Most moirés are incidental: they arise naturally when two or more gratings, sheets of fabric, wire screens, fences, or dot screens are superposed.
Fence moiré, by Prof. David Eppstein at UC Irwine
In signal processing and imaging, moiré patterns are often known as aliasing. They arise when the sampling rate of a digitized signal is too low to accurately represent the signal. This effect may be a big nuisance in photography and television. In this case in the role of the two gratings play the periodic features in the image and the regular arrangement of pixels in the camera.
Moiré patterns are responsible for adding funky colors to Al Gore's collar in this photo, arguably improving its appearance:
These aliasing artifacts can be easily resolved by either increasing the sampling resolution or by applying proper filtering techniques prior to sampling as described by the Nyquist theorem.
Moiré patterns are commonly treated as a problem. However, they also have many useful applications. For example, their ability to magnify small offsets is exploited for strain and deformation analysis of materials by means of moiré interferometry.
The intentional synthesis of moirés appears to be largely unexplored. The most interesting and sophisticated examples of moiré art I have found to date are animated pictures in children's books:
Invariably, the words magic and amazing are used to describe moiré phenomena, even the most basic ones.
These examples are but a small fraction of what could be done with moirés. Through this blog, I would like to expand the definition of moirés and their applications.