gfxCircle

gfxCircle is a basic geometric drawing primitive found in many graphics libraries, particularly those used for embedded systems and hobbyist electronics. It provides a straightforward method for rendering a circle on a pixel‑based display using integer math, which is efficient for devices with limited processing power and no floating‑point unit. The function typically accepts a center point (x, y), a radius, and a color value. In many implementations it also allows an optional thickness parameter to draw either a filled circle or the outline of a circle.

The most common algorithm used by gfxCircle is the midpoint circle algorithm. This method calculates points along the perimeter by exploiting the symmetry of a circle: for each calculated point (x, y) in one octant, the mirrored points in the other seven octants are drawn automatically. Because the algorithm uses only integer arithmetic and simple additions and subtractions, it is both fast and precise on small displays such as the 128x64 OLEDs and LCDs common in Arduino projects.

The gfxCircle function first appeared in the Adafruit GFX library for Arduino in 2009 and has since been adopted by other lightweight graphics stacks, including the Teensy graphics library and the TouchGFX framework. Its cross‑platform nature makes it a common building block in tutorials for drawing shapes on microcontrollers.

Typical usage looks like this:

x = 64

y = 32

radius = 10

color = 0xFFFF // white in 16‑bit RGB565

gfxCircle(x, y, radius, color)

When a filled circle is desired, many libraries offer a separate filledCircle function, or the circle function accepts a boolean flag or a thickness value of zero. This flexibility allows developers to create both simple outline shapes and solid filled graphics without writing custom rendering code for each case.