canvas-star6-drawMoon.html

<!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');

                // context.arc(400,400,300,0.5*Math.PI,1.5*Math.PI,true);
                // context.moveTo(400,100);
                // context.arcTo(1200,400,400,700,(400-100)*dis(400,100,1200,400)/(1200-400))
                // context.stroke();

                fillMoon(context,2,400,400,300,0)

            }else{
                alert('当前游览器不支持Canvas，请更换游览器后再试！');
            }
        }

        function dis(x1,y1,x2,y2){
            return Math.sqrt((x1-x2)*(x1-x2) + (y1-y2)*(y1-y2))
        }

        // 优化
        // x,y位置 r半径
        function fillMoon(cxt,d,x,y,R,rot,fillColor){
            cxt.save();
                cxt.translate(x,y);
                cxt.rotate(rot*Math.PI/180);
                cxt.scale(R,R);
                pathMoon(cxt,d);
                cxt.fillStyle = fillColor || "#fb5";
                cxt.fill();
            cxt.restore();
        }

        function pathMoon(cxt,d){
            cxt.beginPath();
                cxt.arc(0,0,1,0.5*Math.PI,1.5*Math.PI,true);
                moveTo(0,-1);
                cxt.arcTo(d,0,0,1,dis(0,-1,d,0)/d);
                // 或者使用贝塞尔二次曲线
                // cxt.quadraticCurveTo(1.2,0,0,1)
            cxt.closePath();
        }

        function dis(x1,y1,x2,y2){
            return Math.sqrt((x1-x2)*(x1-x2)+(y1-y2)*(y1-y2))
        }
    </script>
</body>
</html>