# An example of the canvas ellipse() function

Written by Richard Heyes, on 23rd January 2013

This page shows you an example of the canvas ellipse() function and an alternative to it if it's not available

Warning: As of March 2014 the features described on this page have limited browser support - currently only Google Chrome and Opera have working implementations of the `ellipse()` function.

[No canvas support]
An example of the `ellipse()` function

## Introduction

One of the new additions in the canvas v5 API is the `ellipse()` function. This is similar to the `arc()` function which can be used to draw arcs or circles. It takes more arguments than the `arc()` function - in order for you to be able to give the vertical and horizontal radius and also the orientation of the ellipse - but other than that it will work in much the same way. In total it takes eight arguments which are listed below. As with the arc function it's important to remember that the angles are measured in radians - not degrees. There are two functions shown below which you can use to convert to and from radians and degrees.

## Browser support

At the time of writing (23rd January 2013) there are no browsers that have implemented the `ellipse()` function. See below however for up-to-date listings of browsers that do now support the `ellipse()` function and a method of using the `bezierCurveTo()` function to achieve a "pseudo-ellipse".

Chrome:
Yes
Firefox:
No
Internet Explorer:
No
Safari:
No
Opera:
Yes

## An example

This is an example of how the `ellipse()` function will look. It's similar to the arc method so if you're comfortable with that then you shouldn't have any problems. It takes extra arguments - namely two radius arguments and a rotation argument (ie so you can have the longer bit of the ellipse at any angle). The arguments are:

1. x - The X coordinate
2. y - The Y coordinate
3. rx - The X radius of the ellipse
4. ry - The Y radius of the ellipse
5. rotation - How much the ellipse is rotated
6. start - The start angle*
7. end - The end angle*
8. anticlockwise - Whether the ellipse is drawn clockwise (false) or anticlockwise (true)**

* Angles are measured in radians - 1 degree = (Pi / 180) - there's a few functions to convert degrees to radians and back below
** If you're drawing a full ellipse the direction is less important. But if you're only drawing part of the ellipse it is!

```context.beginPath() ;

var x             = 150;         // The X coordinate
var y             = 75;          // The Y cooordinate
var rx            = 125;         // The X radius
var ry            = 50;          // The Y radius
var rotation      = 0;           // The rotation of the ellipse (in radians)
var start         = 0;           // The start angle (in radians)
var end           = 2 * Math.PI; // The end angle (in radians)
var anticlockwise = false;       // Whether the ellipse is drawn in a clockwise direction or
// anticlockwise direction

context.ellipse(x, y, rx, ry, rotation, start, end, anticlockwise);

context.stroke() ;
```

This code creates the ellipse that is shown at the top of the page.

Here are the functions to convert degrees to radians and back again. They are added to the `Number` object so that you can use them as the examples show.

```<script>
Number.prototype.toRadians = function ()
{
return this * (Math.PI / 180);
}

Number.prototype.toDegrees = function ()
{
return this * (180 / Math.PI);
}

num = 360; // 6.28 radians

num = Math.PI / 2; // 90 degrees
</script>
```

## An interactive example of the ellipse() function

As of 2nd September 2013 the `ellipse()` function has been added to the Google Chrome Canary version. So if you're running this or a later version that has support for the `ellipse()` function you can have a play with this interactive ellipse example .

## How can I get ellipses without the ellipse() function?

[No canvas support]

If you're hankering after the `ellipse()` function there is a limited way in which you can get ellipses right now by using the `bezierCurveTo()` function. It involves drawing two bezier curves - the top and the bottom of the ellipse. An example of this method is shown. This could be wrapped into a function that mimics the native API function that's coming - or it could just be added to a regular function to make it easier to use repeatedly.

```<script>
context = document.getElementById("cvs").getContext('2d');

context.beginPath();

context.lineWidth = 1;
context.strokeStyle = 'black';
context.fillStyle = 'red';

// The start point - the center/left of the ellipse
context.moveTo(10,50);

// Draw the top half - a bezier curve to the center right of the ellipse
context.bezierCurveTo(10,10,240,10,240,50);

// Draw the bottom half - a bezier curve back to the center left of the ellipse
context.bezierCurveTo(240,90,10,90,10,50);
context.stroke();
context.fill();

// This adds the text to the center of the ellipse
context.fillStyle = 'black';
context.font = '20pt Arial';
context.textAlign = 'center';
context.textBaseline = 'middle';
context.fillText('An ellipse!',125,50);
</script>
```