Form
context.globalAlpha
Explanation
The globalAlpha specifies the opacity.
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
<head>
<meta http-equiv="content-type" content="text/html;charset=shift_jis">
<title>Sample</title>
<script type="text/javascript"><!--
function drawPoint()
{
canvasObj = document.getElementById("imgCanvas");
conObj = canvasObj.getContext("2d");
conObj.fillStyle = "rgba(255,0,0,1)";
conObj.fillRect(20,40,200,50);
conObj.globalAlpha = 0.5;
conObj.fillRect(20,100,200,50);
conObj.globalAlpha = 0.25;
conObj.fillRect(20,160,200,50);
}
// --></script>
</head>
<body onload="drawPoint()">
<h1>Sample</h1>
<canvas id="imgCanvas" width="320" height="240" style="border:1px black solid"></canvas>
</body>
</html>