CanvasRenderingContext2D.setTransform() - Web APIs 编辑

The CanvasRenderingContext2D.setTransform() method of the Canvas 2D API resets (overrides) the current transformation to the identity matrix, and then invokes a transformation described by the arguments of this method. This lets you scale, rotate, translate (move), and skew the context.

Note: See also the transform() method; instead of overriding the current transform matrix, it multiplies it with a given one.

Syntax

ctx.setTransform(a, b, c, d, e, f);
ctx.setTransform(matrix);

The transformation matrix is described by: $\left[ \begin{array}{ccc} a & c & e \\ b & d & f \\ 0 & 0 & 1 \end{array} \right]$

Parameters

setTransform() has two types of parameter that it can accept. The older type consists of several parameters representing the individual components of the transformation matrix to set:

a (m11)

Horizontal scaling. A value of 1 results in no scaling.

b (m12)

Vertical skewing.

c (m21)

Horizontal skewing.

d (m22)

Vertical scaling. A value of 1 results in no scaling.

e (dx)

Horizontal translation (moving).

f (dy)

Vertical translation (moving).

The newer type consists of a single parameter, matrix, representing a 2D transformation matrix to set (technically, a DOMMatrixInit object; any object will do as long as it contains the above components as properties).

Examples

Skewing a shape

This example skews a rectangle both vertically (.2) and horizontally (.8). Scaling and translation remain unchanged.

HTML

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

JavaScript

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

ctx.setTransform(1, .2, .8, 1, 0, 0);
ctx.fillRect(0, 0, 100, 100);

Result

Retrieving and passing a DOMMatrix object

In the following example, we have two <canvas> elements. We apply a transform to the first one's context using the first type of setTransform() and draw a square on it, then retrieve the matrix from it using CanvasRenderingContext2D.getTransform().

We then apply the retrieved matrix directly to the second canvas context by passing the DOMMatrix object directly to setTransform() (i.e. the second type), and draw a circle on it.

HTML

<canvas width="240"></canvas>
<canvas width="240"></canvas>

CSS

canvas {
  border: 1px solid black;
}

JavaScript

const canvases = document.querySelectorAll('canvas');
const ctx1 = canvases[0].getContext('2d');
const ctx2 = canvases[1].getContext('2d');

ctx1.setTransform(1, .2, .8, 1, 0, 0);
ctx1.fillRect(25, 25, 50, 50);

let storedTransform = ctx1.getTransform();
console.log(storedTransform);

ctx2.setTransform(storedTransform);
ctx2.beginPath();
ctx2.arc(50, 50, 50, 0, 2 * Math.PI);
ctx2.fill();

Result

Specifications

Browser compatibility

BCD tables only load in the browser

See also

• The interface defining this method: CanvasRenderingContext2D
• CanvasRenderingContext2D.transform()