What is the Moire Effect? A Comprehensive Guide
Defining Moire: More Than Just a Glitch
The term "Moiré" comes from the French word for a type of textile, typically silk, with a rippled or watered appearance. In the 18th and 19th centuries, this effect was highly prized. By pressing two layers of fabric together (or folding one layer back on itself) and applying steam and pressure, weavers created beautiful, shimmering wave patterns that changed as the wearer moved.
In the physical world, this effect was desired for its beauty. In the digital world, it is almost always an unwanted error that degrades image quality.
The Mathematics of Moiré
Mathematically, a Moiré pattern is a secondary and visually evident superimposed pattern created, for example, when two identical (usually transparent) patterns on a flat or curved surface (such as closely spaced straight lines drawn radiating from a point or taking the form of a grid) are overlaid while displaced or rotated a small amount from one another.
The equation describing the pattern involves the difference in the frequency vectors of the two grids. When these frequencies are similar, a "beat pattern" emerges—much like the wavering sound you hear when two musical notes are slightly out of tune.
Where You'll See It
1. Photography (The Sensor Clash)
As discussed in our other articles, this is the clash between the sensor grid and the subject's texture. It is most common in:
- Fashion: Suits with tight pinstripes, synthetic sports fabrics, and bridal veils.
- Architecture: Skyscrapers with many windows, brick walls, ventilation grates, and fences.
2. Printing (The Rosette Pattern)
Commercial printing uses "halftones"—grids of tiny dots in Cyan, Magenta, Yellow, and Black (CMYK). To avoid Moiré, printers rotate the screen angle of each color (e.g., Black at 45°, Magenta at 75°).
If these angles are slightly off, or if you scan a printed image (which already has a dot grid) with a scanner (which has its own sensor grid), you get massive, ugly interference patterns. This is a nightmare for archivists digitizing old magazines or newspapers.
3. Video Games and 3D Rendering
In 3D graphics, textures on distant objects often create Moiré. Imagine a tiled floor extending into the horizon. As the tiles get smaller in the distance, they eventually become smaller than the screen's pixels.
To solve this, game developers use Mipmapping (pre-calculating lower-resolution versions of textures) and Anisotropic Filtering. If you turn these settings off in your game, you will see "shimmering" or "crawling" textures—that's Moiré in motion.
4. Marine Navigation
Surprisingly, Moiré isn't always bad. Moiré Safety Beacons are used to guide ships under bridges. These beacons contain two grids of lines. When a ship is perfectly aligned with the safe channel, the captain sees a vertical black band. If the ship drifts left or right, the Moiré pattern shifts to show arrows pointing back to the center. It's a low-tech, fail-safe navigation system powered entirely by physics!
The Technical Solution: Low-Pass Filters
For years, camera manufacturers installed "Optical Low-Pass Filters" (OLPF) in front of sensors. These filters are made of birefringent crystals (like lithium niobate) that split a single point of light into two or four points.
This intentionally blurs the image slightly to ensure no detail is fine enough to cause aliasing. It's a compromise: you sacrifice a tiny bit of maximum sharpness to guarantee you won't get Moiré.
However, the modern trend is "Sharpness is King." High-resolution cameras (45MP+) often remove this filter, assuming the resolution is high enough to resolve most patterns. This puts the burden on the photographer to detect Moiré on location, or on software like Morie to fix it in post.