Squircle

A squircle is a shape intermediate between a square and a circle. There are at least two definitions of "squircle" in use, the most common of which is based on the superellipse. The word "squircle" is a portmanteau of the words "square" and "circle". Squircles have been applied in design and optics.
Superellipse-based squircle.
In a Cartesian coordinate system, the superellipse is defined by the equation
where and are the semi-major and semi-minor axes, and are the and coordinates of the centre of the ellipse, and is a positive number. The squircle is then defined as the superellipse with and . Its equation is:
where is the minor radius of the squircle. Compare this to the equation of a circle. When the squircle is centred at the origin, then , and it is called Lamé's special quartic.
The area inside the squircle can be expressed in terms of the gamma function as
where is the minor radius of the squircle, and is the lemniscate constant.
-norm notation.
In terms of the -norm on , the squircle can be expressed as:
where , is the vector denoting the centre of the squircle, and . Effectively, this is still a "circle" of points at a distance from the centre, but distance is defined differently. For comparison, the usual circle is the case , whereas the square is given by the case (the supremum norm), and a rotated square is given by (the taxicab norm). This allows a straightforward generalization to a spherical cube, or "sphube", in , or "hypersphubes" in higher dimensions.
Fernández–Guasti squircle.
Another squircle comes from work in optics. It may be called the Fernández–Guasti squircle, after one of its authors, to distinguish it from the superellipse-related squircle above. This kind of squircle, centred at the origin, can be defined by the equation:
where is the minor radius of the squircle, is the squareness parameter, and and are in the interval . If , the equation is a circle; if , this is a square. This equation allows a smooth parametrization of the transition from a circle to a square, without involving infinity.
Similar shapes.
A shape similar to a squircle, called a "rounded square", may be generated by separating four quarters of a circle and connecting their loose ends with straight lines, or by separating the four sides of a square and connecting them with quarter-circles. Such a shape is very similar but not identical to the squircle. Although constructing a rounded square may be conceptually and physically simpler, the squircle has the simpler equation and can be generalised much more easily. One consequence of this is that the squircle and other superellipses can be scaled up or down quite easily. This is useful where, for example, one wishes to create nested squircles.
Another similar shape is a "truncated circle", the boundary of the intersection of the regions enclosed by a square and by a concentric circle whose diameter is both greater than the length of the side of the square and less than the length of the diagonal of the square (so that each figure has interior points that are not in the interior of the other). Such shapes lack the tangent continuity possessed by both superellipses and rounded squares.
Uses.
Squircles are useful in optics. If light is passed through a two-dimensional square aperture, the central spot in the diffraction pattern can be closely modelled by a squircle or supercircle. If a rectangular aperture is used, the spot can be approximated by a superellipse.
Squircles have also been used to construct dinner plates. A squircular plate has a larger area (and can thus hold more food) than a circular one with the same radius, but still occupies the same amount of space in a rectangular or square cupboard.
Many Nokia phone models have been designed with a squircle-shaped touchpad button.
Italian car manufacturer Fiat used numerous squircles in the interior and exterior design of the third generation Panda.
Apple Inc. uses a shape that resembles a squircle as the shape of app icons in iOS, iPadOS, and macOS (as of macOS Big Sur), but it is not actually a squircle but an approximation of a quintic superellipse. The same shape is seen on the home button in iOS devices with a home button but not Touch ID (currently only the iPod Touch).
One of the shapes for adaptive icons available in the Android "Oreo" operating system is a squircle.
The logo used by Instagram since 2016 includes a squircle forming the outline of a camera.