Moiré no more

Shift Happens newsletter

August 31, 2021

Moiré no more

I learned a thing long time ago, and it was: Once you print something, you can’t get it back.

On the surface, this statement doesn’t make any sense. Scanners are cheap, and in 2021 some digital cameras are as good as scanners, too. Optical character recognition is so fast that it can happen in real time, leaving enough processor power for simultaneous translation to a different language. And I myself bragged about scanning almost 400 printed items and putting them up on the Internet Archive the other day.

But something happens to photos in particular when they get printed. Pixels only make sense on displays. To survive in real life, pixels have to get translated into halftones – complex patterns of translucent cyan, magenta, yellow, and black dots that overlap, vary in size, and are rotated at strange, yet deliberate angles.

Here’s a simulation of printing a photo with smaller and smaller halftone dots. When the dots are big, you can only get a sense of the photo by squinting. But as you make the dots smaller, squinting becomes unnecessary – the photo on paper looks exactly like it did onscreen even as, under the hood, it’s put together in a very different way.

Grab a magnifying lens and examine any photo printed in a newspaper or a book, and you will see a similar complex pattern of dots. But if you find such a printed photo and scan it, you don’t automatically get its pixels back. No, you get new, smaller pixels that replicate the exact nature of halftone dots. And with those confused pixels, you also get one more thing: a warm hug from something called moiré.

Moiré is named after a type of material where a special process adds an organic, shimmering, and generally admired effect:

Moiré in silk is intentional, but the modern effect named after it – occurring when two similar frequencies overlap and create strange, pulsating patterns – is almost always something to avoid.

A version of moiré happens when you try to hum with your vacuum cleaner, and your hum gets really close in pitch (seriously, please do try this). It happens when you try to see through two chainlink fences – the links get out of and in phase with each other, creating a weird effect:

And it happens a lot in digital scanning and photography, because the sensor elements in scanners and cameras are on a grid, and therefore have a frequency of their own – a frequency another frequency can interfere with.

This is why taking a photo of just one fence with a digital camera can give you this:

This is why nobody will allow you to wear stripes on television:

This is why taking a photo of a completely white screen at an angle can result in weird splotches of colour:

And this is why dealing with a printed photo can be such a pain in the ass.

First of all, moiré can already happen when scanning a printed photo. It can happen later when the photo is resized. And when printing an already halftoned image and having it halftoned again, possibilities for moiré to appear are basically endless.

For all of these reasons, I tried to avoid putting once-printed things in my book. I reached out to newspaper archives for original photos, rather than scanning newspapers. I did the same for books. When I needed to, I took my own photos or asked others to take them for me.

But there were times when I didn’t have a choice. You know how much I care for a good photograph, and right here was another perfect example: an evocative hundred-years-old shot dripping in what used to be the future, coming from a company that no longer exists, scanned from perhaps the only remaining copy of said company’s old internal newsletter:

It was a great photo from a distance. But when I zoomed in, the halftone dots were clearly visible:

And even simply resizing the scan was like buying a ticket to Weird Pattern Central:

What to do then?

There is a tried-and-true way to remove halftones, and it’s the digital equivalent of something I mentioned above: squinting. If you blur the photo enough, the dots disappear:

That might be good in some cases, for example when the reproduction is small in size. But it’s a blunt, naïve technique. Blurring doesn’t just remove halftones – it also removes a lot more information around them. On the way to lose the dots, you lose a lot more.

All the demoireing guides tell you to apply a sharpening filter after this to compensate, but that’s like reheating cold pizza. You will never get back to where you were before, no matter how hard you try:

I procrastinated and dreaded removing moiré on this and a few photos like it, because I was worrying about them becoming too blurry.

But some of my wait was also deliberate. The world of digital image processing is not standing still. In recent years, there have been many examples of machine learning algorithms that seemed rather… magical. Machine learning cannot recover information from a blurred photo, but it can add new...