CanvasRenderingContext2D.rotate()
The
CanvasRenderingContext2D.rotate()
method of the Canvas 2D API adds a rotation to the transformation matrix.
Syntax
void ctx.rotate(angle);
Parameters
angle
-
The rotation angle, clockwise in radians. You can use
degree * Math.PI / 180
to calculate a radian from a degree.
The rotation center point is always the canvas origin. To change the center point, you
will need to move the canvas by using the
translate()
method.
Examples
Rotating a shape
This example rotates a rectangle by 45°. Note that the center of rotation is the top-left corner of the canvas, and not a location relative to any shape.
HTML
<canvas id="canvas"></canvas>
JavaScript
const canvas = document.getElementById('canvas');
const ctx = canvas.getContext('2d');
// Point of transform origin
ctx.arc(0, 0, 5, 0, 2 * Math.PI);
ctx.fillStyle = 'blue';
ctx.fill();
// Non-rotated rectangle
ctx.fillStyle = 'gray';
ctx.fillRect(100, 0, 80, 20);
// Rotated rectangle
ctx.rotate(45 * Math.PI / 180);
ctx.fillStyle = 'red';
ctx.fillRect(100, 0, 80, 20);
// Reset transformation matrix to the identity matrix
ctx.setTransform(1, 0, 0, 1, 0, 0);
Result
The center of rotation is blue. The non-rotated rectangle is gray, and the rotated rectangle is red.
Rotating a shape around its center
This example rotates a shape around its center point. To do this, the following steps are applied to the matrix:
-
First,
translate()
moves the matrix's origin to the shape's center. rotate()
rotates the matrix by the desired amount.-
Finally,
translate()
moves the matrix's origin back to its starting point. This is done by applying the values of the shape's center coordinates in a negative direction.
HTML
<canvas id="canvas"></canvas>
JavaScript
The shape is a rectangle with its corner at (80, 60), a width of 140, a height of 30. Its horizontal center is at (80 + 140 / 2), or 150. Its vertical center is at (60 + 30 / 2), or 75. Thus, the center point is at (150, 75).
const canvas = document.getElementById('canvas');
const ctx = canvas.getContext('2d');
// Non-rotated rectangle
ctx.fillStyle = 'gray';
ctx.fillRect(80, 60, 140, 30);
// Matrix transformation
ctx.translate(150, 75);
ctx.rotate(Math.PI / 2);
ctx.translate(-150, -75);
// Rotated rectangle
ctx.fillStyle = 'red';
ctx.fillRect(80, 60, 140, 30);
Result
The non-rotated rectangle is gray, and the rotated rectangle is red.
Specifications
Specification |
---|
HTML Standard # dom-context-2d-rotate-dev |
Browser compatibility
BCD tables only load in the browser
See also
- The interface defining this method:
CanvasRenderingContext2D