Squircle Squircle centered on the origin (a=0=b) with minor radius r=1: $x^{4}+y^{4}=1$ 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

$\left|{\frac {x-a}{r_{a}}}\right|^{n}\!+\left|{\frac {y-b}{r_{b}}}\right|^{n}\!=1,\,$ where ra and rb are the semi-major and semi-minor axes, a and b are the x and y coordinates of the center of the ellipse, and n is a positive number. The squircle can be defined as the superellipse with ra = rb and n = 4. Its equation is:

$\left(x-a\right)^{4}+\left(y-b\right)^{4}=r^{4}$ where r is the minor radius of the squircle. Compare this to the equation of a circle. When the squircle is centered at the origin, then a = b = 0, and it is called Lamé's special quartic.

In terms of the p-norm $\|\cdot \|_{p}$ on $\mathbb {R} ^{2}$ , the squircle can be expressed as:

$\|\mathbf {x} -\mathbf {x} _{c}\|_{p}=r$ where p = 4, $\mathbf {x} _{c}=(a,b)$ is the vector denoting the center of the squircle, and $\mathbf {x} =(x,y)$ . Effectively, this is still a "circle" of points at a distance r from the center, but distance is defined differently. For comparison, the usual circle is the case p = 2, whereas the square is given by the $p\to \infty$ case (the supremum norm), and a rotated square is given by p = 1 (the taxicab norm). This allows a straightforward generalization to a spherical cube, or "sphube", in $\mathbb {R} ^{3}$ , or "hypersphubes" in higher dimensions.

The area inside the squircle can be expressed in terms of the gamma function Γ(x) as

$\mathrm {Area} =4r^{2}{\frac {\left(\Gamma \left(1+{\tfrac {1}{4}}\right)\right)^{2}}{\Gamma \left(1+{\tfrac {2}{4}}\right)}}={\frac {8r^{2}{\big (}\Gamma {\big (}{\frac {5}{4}}{\big )}{\big )}^{2}}{\sqrt {\pi }}}=S{\sqrt {2}}r^{2}\approx 3.708r^{2},$ where r is the minor radius of the squircle, and S is the lemniscate constant.

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, centered at the origin, can be defined by the equation:

$r^{2}x^{2}+r^{2}y^{2}-s^{2}x^{2}y^{2}=r^{4}$ where r is the minor radius of the squircle, s is the squareness parameter, and x and y are in the interval [-r,r]. If s = 0, the equation is a circle; if s=1, this is a square. This equation allows a smooth parameterization 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. This is even more true of a square plate, but there are various problems (such as fragility, and difficulty of wiping up sauce[full citation needed]) associated with the corners of square plates.

Nokia is closely associated with the squircle, having used it as a touchpad button in many phones.

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 in its iOS mobile operating system as the shape of app icons, but it is not actually a squircle but an approximation of a quintic superellipse. The same shape also used to be seen in iOS devices (currently only in the iPod Touch) on the home button.

One of the shapes for adaptive icons available in the Android "Oreo" operating system is a squircle.