canvas上设置透明度
<!DOCTYPE html>
<html>
<head>
<meta http-equiv="content-type" content="text/html;charset=utf-8">
<title></title>
</head>
<body>
<canvas id="drawingCanvas" style="width:400px;height:300px;background:red;"></canvas>
<script>
var canvas=document.getElementById("drawingCanvas");
var ctx=canvas.getContext('2d');
function markSpindle(){
drawingCanvas.onmousedown=function(ev){
var ev=ev||window.event;
var left=ev.clientX;
document.onmousemove=function(ev){
ctx.fillStyle='white';
ctx.globalAlpha=.1;
ctx.fillRect(left,0,ev.clientX-left,200);
}
document.onmouseup=function(){
document.onmousemove = null;
document.onmouseup = null;
}
}
}
markSpindle();
</script>
</body>
</html>
鼠标移动的过程中,透明度不均匀,怎样可以设置让透明度均匀,求大神指导