CanvasRenderingContext2D.arc()

The CanvasRenderingContext2D.arc() method of the Canvas 2D API adds an arc to the path which is centered at (x, y) position with radius r starting at startAngle and ending at endAngle going in the given direction by anticlockwise (defaulting to clockwise).

Синтаксис

void ctx.arc(x, y, radius, startAngle, endAngle, anticlockwise);

Параметры

x

The x coordinate of the arc's center.

y

The y coordinate of the arc's center.

radius

The arc's radius.

startAngle

The angle at which the arc starts, measured clockwise from the positive x axis and expressed in radians.

endAngle

The angle at which the arc ends, measured clockwise from the positive x axis and expressed in radians.

anticlockwise Необязательный

An optional Boolean which, if true, causes the arc to be drawn counter-clockwise between the two angles. By default it is drawn clockwise.

Examples

Using the arc method

This is just a simple code snippet drawing a circle.

HTML

<canvas id="canvas"></canvas>

JavaScript

var canvas = document.getElementById("canvas");
var ctx = canvas.getContext("2d");

ctx.beginPath();
ctx.arc(75, 75, 50, 0, 2 * Math.PI);
ctx.stroke();

Edit the code below and see your changes update live in the canvas:

<canvas id="canvas" width="400" height="200" class="playable-canvas"></canvas>
<div class="playable-buttons">
  <input id="edit" type="button" value="Edit" />
  <input id="reset" type="button" value="Reset" />
</div>
<textarea id="code" class="playable-code">
ctx.beginPath();
ctx.arc(50, 50, 50, 0, 2 * Math.PI, false);
ctx.stroke();</textarea>

var canvas = document.getElementById("canvas");
var ctx = canvas.getContext("2d");
var textarea = document.getElementById("code");
var reset = document.getElementById("reset");
var edit = document.getElementById("edit");
var code = textarea.value;

function drawCanvas() {
  ctx.clearRect(0, 0, canvas.width, canvas.height);
  eval(textarea.value);
}

reset.addEventListener("click", function() {
  textarea.value = code;
  drawCanvas();
});

edit.addEventListener("click", function() {
  textarea.focus();
})

textarea.addEventListener("input", drawCanvas);
window.addEventListener("load", drawCanvas);

Different shapes demonstrated

In this example different shapes are drawn to show what is possible when using arc().

<canvas id="canvas" width="150" height="200"></canvas>

var canvas = document.getElementById('canvas');
var ctx = canvas.getContext('2d');

// Draw shapes
for (i=0;i<4;i++){
  for(j=0;j<3;j++){
    ctx.beginPath();
    var x              = 25+j*50;               // x coordinate
    var y              = 25+i*50;               // y coordinate
    var radius         = 20;                    // Arc radius
    var startAngle     = 0;                     // Starting point on circle
    var endAngle       = Math.PI+(Math.PI*j)/2; // End point on circle
    var anticlockwise  = i%2==1;                // Draw anticlockwise
   
    ctx.arc(x,y,radius,startAngle,endAngle, anticlockwise);
   
    if (i>1){
      ctx.fill();
    } else {
      ctx.stroke();
    }
  }
}

Screenshot	Live sample

Specifications

Browser compatibility

Gecko-specific notes

Starting with Gecko 2.0 (Firefox 4 / Thunderbird 3.3 / SeaMonkey 2.1):

• The anticlockwise parameter is optional,
• Specifying a negative radius now throws a IndexSizeError error ("Index or size is negative or greater than the allowed amount").

See also

• The interface defining it, CanvasRenderingContext2D