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<title>Document</title>
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<body onload="draw()">
<canvas id="canvas" width="500" height="500"></canvas>
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function draw(){
var canvas=document.getElementById(‘canvas‘);
var context=canvas.getContext("2d");
var g1=context.createLinearGradient(0,0,0,200);
g1.addColorStop(0,"rgb(22,22,5)");
g1.addColorStop(1,"rgb(45,230,7)");
context.fillStyle=g1;
context.fillRect(0,0,500,500);
var g2=context.createLinearGradient(0,0,300,300);
g2.addColorStop(0,‘rgba(0,0,255,0.5)‘);
g2.addColorStop(1,‘rgba(255,45,6,0.5)‘);
for(var i=0;i<10;i++)
{
context.beginPath();
context.fillStyle=g2;
context.arc(i*25,i*25,i*10,0,Math.PI*2,true);////6个参数分别是 起点横坐标，起点纵坐标，圆的半径，起始弧度，结束弧度（2PI刚好是360度），是否顺时针
context.closePath();
context.fill();
}
}
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原文：http://www.cnblogs.com/qiuchunxia/p/5815010.html