一个菜鸟是如何用curveTo画出双曲线的

var mousedown = false;//用于判断鼠标是否被按下
function Point(x, y) {//创建Point类
this.x = x;
this.y = y;
}
function Shuangquxian(a, b) {//创建双曲线类
this.a = a;
this.b = b;//a、b就是双曲线的a、b咯
//下面的dd是贝赛尔曲线的端点,jd是贝赛尔曲线的控制点（即通过dd作切线后切线的 交点）
this.dd = [];
this.jd = [];
for (var i = 0; i<=3; i++) {
this.dd[i] = new Point(0, 0);
this.jd[i] = new Point(0, 0);
}
with (this) {//给各端点的x坐标赋值
c = 320-a;//这里减去320是因为场景是640×480的
dd[0].x = a;
dd[1].x = a+c/12;
dd[2].x = a+c/6;
dd[3].x = a+c;
}
for (var j = 0; j<=3; j++) {
this.dd[j].sqxfz(this.a, this.b);//按双曲线方程计算端点的y坐标值
}
with (this) {//计算出各控制点的坐标，用的是sqxjd方法
jd[1].x = dd[0].x;
jd[1].y = sqxqx(dd[1]).k*dd[0].x+sqxqx(dd[1]).t;
jd[2] = qxjd(dd[1], dd[2]);
jd[3] = qxjd(dd[2], dd[3]);
}
}
Point.prototype.sqxfz = function(a, b) {//双曲线赋值方法，用于计算已知x坐标的点的y坐标值
this.y = Math.sqrt(this.x*this.x*b*b/(a*a)-b*b);
};
Shuangquxian.prototype.sqxqx = function(p) {//求双曲线切线的方法，参数p是双曲线上任意一点
var a = this.a, b = this.b;
return {k:b*b*p.x/(a*a*p.y), t:-b*b/p.y};
};
Shuangquxian.prototype.qxjd = function(a, b) {//求两条切线的交点，a、b是两个切线的切点
var fc1 = this.sqxqx(a);
var fc2 = this.sqxqx(b);
var x = (fc2.t-fc1.t)/(fc1.k-fc2.k);
var y = fc1.k*x+fc1.t;
return new Point(x, y);
};
_root.createEmptyMovieClip("aaa", 1);//创建一个空mc用来放整个图形，这样容易把图的原点放到场景中心
aaa.createEmptyMovieClip("xzhou", 1);
aaa.createEmptyMovieClip("yzhou", 2);
with (aaa) {//画出坐标系，并把原点放到场景中间
xzhou.lineStyle(1, 0x000000, 100);
yzhou.lineStyle(1, 0x000000, 100);
xzhou.moveTo(-320, 0);
xzhou.lineTo(320, 0);
yzhou.moveTo(0, -240);
yzhou.lineTo(0, 240);
_x = 320;
_y = 240;
}
aaa.huaxian = function(a, b) {
this.sqx = new Shuangquxian(a, b);
this.createEmptyMovieClip("ab", 3);
with (this) {//下面就是画双曲线了，用了8段贝赛尔曲线，控制点就是双曲线对象的jd属性，端点就是dd属性
ab.lineStyle(1, 0xFF0000, 100);
ab.moveTo(sqx.dd[0].x, sqx.dd[0].y);
for (var k = 1; k<=3; k++) {
ab.curveTo(sqx.jd[k].x, sqx.jd[k].y, sqx.dd[k].x, sqx.dd[k].y);
}
ab.moveTo(sqx.dd[0].x, sqx.dd[0].y);
for (var k = 1; k<=3; k++) {
ab.curveTo(sqx.jd[k].x, -sqx.jd[k].y, sqx.dd[k].x, -sqx.dd[k].y);
}
ab.moveTo(-sqx.dd[0].x, sqx.dd[0].y);
for (var k = 1; k<=3; k++) {
ab.curveTo(-sqx.jd[k].x, sqx.jd[k].y, -sqx.dd[k].x, sqx.dd[k].y);
}
ab.moveTo(-sqx.dd[0].x, sqx.dd[0].y);
for (var k = 1; k<=3; k++) {
ab.curveTo(-sqx.jd[k].x, -sqx.jd[k].y, -sqx.dd[k].x, -sqx.dd[k].y);
}
}
};
//下面的代码用鼠标控制双曲线的a值和b值，这里设置的a＝b
onMouseDown = function () {
_root.mousedown = true;
};
onMouseMove = function () {
if (mousedown) {
aaa.huaxian(Math.abs(_xmouse-320), Math.abs(_xmouse-320));
updateAfterEvent();
}
};
onMouseUp = function () {
_root.mousedown = false;
if (_ymouse<=250 && _ymouse>=230) {
aaa.huaxian(Math.abs(_xmouse-320), Math.abs(_xmouse-320));
}
};