I scanned in some pictures from a yearbook, and some of the scans picked up some checkerboard pattern of artifacts. There’s a lot of variability in the size of the patterns, as you can see in this example. And in a lot of the scans they’re hardly noticeable or even don’t exist.
Uh-huh. Thanks.
The pictures in the yearbook are printed using a halftone screen - a grid of tiny black dots, all different sizes, that trick the eye into thinking it’s seeing a grayscale image.
The scanner also sees in a grid when it sweeps across the image to turn it into pixels. Since you now have two grids (the halftone screen and the scanner’s pixels), and they’re probably pretty close to each other in dots per inch (otherwise you wouldn’t be seeing the Moire pattern), any little misalignment between the two will result in different interference patterns showing up.
Takeaway: increase the scanning resolution in the scanning software, then use software to downsample to the image file size/dpi/pixel size that you want. If you’re already at the scanner’s resolution limit, you might want to play with scanning the image in different modes instead, say, color, or grayscale, depending on what you scanned it in the first place.
To get rid of them, you need to do something called descreening. Some scanners can do this internally, or there are plugins for Photoshop that do it, if your version doesn’t have it built in.
gotpasswords: thank you! I didn’t know about descreening, but, sure enough, my scanner has that as an option. Many many thanks! Lots less moire in my life now!