贝塞尔曲线

获取贝塞尔控制点 

function getCtrlPoint(ps, i, a, b) {
					if (!a || !b) {
						a = 0.25;
						b = 0.25;
					}
					//处理两种极端情形
					if (i < 1) {
						var pAx = ps[0].x + (ps[1].x - ps[0].x) * a;
						var pAy = ps[0].y + (ps[1].y - ps[0].y) * a;
					} else {
						var pAx = ps[i].x + (ps[i + 1].x - ps[i - 1].x) * a;
						var pAy = ps[i].y + (ps[i + 1].y - ps[i - 1].y) * a;
					}
					if (i > ps.length - 3) {
						var last = ps.length - 1
						var pBx = ps[last].x - (ps[last].x - ps[last - 1].x) * b;
						var pBy = ps[last].y - (ps[last].y - ps[last - 1].y) * b;
					} else {
						var pBx = ps[i + 1].x - (ps[i + 2].x - ps[i].x) * b;
						var pBy = ps[i + 1].y - (ps[i + 2].y - ps[i].y) * b;
					}
					return {
						pA: {
							x: pAx,
							y: pAy
						},
						pB: {
							x: pBx,
							y: pBy
						},
						now: {
							x: ps[i].x,
							y: ps[i].y
						},
						t: 0
					}

				}

连接：贝塞尔曲线控制点确定的方法

https://wenku.baidu.com/view/c790f8d46bec0975f565e211.html