CanvasRenderingContext2D.globalAlpha

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句法
范例
规范
浏览器兼容性
另请参阅
相关话题

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 在 Canvas Tutorial .

句法

ctx.globalAlpha = value;

选项

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.

范例

Drawing translucent shapes

此范例使用 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);

结果

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 特性到 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

规范

规范	状态	注释
HTML 实时标准 The definition of 'CanvasRenderingContext2D.globalAlpha' in that specification.	实时标准

浏览器兼容性

The compatibility table on this page is generated from structured data. If you'd like to contribute to the data, please check out https://github.com/mdn/browser-compat-data and send us a pull request. 更新 GitHub 上的兼容性数据

	桌面						移动
	Chrome	Edge	Firefox	Internet Explorer	Opera	Safari	Android webview	Chrome for Android	Firefox for Android	Opera for Android	Safari on iOS	Samsung Internet
`globalAlpha`	Chrome Yes	Edge 12	Firefox 1.5	IE Yes	Opera Yes	Safari Yes	WebView Android Yes	Chrome Android Yes	Firefox Android 4	Opera Android Yes	Safari iOS Yes	Samsung Internet Android Yes

图例

完整支持

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.

另请参阅

The interface defining this property: CanvasRenderingContext2D
CanvasRenderingContext2D.globalCompositeOperation

元数据

最后修改： 2019 年 3 月 23 日

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句法

选项

范例