CanvasRenderingContext2D.globalAlpha - Web APIs 编辑

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

Specifications

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 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.

See also

• The interface defining this property: CanvasRenderingContext2D
• CanvasRenderingContext2D.globalCompositeOperation