Concentric circles going wavy

April 10, 2005 12:52 AM

Is there a name for the phenomenon of concentric circles in computer images creating weird wavy distortions when you shrink the image? Example here: big / small.
posted by Space Coyote to computers & internet (16 comments total)

Moiré. Self link: I put together a few nice links on this some time ago (and it's easiest just to point you to the post).
posted by taz at 12:58 AM on April 10, 2005

You rock, taz, thanks :)
posted by Space Coyote at 1:22 AM on April 10, 2005

: )

Also, reducing images in general means that you lose information and gain artifacts, which can lead to a moiré pattern developing. The larger image you are showing already has distortions in the concentric rings that become more obvious/exaggerated in a reduced size.
posted by taz at 1:28 AM on April 10, 2005

So I've never heard the word out loud. How is it pronounced?
posted by cillit bang at 2:59 AM on April 10, 2005

The pronunciation I'm familiar with is "mwah-ray", but I am not French.
posted by Kirth Gerson at 3:02 AM on April 10, 2005

Me, too. Audio.
posted by taz at 3:20 AM on April 10, 2005

The Moiré effect is a type of aliasing. Check out these links to learn more about it and how anti-aliasing can be used to reduce it.
posted by driveler at 6:42 AM on April 10, 2005

if you're trying to shrink images while avoiding this, try using better interpolation (the relevant menu option in whatever tool youre using might be "cubic interpolation").

you can see the same effect all over once you start looking - lace curtains, fences, etc.
posted by andrew cooke at 7:11 AM on April 10, 2005

...and the surface of almost calm water. Moiré patterns skim across lakes at astonishing speeds. They look like waves, but in fact they are constructive/destructive interference patterns in the much slower water waves.
posted by weapons-grade pandemonium at 7:49 AM on April 10, 2005

The degree of aliasing might also have something to do with the browser. I compared the big/small images you posted and in my IE 6.0 (updated) the smaller image appeared to be cleaner.
posted by RMALCOLM at 8:36 AM on April 10, 2005

I've been unable to find a high-enough resolution image of that album cover online to be able to make a good version of it. I actually went through my CD collection to make sure it really doesn't look that way normally.

Andrew: yeah, GIMP and actually even iTunes' album cover art did a better job of shrinking it than whatever Amazon uses, but I was mostly interested in the cool pattern that came when you did shrink it without using any complex sampling methods.
posted by Space Coyote at 8:46 AM on April 10, 2005

When the lines on your screen
Make more lines in between
That's a moiré!
posted by mendel at 9:25 AM on April 10, 2005

mendel, I love love love that!

This is one of the first things I learned about when I started working with science journals. Unintentional moirés can result when halftones and screen tints are made with incorrectly aligned screens during printing. Or it can happen there's a pattern already existing in a photo that interacts badly with the way the dots of ink happen to distribute. This is a Very Bad Thing because the image as it was originally intended is now distorted, so anyone responsible for quality checking the art are always on the lookout for it.
posted by melissa may at 9:58 AM on April 10, 2005

The Moiré effect is a type of aliasing.

I think you meant to say that it's a side effect of aliasing. It's not just for circles, either -- interference can happen any time you have two regular patterns overlaid. In this case the two patterns are the circles and the sampling pattern of the scanning and/or sizing algorithm.

Also, mendel rules.
posted by jjg at 10:32 AM on April 10, 2005

If you want "That's a moiré!" on a t-shirt perspicuity has it. Warning: it's inferior to mendel's version.

When an eel bites your leg and the pain makes you beg that's a moray
posted by Aknaton at 1:14 PM on April 10, 2005

*kisses mendel*
posted by taz at 10:25 PM on April 10, 2005