In digital image processing, moiré is the name of the unwanted, wavy artifact created by the interference between a pattern on the object and the shape of the light sensors on the imaging device. A similar effect is produced when converging lines in a video image are nearly parallel to the scanning lines of the analog monitor or television used to render it.
In mathematics, physics, and art, a moiré pattern or moiré fringes are large-scale interference patterns that can be produced when an opaque ruled pattern with transparent gaps is overlaid on another similar pattern. For the moiré interference pattern to appear, the two patterns must not be completely identical, but rather e.g. displaced, rotated, or have a slightly different pitch.
Moiré patterns appear in many different situations. In printing, the printed pattern of dots can interfere with the image. In television and digital photography, a pattern on an object being photographed can interfere with the shape of the light sensors to generate unwanted artifacts. They are also sometimes created deliberately – in micrometers, they are used to amplify the effects of very small movements.
In physics, its manifestation is the beat phenomenon that occurs in many wave interference conditions.
Moiré patterns are often an artifact of images produced by various digital imaging and computer graphics techniques, for example when scanning a halftone picture or ray tracing a checkered plane (the latter being a special case of aliasing, due to undersampling a fine regular pattern). This can be overcome in texture mapping through the use of mipmapping and anisotropic filtering.
The drawing on the upper right shows a moiré pattern. The lines could represent fibers in moiré silk or lines drawn on paper or on a computer screen. The nonlinear interaction of the optical patterns of lines creates a real and visible pattern of roughly parallel dark and light bands, the moiré pattern, superimposed on the lines.
The moiré effect also occurs between overlapping transparent objects. For example, an invisible phase mask is made of a transparent polymer with a wavy thickness profile. As light shines through two overlaid masks of similar phase patterns, a broad moiré pattern occurs on a screen some distance away. This phase moiré effect and the classical moiré effect from opaque lines are two ends of a continuous spectrum in optics, which is called the universal moiré effect. The phase moiré effect is the basis for a type of broadband interferometer in x-ray and particle-wave applications. It also provides a way to reveal hidden patterns in invisible layers.