CanvasRenderingContext2D.globalAlpha

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.

Syntax

<var>ctx.globalAlpha = value;</var>

Options

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.

Examples

Using the globalAlpha property

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

HTML

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

JavaScript

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:

<canvas id="canvas" width="400" height="200" class="playable-canvas"></canvas>
<div class="playable-buttons">
  <input id="edit" type="button" value="Edit" />
  <input id="reset" type="button" value="Reset" />
</div>
<textarea id="code" class="playable-code" style="height:120px;">
ctx.globalAlpha = 0.5;

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

ctx.fillStyle = "red";
ctx.fillRect(50, 50, 100, 100);</textarea>

var canvas = document.getElementById("canvas");
var ctx = canvas.getContext("2d");
var textarea = document.getElementById("code");
var reset = document.getElementById("reset");
var edit = document.getElementById("edit");
var code = textarea.value;

function drawCanvas() {
  ctx.clearRect(0, 0, canvas.width, canvas.height);
  eval(textarea.value);
}

reset.addEventListener("click", function() {
  textarea.value = code;
  drawCanvas();
});

edit.addEventListener("click", function() {
  textarea.focus();
})

textarea.addEventListener("input", drawCanvas);
window.addEventListener("load", drawCanvas);

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.beginPath();
  ctx.arc(75,75,10+10*i,0,Math.PI*2,true);
  ctx.fill();
}

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

Screenshot	Live sample

Specifications

Browser compatibility

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 besides this property.

See also

• The interface defining it, CanvasRenderingContext2D
• CanvasRenderingContext2D.globalCompositeOperation