CanvasRenderingContext2D.lineDashOffset

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The CanvasRenderingContext2D.lineDashOffset property of the Canvas 2D API sets the line dash pattern offset or "phase" to achieve a "marching ants" effect, for example.

Syntax

ctx.lineDashOffset = value;
value
A float specifying the amount of the offset. Initially 0.0.

Examples

Using the lineDashOffset property

This is just a simple code snippet which uses the lineDashOffset property to draw a dashed line.

HTML

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

JavaScript

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

ctx.setLineDash([4, 16]);
ctx.lineDashOffset = 2;

ctx.beginPath();
ctx.moveTo(0, 100);
ctx.lineTo(400, 100);
ctx.stroke();

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

Playable code
<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" style="height: 120px">
ctx.setLineDash([4, 16]);
ctx.lineDashOffset = 2;

ctx.beginPath();
ctx.moveTo(0,100);
ctx.lineTo(400, 100);
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);

Marching ants

The marching ants effect is an animation technique often found in selection tools of computer graphics programs. It helps the user to distinguish the selection border from the image background by animating the border.

var canvas = document.getElementById('canvas');
var ctx = canvas.getContext('2d');
var offset = 0;

function draw() {
  ctx.clearRect(0,0, canvas.width, canvas.height);
  ctx.setLineDash([4, 2]);
  ctx.lineDashOffset = -offset;
  ctx.strokeRect(10, 10, 100, 100);
}

function march() {
  offset++;
  if (offset > 16) {
    offset = 0;
  }
  draw();
  setTimeout(march, 20);
}

march();

Specifications

Specification Status Comment
HTML Living Standard
The definition of 'CanvasRenderingContext2D.lineDashOffset' in that specification.
Living Standard  

Browser compatibility

Update compatibility data on GitHub
DesktopMobile
ChromeEdgeFirefoxInternet ExplorerOperaSafariAndroid webviewChrome for AndroidEdge MobileFirefox for AndroidOpera for AndroidiOS SafariSamsung Internet
Basic supportChrome Full support YesEdge Full support 12Firefox Full support 27
Full support 27
Full support 7
Alternate Name
Alternate Name Uses the non-standard name: mozDashOffset
IE Full support 11Opera Full support YesSafari Full support YesWebView Android Full support YesChrome Android Full support YesEdge Mobile Full support YesFirefox Android Full support 27
Full support 27
Full support 7
Alternate Name
Alternate Name Uses the non-standard name: mozDashOffset
Opera Android Full support YesSafari iOS Full support YesSamsung Internet Android Full support Yes

Legend

Full support  
Full support
Uses a non-standard name.
Uses a non-standard name.

Gecko-specific notes

  • Starting with Gecko 7.0 (Firefox 7.0 / Thunderbird 7.0 / SeaMonkey 2.4), the non-standard and deprecated property mozDashOffset has been implemented. This property will be deprecated and removed in the future, see bug 931389. Use lineDashOffset instead.

WebKit-specific notes

  • In WebKit-based browsers (e.g. Safari), the non-standard and deprecated property webkitLineDashOffset is implemented besides this method. Use lineDashOffset instead.

See also

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