Moire

The nearby image shows two versions of the same picture. (For those who are interested, this is a scan of issue 22 of Marvel Comics UK Transformers comic - so I was a fan years ago! The purple stuff is part of a robot called Shockwave.) The leftmost image shows the "perfect" original image - how it would appear to you looking at the comic from a typical viewing distance. The image on the right is the same image suffering from "moire" - this gives it the horrible square pattern that appears upon it. This is an extremely good example of moire. The effect is particularly unpleasant.

Okay, so that's what it looks like. But what causes it? Consider the next picture, showing two versions of another section of Shockwave. The small image is at 100% - i.e. how you would normally see it. The bigger image is the same section at twice the size - like when you press your nose against the page. You should be able to see quite clearly that what looks like mostly a smooth colour is in fact composed of patterns of dots. This is true of most printed media such as magazines and especially newspapers, which often rely on a technique called "halftoning" to produce shades of grey using patterns of black dots.

So how do we get the chequered pattern? Imagine you scan an image at the same resolution it was printed. Every pixel should correspond to one dot on the printed page. Moire doesn't occur. If you instead scan at less than the printing resolution, as you frequently will, you can only sample a small number of the printed dots - say, every third dot. If the printing process uses a pattern to produce certain colours, then it is possible for the scanner to grab dots at regular intervals which all have similar colours and form a regular pattern. This gives you the square effect of moire.

What about photos though? You may have noticed that scanning photos you've taken and had developed nearly always works fine, with no moire. The reason is that while newspapers and magazines use dots to produce the image, original photos use a continuous printing technique without dots, so moire is unlikely to occur.

(This isn't strictly accurate, as photographic films have a particular grain, which is dots which actually respond to light. The scale of the dots and the printing techniques eliminate the appearance of dots for most standard films though (ISO 100/200/400).)

The wonderful thing about moire is that it may occur at more than just the scanning stage. Because printers, monitors and graphics cards effectively do the same sort of regular dot-based sampling as scanners, your scanned image may appear to have moire when viewed at certain sizes on screen or if printed. In my example of moire above, the perfect image is my original scan resized, whereas the image with moire is a screen-grab of the same picture viewed at about 30% size.

Change resolution

The first thing to try is change the resolution you are scanning or printing at. If you are viewing on-screen, change the viewing size or actual size of the image (a different percentage size). It can be worth trying resolutions above and below your desired resolution, until you find a suitable one. If possible, try odd sizes too, not just double or half sizes. For instance, instead of scanning at 300dpi, try 290dpi if you can. That may make enough of a difference, without changing resolution too much.

Resize the image

Essentially the same as the previous approach, but from a slightly different angle, you could try using your image processing program's resize option if you already have an image with moire. Sometimes the effect can be eliminated or lessened by changing the image size (effectively you are resampling the image avoiding the regular pattern of the moire).

Scan at an angle

An unusual one this, but if you scan the picture at an angle, say 30 degrees, and then use your software to alter the image to be level again after scanning, moire is often removed. The tricky part of this is making sure the scan level again afterwards, but a bit of trial and error should get it close enough. Obviously, this doesn't help if an image is printing with moire.

Blur the image

Not an ideal solution, but if the moire effect is fairly small, you might be able to get away with applying a light blur to the image, which should hide the moire while leaving some detail still present. Some programs have a tool to remove patterns from scans, although these tend to be variations on blurs. If you mask off the part of the image which is most affected, and then apply a medium gaussian blur, this is probably just as effective, and it gives you more control.

Remove the affected part

Another less practical solution, but it is worth considering whether you really need all of your image. For instance, the background may often suffer if it is a large block of particular colour. In this case, you could put your own background in the image to circumvent the problem altogether. Similarly, if a part you want is affected, painting or cloning over the affected area can remove it with a bit of work on your part.