The CanvasRenderingContext2D.globalAlpha property of the Canvas 2D API specifies the alpha value that is applied to shapes and images before they are drawn onto the canvas. The value is in the range from 0.0 (fully transparent) to 1.0 (fully opaque).

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


ctx.globalAlpha = value;


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


Using the globalAlpha property

This is just a simple code snippet using the globalAlpha property to draw two semi transparent rectangles.


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


var canvas = document.getElementById('canvas');
var 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);

Edit the code below and see your changes update live in the canvas:

A globalAlpha example

In this example, a background of four different colored squares is drawn. On top of these, there is a set of semi-transparent circles. The globalAlpha property is set at 0.2 which will be used for all shapes from that point on. Every step in the for loop draws a set of circles with an increasing radius. The final result is a radial gradient. By overlaying ever more circles on top of each other, the transparency of the circles that have already been drawn are reduced. By increasing the step count and in effect drawing more circles, the background would completely disappear from the center of the image.

var ctx = document.getElementById('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 semi transparent circles
for (i = 0; i < 7; i++){
  ctx.arc(75, 75, 10 + 10 * i, 0, Math.PI * 2, true);

ScreenshotLive sample


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

Browser compatibility

Update compatibility data on GitHub
ChromeEdgeFirefoxInternet ExplorerOperaSafariAndroid webviewChrome for AndroidEdge MobileFirefox for AndroidOpera for AndroidiOS SafariSamsung Internet
Basic supportChrome Full support YesEdge Full support 12Firefox Full support YesIE Full support YesOpera Full support YesSafari Full support YesWebView Android Full support YesChrome Android Full support YesEdge Mobile Full support YesFirefox Android Full support YesOpera Android Full support YesSafari iOS Full support YesSamsung Internet Android Full support Yes


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.
  • In WebKit- and Blink-based Browsers, a non-standard and deprecated method ctx.setAlpha() is implemented besides this property.

See also

Document Tags and Contributors

Contributors to this page: fscholz, NemoNobobyPersonne, nmve, erikadoyle, nearwood
Last updated by: fscholz,