Se nel giro di due giorni ti imbatti più volte in un argomento non proprio quotidiano, io penso che ci sia un motivo… ;-)

a moiré pattern is an interference pattern created, for example, when two grids are overlaid at an angle, or when they have slightly different mesh sizes.

Moiré patterns are often an undesired 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).

The nonlinear interaction of the optical patterns of lines creates a real and visible pattern of roughly horizontal dark and light bands, the moiré pattern, superimposed on the lines.^[1] More complex line moiré patterns are created if the lines are curved or not exactly parallel. Moiré patterns revealing complex shapes, or sequences of symbols embedded in one of the layers (in form of periodically repeated compressed shapes) are created with shape moiré, otherwise called band moiré patterns. One of the most important properties of shape moiré is its ability to magnify tiny shapes along either one or both axes, that is, stretching. A common 2D example of moiré magnification occurs when viewing a chain-link fence through a second chain-link fence of identical design. The fine structure of the design is visible even at great distances.

Su mathematik.com c’è una versione in movimento molto suggestiva…

Queste distorsoni compaiono quando si ridimensionano alcune immagini contenenti linee quasi parallele o pattern concentrici…

cambridgeincolour.com contiene un breve tutorial con alcune spiegazioni interessanti (anche se ovviamente basta cercare un atitmo in rete per trovarne altre…)

Il problema dell'effetto Moirè è presente anche nella stampa tradizionale su carta.

Halftone is the reprographic technique that simulates continuous tone imagery through the use of dots, varying either in size or in spacing. 'Halftone' can also be used to refer specifically to the image that is produced by this process.

Where continuous tone imagery contains an infinite range of colors or greys, the halftone process reduces visual reproductions to a binary image that is printed with only one color of ink. This binary reproduction relies on a basic optical illusion—that these tiny halftone dots are blended into smooth tones by the human eye. At a microscopic level, developed black and white photographic film also consists of only two colors, and not an infinite range of continuous tones. For details, see film grain.

Non sapevo che non si potesse stampare con retinature quadrate variando solamente la distanza tra i punti (FM Frequency Modulation) o il diametro (AM Amplitude Modulation), proprio a causa dell’effetto Moirè.

In process color prepress and printing, the angle at which the rows of halftone dots run in relation to the horizontal. In order to eliminate undesirable moiré patterns when the four color separation halftones are overprinted in multi-color printing, each screen needs to be placed at a different angle, as the dots of one color interfere with those of another color, creating the distinct moiré patterns. Ideally, moiré is kept minimal when screens are 30º from each other. However, since there is only a total of 90º (at least for perfectly round dots) in which to rotate the screens, each screen can't be 30º from each other when printing four colors (30 x 4 = 120º). Experience, though, has resulted in a standard set of default screen angles which work very well in a wide variety of applications. The screen angle of the yellow separation is 0º, or perfectly horizontal. The magenta separation is 15º from the horizontal. The black separation is 45º from the horizontal, and the cyan separation is 75º from horizontal. Generally speaking, the further a separation is from either the horizontal or vertical axis, the less intrusive it tends to be. (Even in black-and-white halftone production, a perfectly horizontal screen angle results in more of a visual discernment of the individual dots than does a 45º angle.) Therefore, yellow, which is the lightest color, is best left along one of these axes, while black, the darkest color, is best kept as far from both as possible (or 45º, the midway point between vertical and horizontal). Depending on the application, these angles may be varied, but only by a color separator who knows what s/he is doing.
from: http://printwiki.org/Screen_Angles

Altre info sono su http://en.wikipedia.org/wiki/Screening_(printing) e http://www.dtp-aus.com/hlftone.htm