any image, including scanned ones. One popular technique used to help minimize the impact of issues such as moiré patterns is to scan the image at double the resolution required with the descreen filter on, then resize the image to one-half of the current size, and finally apply a common filter known as the Unsharp Mask. Although its name might not suggest it, the Unsharp Mask is a filter used to restore the sharpness in an image, some of which might be lost after sampling an image to a smaller size.
Filters.bmp: Image-editing programs like Adobe Photoshop include a variety of different filters that can be used to improve scan quality.
If you’ve already scanned an image without the descreen option and no longer have the original hardcopy, all is not lost. Another popular way to remove moiré patterns is to slightly blur the image, causing the dot or crosshatch pattern to blend more naturally. Applications like Photoshop include a variety of blur-related filters on the Filter menu, but one of the most popular is known as the Gaussian Blur. Try using this filter on the full-sized image with the moiré pattern, and then resize the image and apply the Unsharp Mask filter to complete the process.
Unlike a traditional photograph, images and text found in magazines and newspapers are printed using a half-tone pattern. While this is not generally discernable to the human eye under normal reading conditions, look closely enough (perhaps with a magnifying glass) and you’ll notice that the image is actually made up of a dot-like or crosshatch pattern. Unfortunately, this pattern is most evident when you scan either of the above as a traditional colour or grayscale photograph, and seriously compromises the quality of the scan. To the uninitiated, this “effect” is commonly referred to as a moiré pattern.
The great news is that why moiré patterns represent a rather serious obstacle, most scanning software includes a simple and effective filter to tackle the problem. Known as a descreen filter, this is often implemented as a simple checkbox or drop-down menu option in the advanced settings of the scanning software. Depending on your scanning software, it may also include an option to scan from a magazine, which will implement the descreen filter automatically. While scanning an image using the descreen option will take considerably longer than without, the results speak for themselves, as shown below. Note that the descreen filter is not necessary (or useful) when scanning traditional photographs, since these do not suffer from the same moiré pattern issue.