                        课程
                    
                        /前端开发
                        
                            /Html5
                        
                        /Canvas绘图详解

为什么我画的月亮会缺了一点？

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<script>
    window.onload = function() {
        var canvas = document.getElementById("canvas");

        canvas.width = 800;
        canvas.height = 800;

        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();

        function dis(x1,y1,x2,y2){
            return Math.sqrt((x1-x2)*(x1-x2),(y1-y2)*(y1-y2));
        }
    }
</script>
<body>
<canvas id="canvas" style="margin: 50px auto ;display: block; border:1px solid black;"></canvas>
</body>
</html>