Canvas Tutorial: an arc is defined by two angles

The arc is the basis for many geometric figures, but it always has a symmetrical shape.

Simple plot, elaborate uses

We use the curve methods for making figures that are not symmetrical.

The arc method depends on a position and two angles

The arc function has six parameters:

  1. The starting point horizontally.
  2. The vertical starting point.
  3. The radius.
  4. The starting angle.
  5. The endingl angle.
  6. A flag to reverse the direction of rotation: false sense of clockwise, true otherwise.

This gives the following syntax:

arc(x, y, radius, startingangle, endingangle,  [true | false])

Example of arc instruction:

context.arc(100,5, 70, Math.PI, 1,5 * Math.PI,  true);

The point x, y can be the beginning of a figure, in this case the beginPath method is called befor arc(). The arc can also be seen as an extension of a figure already drawn, then it retrieves the attributes of it.

A simple demo...

The starting angle is 0 and the final angle is PI. The radius is 70 pixels. The rotation direction is false (clock-wise):

The code:

context.arc(150, 5, 70, 0, Math.PI, false);

The direction of rotation may be changed

By default the rotation is clock-watch and it is false. We can reverse the direction with an optional sixth parameter.

If we give a true sixth parameter, the arc will be drawn in the opposite direction, counterclockwise.
Example as above but with a direction of rotation reversed:

Code:

 context.arc(150, 75, 70, 0, Math.PI, true);

How to define the angles of an arc

The angle has a real value ranging from 0 to 2 times PI, that is expressed in JavaScript by 2 * Math.PI.
The latter value corresponds to 360 ° (but the angle values ​​are not expressed in degrees).
The above example with initial angle of PI and final double PI:

Code:

context.arc(150, 75, 70, Math.PI, 2 * Math.PI, false);

And we face the previous arc as we have in each case the complement of the arc of the first example.

Making a arc to the left

When we trace in the direction of the clock, a semicircle has a starting of 0 and a ending angle of PI.
A circle is 0 to 2 times PI.
To draw a semicircle on the left, so we start to PI / 2 and ends at 1.5 times PI. Proof:

To draw an arc on the right, is assigned the value true to the parameter in the direction of rotation or we take the values ​​of angles of 1.5 IP and 0.5 IP.

Arc in motion

To see how this works, you go into an animation to vary the starting angle and the final angle from 0 to 2*PI.
(Does not work under Chrome).

Value of initial and final angle:

Code of the demonstration:

var angle = 0;
var delay; 

function drawAngle(ctxa)
{
  ctxa.arc(150, 50, 40, angle, angle);
  ctxa.stroke();
  angle+=0.1;
  if(angle > 2 * Math.PI) { clearInterval(delay);return;}
  document.getElementById("angle").value = Math.round(angle * 100)/100;
}  

function setAngle()
{
  angle=0;
  var canvas = document.getElementById("canvas4"); 
  var ctxa = canvas.getContext("2d");
  ctxa.lineWidth="3";
  ctxa.beginPath();
  delay = setInterval(function(){ drawAngle(ctxa); }, 100);
}

setAngle();

Interactive demonstration

How to choose a starting and ending angle, how to choose a sense of rotation, and the interaction between all these factors...
The best way to understand them is an interactive demonstration .

The Canvas tag
X max: 400
Y max: 200
Angle max: 2 PI = 6,29
Radius max: 200
Canvas requires a modern browser: Chrome, Firefox, IE9, Safari.
Defining an arc

Starting angle Ending angle Rotating direction: true false

Coordinates x= y= Radius (0-100)

The HTML code: 

<canvas id="canvasDemo" width="402" height="202" style="border:1px solid #CCC;background:white">
  Canvas requires a modern browser: Chrome, Firefox, IE9, Safari.
</canvas>

The JavaScript code: 

function setArcDemo()
{
  var canvas = document.getElementById("canvasDemo"); 
  var context = canvas.getContext("2d");
  
  var x = document.getElementById("x").value;
  var y = document.getElementById("y").value;
  var radius = document.getElementById("radius").value;
  var initial = document.getElementById("startangle").value;
  var final = document.getElementById("endangle").value;
  if(document.getElementById("clock1").checked)
  	clock = true;
  else	
  	clock = false;
  context.beginPath();
  context.lineWidth="2";
  context.arc(x, y, radius, initial, final, clock)
  context.stroke();
}

function erase()
{
  var canvas = document.getElementById("canvasDemo"); 
  var context = canvas.getContext("2d");
  context.clearRect(0,0, canvas.width, canvas.height); 
}

See also