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.
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.
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
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.
const canvas = document.getElementById('canvas');
const ctx = canvas.getContext('2d');
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';
ctx.globalAlpha = 0.2;
for (let i = 0; i < 7; i++) {
ctx.beginPath();
ctx.arc(75, 75, 10 + 10 * i, 0, Math.PI * 2, true);
ctx.fill();
}