How to create circle with Bézier curves?

Question

We have a start point (x, y) and a circle radius. There also exists an engine that can create a path from Bézier curve points.

How can I create a circle using Bézier curves?

Answer 1

Many answers already but I found a small online article with a very good cubic bezier approximation of a circle. In terms of unit circle c = 0.55191502449 where c is the distance from the axis intercept points along the tangents to the control points.

As a single quadrant for the unit circle with the two middle coordinates being the control points. (0,1),(c,1),(1,c),(1,0)

The radial error is just 0.019608% so I just had to add it to this list of answers.

The article can be found here Approximate a circle with cubic Bézier curves

Answer 2

The answers to the question are very good, so there's little to add. Inspired by that I started to make an experiment to visually confirm the solution, starting with four Bézier curves, reducing the number of curves to one. Amazingly I found out that with three Bézier curves the circle looked good enough for me, but the construction is a bit tricky. Actually I used Inkscape to place the black 1-pixel-wide Bézier approximation over a red 3-pixel-wide circle (as produced by Inkscape). For clarification I added blue lines and surfaces showing the bounding boxes of the Bézier curves.

To see yourself, I'm presenting my results:

The 1-curve graph (which looks like a drop squeezed in a corner, just for completeness) :

The 2-curve graph:

The 3-curve graph:

The 4-curve graph:

(I wanted to put the SVG or PDF here, but that isn't supported)

Answer 3

Covered in the comp.graphics.faq

Excerpt:

Subject 4.04: How do I fit a Bezier curve to a circle?

Interestingly enough, Bezier curves can approximate a circle but not perfectly fit a circle. A common approximation is to use four beziers to model a circle, each with control points a distance d=r*4*(sqrt(2)-1)/3 from the end points (where r is the circle radius), and in a direction tangent to the circle at the end points. This will ensure the mid-points of the Beziers are on the circle, and that the first derivative is continuous.
The radial error in this approximation will be about 0.0273% of the circle's radius.

Michael Goldapp, "Approximation of circular arcs by cubic polynomials" Computer Aided Geometric Design (#8 1991 pp.227-238)

Tor Dokken and Morten Daehlen, "Good Approximations of circles by curvature-continuous Bezier curves" Computer Aided Geometric Design (#7 1990 pp. 33-41). http://www.sciencedirect.com/science/article/pii/016783969090019N (non free article)

Also see the non-paywalled article at http://spencermortensen.com/articles/bezier-circle/

Browsers and Canvas Element.

Note that some browsers use Bezier curves to their canvas draw arc, Chrome uses (at the present time) a 4 sector approach and Safari uses an 8 sector approach, the difference is noticeable only at high resolution, because of that 0.0273%, and also only truly visible when arcs are drawn in parallel and out of phase, you'll notice the arcs oscillate from a true circle. The effect is also more noticeable when the curve is animating around it's radial center, 600px radius is usually the size where it will make a difference.

Certain drawing API's don't have true arc rendering so they also use Bezier curves, for example the Flash platform has no arc drawing api, so any frameworks that offer arcs are generally using the same Bezier curve approach.

Note that SVG engines within browsers may use a different drawing method.

Other platforms

Whatever platform you are trying to use, it's worth checking to see how arc drawing is done, so you can predict visual errors like this, and adapt.

Answer 4

The other answers have covered the fact that a true circle is not possible. This SVG file is an approximation using Quadratic Bezier curves, and is the closest thing you can get: http://en.wikipedia.org/wiki/File:Circle_and_quadratic_bezier.svg

Here's one with Cubic Bezier curves: http://en.wikipedia.org/wiki/File:Circle_and_cubic_bezier.svg

Answer 5

It is not possible. A Bezier is a cubic (at least... the most commonly used is). A circle cannot be expressed exactly with a cubic, because a circle contains a square root in its equation. As a consequence, you have to approximate.

To do this, you have to divide your circle in n-tants (e.g.quadrants, octants). For each n-tant, you use the first and last point as the first and last of the Bezier curve. The Bezier polygon requires two additional points. To be fast, I would take the tangents to the circle for each extreme point of the n-tant and choose the two points as the intersection of the two tangents (so that basically your Bezier polygon is a triangle). Increase the number of n-tants to fit your precision.

Answer 6

It's a heavy approximation that will look reasonable or terrible depending on the resolution and precision but I use sqrt(2)/2 x radius as my control points. I read a rather long text how that number is derived and it's worth reading but the formula above is the quick and dirty.