正多边形绘制

参考：正多边形的数学推导及绘制

/*
	x 外接圆圆心 x坐标
	y 外接圆圆心 y坐标
	r 外接圆半径
	startAngle 起始旋转角度
*/
function drawPolygon(centerX, centerY, radius, sides, startAngle) {
	let angle = startAngle || 0;
	let points = []
	
	// 求各个顶点的坐标
	for (let i = 0; i < sides; i++) {
		x = centerX + radius * Math.sin(angle);
		y = centerY - radius * Math.cos(angle);
		angle += 2*Math.PI/sides;
		points.push({x: x, y: y});
	}
	console.log(points);
	// 顶点坐标连线，先移动到第一个顶点
	context.moveTo(points[0].x, points[0].y);
	// 依次开始连线
	for (let i = 1; i < sides; i++) {
		context.lineTo(points[i].x, points[i].y);
	}
	// 关闭路径，使得最后一点和第一点闭合
	context.closePath();
	context.stroke();
}


drawPolygon(100, 150, 100, 3);
drawPolygon(300, 150, 100, 4, 45/180*Math.PI);
drawPolygon(500, 150, 100, 5);
drawPolygon(700, 150, 100, 6, 30/180*Math.PI);
drawPolygon(100, 400, 100, 7);
drawPolygon(300, 400, 100, 8, 22.5/180*Math.PI);
drawPolygon(500, 400, 100, 9);
drawPolygon(700, 400, 100, 10, 18/180*Math.PI);