CanvasRenderingContext2D.lineDashOffset

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:

<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.

<canvas id="canvas" class="playable-canvas"></canvas>

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

Browser compatibility

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

• The interface defining it, CanvasRenderingContext2D
• CanvasRenderingContext2D.getLineDash()
• CanvasRenderingContext2D.setLineDash()