Moiré: When a Wallpaper Starts to Shimmer
Fine stripes, fabric and window grids can turn into wavy bands once a large image is shrunk to fit a screen. The cause is arithmetic, and so is most of the fix.
A photo of a glass tower can look clean at full size and then, set as wallpaper on a 1920×1080 laptop, grow soft ripples across the facade that were never there. Those ripples are moiré, and they appear when a repeating pattern in the image — window mullions, corduroy, a woven basket, the rows of a vineyard seen from the air — is finer than the grid of pixels it is being squeezed into. A 6000-pixel-wide file shown on a 1920-pixel screen has to discard roughly two of every three columns, and if the pattern repeats at a spacing close to what survives, the leftovers line up into slow bands.
How the image is shrunk matters as much as what is in it. A careful downscale averages neighbouring pixels first, which smooths stripes into an even tone before they can interfere; a quick one, the kind some systems use to save time when they set a wallpaper, simply samples every few pixels and keeps whatever lands there. That is why the same file can shimmer on one machine and look fine on another, and why the effect sometimes changes when you switch between Fill and Fit. Display scaling adds a second pass: at 125 or 150 percent the image is resized again, and a pattern that was safe at 100 percent can start to beat.
The practical answers are modest. Resize the image yourself to the exact screen resolution in an editor that uses a smoothing filter, such as bicubic or Lanczos, and the system has nothing left to guess. If a picture is mostly fine stripes, a very slight blur before resizing costs almost no visible detail and removes the raw material for interference. When choosing from a gallery, a glance at the thumbnail also helps: patterns that already wobble at small size will wobble on the desktop too, only larger.
