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.
setTransform(a, b, c, d, e, f)
setTransform(matrix)
The transformation matrix is described by:
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).
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
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