canvas如何来绘制一个椭圆形?canvas画椭圆的方法总结(2)-木庄网络博客

二、canvas画椭圆之使用贝赛尔曲线绘制椭圆

这种方法绘制椭圆是把一个椭圆分成了4条贝塞尔曲线，用他们连成了一个椭圆。

canvas绘制椭圆的代码如下：

<!DOCTYPE html>
<html>
<head>
    <meta charset="UTF-8">
    <title>椭圆</title>
</head>
<body>
<canvas id="canvas" style="border:1px solid #aaa;display:block;margin:50px auto;">
    当前浏览器不支持Canvas，请更换浏览器后再试
</canvas>
<script>
    var canvas = document.getElementById("canvas");
    canvas.width = 600;
    canvas.height = 600;
    var context = canvas.getContext("2d");
        context.lineWidth = 10;
        context.strokeStyle="black";
        BezierEllipse2(context, 470, 200, 100, 20); //椭圆
    function BezierEllipse2(ctx, x, y, a, b){
        var k = .5522848,
        ox = a * k, // 水平控制点偏移量
        oy = b * k; // 垂直控制点偏移量</p> <p> ctx.beginPath();
        //从椭圆的左端点开始顺时针绘制四条三次贝塞尔曲线
        ctx.moveTo(x - a, y);
        ctx.bezierCurveTo(x - a, y - oy, x - ox, y - b, x, y - b);
        ctx.bezierCurveTo(x + ox, y - b, x + a, y - oy, x + a, y);
        ctx.bezierCurveTo(x + a, y + oy, x + ox, y + b, x, y + b);
        ctx.bezierCurveTo(x - ox, y + b, x - a, y + oy, x - a, y);
        ctx.closePath();
        ctx.stroke();
    };
</script>
</body>
</html>

canvas椭圆效果如下：

2345截图20180920114034.png

三、canvas画椭圆之使用两条贝赛尔曲线画出椭圆

canvas绘制椭圆的代码如下：

//椭圆 
CanvasRenderingContext2D.prototype.oval = function (x, y, width, height) {
 var k = (width/0.75)/2, 
w = width/2, 
h = height/2; 
this.beginPath(); 
this.moveTo(x, y-h); 
this.bezierCurveTo(x+k, y-h, x+k, y+h, x, y+h); 
this.bezierCurveTo(x-k, y+h, x-k, y-h, x, y-h); 
this.closePath(); return this; 
}