A moire pattern is a (usually undesired) interference pattern on a digital image which is the result of overlapping grids, lines, dots and other repetitive details.
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. The halftone dots are printed entirely in fine dots which cause optical problems in a scanned image because the scanned image is also composed of fine dots. When the two patterns are superimposed, there is often a resulting moiré pattern.
Many scanner driver programs provide a ‘descreen’ filter to remove moire patters during the process of creating digital images from halftone images.
There are various techniques within Photoshop for removing moire patterns. Here is an example of one of these methods, as provided by http://photographylife.com
- Select the area affected by moire pattern
- Select colour of moire area as foreground colour using eyedropper tool
- Create empty layer
- Select Mode as Colour for this layer
- Fill the new layer area with foreground colour
- The rainbowing effect should now be removed from the image
- Keep the area selected, but hide the new layer you’ve just been working on
- Select layer of the original image with moire on
- Create new layer, and again blend mode should be changed to colour
- Again fill the new layer, but this time use White as the colour
- Select the original layer and go to Layer->New Adjustment Layer->Hue/Saturation
- Adjust the hue and saturation levels until the moire pattern has been removed (ensure view is at 100%).
- Turn the second (white) later back on
- Merge all layers just worked on
Before (with Moire):
After (without Moire):
Interestingly, moire patterns are almost never seen in images of nature – they prodominantly appear in man-made objects and materials such as fabrics and architecture.
Many currency notes use fine circular and wavy designs as these are are likely to exhibit a moiré pattern when scanned and printed using a digital scanner.