在一个区域里取一个点，并向外引一条射线。这条射线会与组成这个区域的线条相交。而区域的线条都是有方向的，假设射线与一个方向的线条相交为+1，与另一个方向相交为-1。如果射线与所有方向的相交的结果的和为非0，则射线的起始点处于多边形的里面；如果为0，则处于多边形的外面
一般地，很少会绘制上图中无规律的图形。但是，在绘制空心图形，如圆环等，非零环绕原则也是适用的
<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title></title>
</head>
<body>
<canvas id="canvas" height="80" width="80">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');
var H = 80,W=80;
if(canvas.getContext){
var cxt = canvas.getContext('2d');
cxt.beginPath();
cxt.arc(40,40,30,0,Math.PI*2,false);
cxt.arc(40,40,15,0,Math.PI*2,true);
cxt.fillStyle = '#058';
cxt.shadowColor = 'gray';
cxt.shadowOffsetX = 3;
cxt.shadowOffsetY = 3;
cxt.shadowBlur = 3;
cxt.fill();
}
</script>
</body>
</html>
接下来，利用非零环绕原则和阴影来绘制一个镂空的剪纸效果
<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title></title>
</head>
<body>
<canvas id="drawing" style="border:1px solid black">
<p>The canvas element is not supported!</p>
</canvas>
<script>
var canvas = document.getElementById('drawing');
if(drawing.getContext){
var cxt = drawing.getContext('2d');
cxt.fillStyle = "#FFF";
cxt.fillRect(0,0,300,150);
cxt.beginPath();
cxt.rect(0,0,300,150);
drawPathRect(cxt, 10, 10, 200, 80);
drawPathTriangle(cxt, 10, 140,30, 100, 50, 140);
cxt.arc(150, 120, 20, 0, Math.PI * 2, true);
cxt.closePath();
cxt.fillStyle = "#058";
cxt.shadowColor = "gray";
cxt.shadowOffsetX = 10;
cxt.shadowOffsetY = 10;
cxt.shadowBlur = 10;
cxt.fill();
//逆时针绘制矩形
function drawPathRect(cxt, x, y, w, h){
cxt.moveTo(x, y);
cxt.lineTo(x, y + h);
cxt.lineTo(x + w, y + h);
cxt.lineTo(x + w, y);
cxt.lineTo(x, y);
}
//逆时针绘制三角形
function drawPathTriangle(cxt, x1, y1, x2, y2, x3, y3){
cxt.moveTo(x1,y1);
cxt.lineTo(x3,y3);
cxt.lineTo(x2,y2);
cxt.lineTo(x1,y1);
}
}
</script>
</body>
</html>
原文链接：https://www.qiquanji.com/post/7888.html
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