<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>Document</title>
</head>
<body>
<canvas id="canvas" style="margin:0 auto;border:1px #ddd solid;">
The current browser does not support Canvas, can replace the browser a try!
</canvas>
<script>
window.onload = function(){
var canvas = document.getElementById('canvas');
canvas.width = 800;
canvas.height = 800;
if(canvas.getContext('2d')){
var context = canvas.getContext('2d');
// begin
var radialGrad = context.createRadialGradient(400,400,0,400,400,500);
radialGrad.addColorStop(0.0,'white');
radialGrad.addColorStop(0.25,'yellow');
radialGrad.addColorStop(0.5,'green');
radialGrad.addColorStop(0.75,'blue');
radialGrad.addColorStop(1.0,'black');
context.fillStyle = radialGrad;
context.fillRect(0,0,800,800)
}else{
alert('当前游览器不支持Canvas，请更换游览器后再试！');
}
}
</script>
</body>
<script>
/*总结
step 1
// 内圆的圆心（x0,y0） 内圆半径r0
var grd = context.creatRadialGradient(x0,y0,r0,x1,x1,r1)
step 2
//stop的值域[0,1]
grd.addColorStop(stop,color)
*/
</script>
</html>
