CanvasRenderingContext2D.scale() - Web APIs 编辑

The CanvasRenderingContext2D.scale() method of the Canvas 2D API adds a scaling transformation to the canvas units horizontally and/or vertically.

By default, one unit on the canvas is exactly one pixel. A scaling transformation modifies this behavior. For instance, a scaling factor of 0.5 results in a unit size of 0.5 pixels; shapes are thus drawn at half the normal size. Similarly, a scaling factor of 2.0 increases the unit size so that one unit becomes two pixels; shapes are thus drawn at twice the normal size.

Syntax

void ctx.scale(x, y);

Parameters

x

Scaling factor in the horizontal direction. A negative value flips pixels across the vertical axis. A value of 1 results in no horizontal scaling.

y

Scaling factor in the vertical direction. A negative value flips pixels across the horizontal axis. A value of 1 results in no vertical scaling.

Examples

Scaling a shape

This example draws a scaled rectangle. A non-scaled rectangle is then drawn for comparison.

HTML

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

JavaScript

The rectangle has a specified width of 8 and a height of 20. The transformation matrix scales it by 9x horizontally and by 3x vertically. Thus, its final size is a width of 72 and a height of 60.

Notice that its position on the canvas also changes. Since its specified corner is (10, 10), its rendered corner becomes (90, 30).

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

// Scaled rectangle
ctx.scale(9, 3);
ctx.fillStyle = 'red';
ctx.fillRect(10, 10, 8, 20);

// Reset current transformation matrix to the identity matrix
ctx.setTransform(1, 0, 0, 1, 0, 0);

// Non-scaled rectangle
ctx.fillStyle = 'gray';
ctx.fillRect(10, 10, 8, 20);

Result

The scaled rectangle is red, and the non-scaled rectangle is gray.

Flipping things horizontally or vertically

You can use scale(-1, 1) to flip the context horizontally and scale(1, -1) to flip it vertically. In this example, the words "Hello world!" are flipped horizontally.

Note that the call to fillText() specifies a negative x coordinate. This is to adjust for the negative scaling factor: -280 * -1 becomes 280, and text is drawn leftwards from that point.

HTML

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

JavaScript

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

ctx.scale(-1, 1);
ctx.font = '48px serif';
ctx.fillText('Hello world!', -280, 90);
ctx.setTransform(1, 0, 0, 1, 0, 0);

Result

Specifications

Browser compatibility

BCD tables only load in the browser

See also

• The interface defining this method: CanvasRenderingContext2D