CanvasRenderingContext2D: globalAlpha property
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.
Note: See also the chapter Applying styles and color in the Canvas Tutorial.
Value
Examples
Drawing translucent shapes
This example uses the globalAlpha
property to draw two semi-transparent
rectangles.
HTML
html
<canvas id="canvas"></canvas>
JavaScript
js
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.
js
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 |
---|---|
Specifications
Specification |
---|
HTML Standard # dom-context-2d-globalalpha-dev |
Browser compatibility
BCD tables only load in the browser
Gecko-specific notes
-
Starting with Gecko 5.0, specifying invalid values for
globalAlpha
no longer throws aSYNTAX_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.
See also
- The interface defining this property:
CanvasRenderingContext2D
CanvasRenderingContext2D.globalCompositeOperation