HTML canvas globalAlpha 属性

示例

首先，绘制一个红色矩形，然后设置透明度（globalAlpha）为0.5，然后绘制一个蓝色和一个绿色矩形：

<canvas id="myCanvas" width="300" height="150" style="border:1px solid #d3d3d3;">您的浏览器不支持 HTML5 canvas 标签。</canvas>
<script>
var c = document.getElementById("myCanvas");
var ctx = c.getContext("2d");
ctx.fillStyle = "red";
ctx.fillRect(20, 20, 75, 50);

// 打开透明度
ctx.globalAlpha = 0.2;
ctx.fillStyle = "blue"; 
ctx.fillRect(50, 50, 75, 50); 
ctx.fillStyle = "green"; 
ctx.fillRect(80, 80, 75, 50);
</script>

JavaScript:

var c = document.getElementById("myCanvas");
var ctx = c.getContext("2d");
ctx.fillStyle = "red";
ctx.fillRect(20, 20, 75, 50);

// 打开透明度
ctx.globalAlpha = 0.2;
ctx.fillStyle = "blue";
ctx.fillRect(50, 50, 75, 50);
ctx.fillStyle = "green";
ctx.fillRect(80, 80, 75, 50);