CanvasRenderingContext2D.lineDashOffset - Web APIs 编辑

The CanvasRenderingContext2D.lineDashOffset property of the Canvas 2D API sets the line dash offset, or "phase."

Note: Lines are drawn by calling the stroke() method.

Syntax

ctx.lineDashOffset = value;

value

A float specifying the amount of the line dash offset. The default value is 0.0.

Examples

Offsetting a line dash

This example draws two dashed lines. The first has no dash offset. The second has a dash offset of 4.

HTML

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

JavaScript

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

ctx.setLineDash([4, 16]);

// Dashed line with no offset
ctx.beginPath();
ctx.moveTo(0, 50);
ctx.lineTo(300, 50);
ctx.stroke();

// Dashed line with offset of 4
ctx.beginPath();
ctx.strokeStyle = 'red';
ctx.lineDashOffset = 4;
ctx.moveTo(0, 100);
ctx.lineTo(300, 100);
ctx.stroke();

Result

The line with a dash offset is drawn in red.

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.

HTML

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

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

BCD tables only load in the browser

See also

• The interface defining this property: CanvasRenderingContext2D
• CanvasRenderingContext2D.getLineDash()
• CanvasRenderingContext2D.setLineDash()
• Applying styles and color