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.
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");
ctx.scale(9, 3);
ctx.fillStyle = "red";
ctx.fillRect(10, 10, 8, 20);
ctx.setTransform(1, 0, 0, 1, 0, 0);
ctx.fillStyle = "gray";
ctx.fillRect(10, 10, 8, 20);
Result
The scaled rectangle is red, and the non-scaled rectangle is gray.
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