canvas绘制贝塞尔曲线

1、绘制二次方贝塞尔曲线

quadraticCurveTo(cp1x,cp1y,x,y); 其中参数cp1x和cp1y是控制点的坐标，x和y是终点坐标

数学公式表示如下：

二次方贝兹曲线的路径由给定点P₀、P₁、P₂的函数B（t）追踪：

$\mathbf{B}(t) = (1 - t)^{2}\mathbf{P}_0 + 2t(1 - t)\mathbf{P}_1 + t^{2}\mathbf{P}_2 \mbox{ , } t \in [0,1]$

<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<title></title>
<style type="text/css">
*{padding: 0;margin:0;}
body{background: #1b1b1b;}
#div1{margin:50px auto; width:300px; height: 300px;}
canvas{background: #fff;}
</style>
<script type="text/javascript">
window.onload = function(){
    var c = document.getElementById('myCanvas');
    var content = c.getContext('2d');
    
    //绘制二次方贝塞尔曲线
    content.strokeStyle ="#FF5D43";
    content.beginPath();
    content.moveTo(0,200);
    content.quadraticCurveTo(75,50,300,200);
    content.stroke();
    content.globalCompositeOperation = 'source-over';    //目标图像上显示源图像
    
    //绘制上面曲线的控制点和控制线，控制点坐标为两直线的交点(75,50)
    content.strokeStyle = '#f0f';
    content.beginPath();
    content.moveTo(75,50);
    content.lineTo(0,200);
    content.moveTo(75,50);
    content.lineTo(300,200);
    content.stroke();
    
};
</script>
</head>
<body>
    <div id="div1">
        <canvas id="myCanvas" width="300" height="200"></canvas>
    </div>
</body>
</html>

2、三次方贝塞尔曲线

bezierCurveTo(cp1x,cp1y,cp2x,cp2y,x,y)　　其中参数cp1x，cp1y表示第一个控制点的坐标, cp2x，cp2y表示第二个控制点的坐标, x，y是终点的坐标；

数学公式表示如下：

P₀、P₁、P₂、P₃四个点在平面或在三维空间中定义了三次方贝兹曲线。曲线起始于P₀走向P₁，并从P₂的方向来到P₃。一般不会经过P₁或P₂；这两个点只是在那里提供方向资讯。P₀和P₁之间的间距，决定了曲线在转而趋进P₃之前，走向P₂方向的“长度有多长”。

$\mathbf{B}(t)=\mathbf{P}_0(1-t)^3+3\mathbf{P}_1t(1-t)^2+3\mathbf{P}_2t^2(1-t)+\mathbf{P}_3t^3 \mbox{ , } t \in [0,1]$

<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<title></title>
<style type="text/css">
*{padding: 0;margin:0;}
body{background: #1b1b1b;}
#div1{margin:50px auto; width:300px; height: 300px;}
canvas{background: #fff;}
</style>
<script type="text/javascript">
window.onload = function(){
    var c = document.getElementById('myCanvas');
    var content = c.getContext('2d');
    
    //三次方贝塞尔曲线
    content.strokeStyle = '#FA7E2A';
    content.beginPath();
    content.moveTo(25,175);
    content.bezierCurveTo(60,80,150,30,170,150);
    content.stroke();
    content.globalCompositeOperation = 'source-over';
    
    //绘制起点、控制点、终点
    content.strokeStyle = 'red';
    content.beginPath();
    content.moveTo(25,175);
    content.lineTo(60,80);
    content.lineTo(150,30);
    content.lineTo(170,150);
    content.stroke();
    
};
</script>
</head>
<body>
    <div id="div1">
        <canvas id="myCanvas" width="300" height="200"></canvas>
    </div>
</body>
</html>