Moire effect

Found on lovely package.com uses the moire effect

In physics, a moiré pattern (pronounced /mwɑːˈreɪ/ or /ˈmɔəreɪ/ in English; [mwaˈʁe] in French) is an interference pattern created, for example, when two grids are overlaid at an angle, or when they have slightly different mesh sizes.

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.

Shape moiré is one type of moiré patterns demonstrating the phenomenon of moiré magnification^[1][2]. 1D shape moiré is the particular simplified case of 2D shape moiré. 1D patterns may appear when superimposing an opaque layer containing tiny horizontal transparent lines on top of a layer containing a complex shape which is periodically repeating along the vertical axis. Shape moiré is sometimes referred as band moiré. The opaque layer with transparent lines is called the revealing layer. The layer containing the periodically repeating shapes is called the base layer. The period of shapes in the base layer is denoted as p_b. The period of transparent lines in the revealing layer is denoted as p_r. The periods of both layers must be sufficiently close. The superimposition image reveals the shapes of the base layer stretched along the vertical axis. The magnified shapes appear periodically along the vertical axis. The dimensions along the horizontal axis are not changed. If the complex shape of the base layer is a sequence of symbols (e.g. a horizontal text) compressed along the vertical axis, then the superimposition of the revealing layer can restore the original proportions of these symbols. The size of optical shapes p_m, magnified along the vertical axis is expressed by the following formula:

Negative values of p_m signify mirrored appearance (the magnified shapes will be inverted along the vertical axis) of the stretched shapes.

When the revealing layer is moved along the vertical axis, the magnified shapes move along the vertical axis at a faster speed. The speedup factor is expressed by the following formula:

Negative values of v_m / v_r signify the movement of optical shapes in reverse direction.

When p_r > p_b, the magnified shapes appear normally, but they move in reverse direction compared to the movement of the revealing layer. See the figure below:

Line moiré is one type of moiré pattern; a pattern that appears when superposing two transparent layers containing correlated opaque patterns. Line moiré is the case when the superposed patterns comprise straight or curved lines. When moving the layer patterns, the moiré patterns transform or move at a faster speed. This effect is calledoptical moiré speedup.