Web/API/CanvasRenderingContext2D/globalAlpha

The CanvasRenderingContext2D.globalAlpha property of the Canvas 2D API specifies the alpha (transparency) value that is applied to shapes and images before they are drawn onto the canvas.

See also the chapter Applying styles and color in the Canvas Tutorial.

Syntax

ctx.globalAlpha = value;

Options

value: A number between 0.0 (fully transparent) and 1.0 (fully opaque), inclusive. The default value is 1.0. Values outside that range, including Infinity and NaN, will not be set, and globalAlpha will retain its previous value.

Examples

Drawing translucent shapes

This example uses the globalAlpha property to draw two semi-transparent rectangles.

HTML

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

JavaScript

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

ctx.globalAlpha = 0.5;

ctx.fillStyle = 'blue';
ctx.fillRect(10, 10, 100, 100);

ctx.fillStyle = 'red';
ctx.fillRect(50, 50, 100, 100);

Result

Overlaying transparent shapes

This example illustrates the effect of overlaying multiple transparent shapes on top of each other. We begin by drawing a solid background composed of four differently colored squares. Next, we set the globalAlpha property to 0.2 (20% opaque); this alpha level will apply to all of our transparent shapes. After that, we use a for loop to draw a series of circles with increasing radii.

With each new circle, the opacity of the previous circles underneath is effectively increased. If we were to increase the step count (and thus draw more circles), the background would eventually disappear completely from the center of the image.

<canvas id="canvas" width="150" height="150"></canvas>

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

// Draw background
ctx.fillStyle = '#FD0';
ctx.fillRect(0, 0, 75, 75);
ctx.fillStyle = '#6C0';
ctx.fillRect(75, 0, 75, 75);
ctx.fillStyle = '#09F';
ctx.fillRect(0, 75, 75, 75);
ctx.fillStyle = '#F30';
ctx.fillRect(75, 75, 75, 75);
ctx.fillStyle = '#FFF';

// Set transparency value
ctx.globalAlpha = 0.2;

// Draw transparent circles
for (let i = 0; i < 7; i++) {
  ctx.beginPath();
  ctx.arc(75, 75, 10 + 10 * i, 0, Math.PI * 2, true);
  ctx.fill();
}

Screenshot	Live sample
[[File:../../../../../../media.prod.mdn.mozit.cloud/attachments/2012/07/09/232/e6c1a9740fb40da8e7cc293ab455a0f6/Canvas_globalalpha.png\|class=internal]]

Specifications

Specification	Status	Comment
HTML Living StandardThe definition of 'CanvasRenderingContext2D.globalAlpha' in that specification.	Living Standard

Browser compatibility

Update compatibility data on GitHub

	Desktop						Mobile
	Chrome	Edge	Firefox	Internet Explorer	Opera	Safari	Android webview	Chrome for Android	Firefox for Android	Opera for Android	Safari on iOS	Samsung Internet
`globalAlpha`	Chrome Full support Yes	Edge Full support 12	Firefox Full support 1.5	IE Full support Yes	Opera Full support Yes	Safari Full support Yes	WebView Android Full support Yes	Chrome Android Full support Yes	Firefox Android Full support 4	Opera Android Full support Yes	Safari iOS Full support Yes	Samsung Internet Android Full support Yes

Legend

Full support: Full support

Gecko-specific notes

Starting with Gecko 5.0, specifying invalid values for globalAlpha no longer throws a SYNTAX_ERR exception; these are now correctly silently ignored.

WebKit/Blink-specific notes

In WebKit- and Blink-based browsers, a non-standard and deprecated method ctx.setAlpha() is implemented in addition to this property.