从三个点找到圆心的算法是什么？

Question

我在圆周上有三个点:

pt A =(Ax,Ay); pt B =(Bx,By); pt C =(Cx,Cy);

如何计算圆的中心？

在Processing(Java)中实现它.

我找到了答案并实施了一个有效的解决方案:

pt A = (A.x, A.y);
pt B = (B.x, B.y);
pt C = (C.x, C.y);

Answer 1

这是我的Java端口,当行列式以非常优雅的IllegalArgumentException方式消失时避开错误条件,我的方法是应对"点数相距很远"或"点在线上"的条件.此外,这会计算相交斜率方法不会做的半径(并应对异常条件).

public class CircleThree
{ 
  static final double TOL = 0.0000001;

  public static Circle circleFromPoints(final Point p1, final Point p2, final Point p3)
  {
    final double offset = Math.pow(p2.x,2) + Math.pow(p2.y,2);
    final double bc =   ( Math.pow(p1.x,2) + Math.pow(p1.y,2) - offset )/2.0;
    final double cd =   (offset - Math.pow(p3.x, 2) - Math.pow(p3.y, 2))/2.0;
    final double det =  (p1.x - p2.x) * (p2.y - p3.y) - (p2.x - p3.x)* (p1.y - p2.y); 

    if (Math.abs(det) < TOL) { throw new IllegalArgumentException("Yeah, lazy."); }

    final double idet = 1/det;

    final double centerx =  (bc * (p2.y - p3.y) - cd * (p1.y - p2.y)) * idet;
    final double centery =  (cd * (p1.x - p2.x) - bc * (p2.x - p3.x)) * idet;
    final double radius = 
       Math.sqrt( Math.pow(p2.x - centerx,2) + Math.pow(p2.y-centery,2));

    return new Circle(new Point(centerx,centery),radius);
  }

  static class Circle
  {
    final Point center;
    final double radius;
    public Circle(Point center, double radius)
    {
      this.center = center; this.radius = radius;
    }
    @Override 
    public String toString()
    {
      return new StringBuilder().append("Center= ").append(center).append(", r=").append(radius).toString();
    }
  }

  static class Point
  {
    final double x,y;

    public Point(double x, double y)
    {
      this.x = x; this.y = y;
    }
    @Override
    public String toString()
    {
      return "("+x+","+y+")";
    }

  }

  public static void main(String[] args)
  {
    Point p1 = new Point(0.0,1.0);
    Point p2 = new Point(1.0,0.0);
    Point p3 = new Point(2.0,1.0);
    Circle c = circleFromPoints(p1, p2, p3);
    System.out.println(c);
  }

}

从这里查看算法:

void circle_vvv(circle *c)
{
    c->center.w = 1.0;
    vertex *v1 = (vertex *)c->c.p1;
    vertex *v2 = (vertex *)c->c.p2;
    vertex *v3 = (vertex *)c->c.p3;
    float bx = v1->xw; float by = v1->yw;
    float cx = v2->xw; float cy = v2->yw;
    float dx = v3->xw; float dy = v3->yw;
    float temp = cx*cx+cy*cy;
    float bc = (bx*bx + by*by - temp)/2.0;
    float cd = (temp - dx*dx - dy*dy)/2.0;
    float det = (bx-cx)*(cy-dy)-(cx-dx)*(by-cy);
    if (fabs(det) < 1.0e-6) {
        c->center.xw = c->center.yw = 1.0;
        c->center.w = 0.0;
        c->v1 = *v1;
        c->v2 = *v2;
        c->v3 = *v3;
        return;
        }
    det = 1/det;
    c->center.xw = (bc*(cy-dy)-cd*(by-cy))*det;
    c->center.yw = ((bx-cx)*cd-(cx-dx)*bc)*det;
    cx = c->center.xw; cy = c->center.yw;
    c->radius = sqrt((cx-bx)*(cx-bx)+(cy-by)*(cy-by));
}

Answer 2

它可以是一个相当深入的计算.这里有一个简单的步骤:http://paulbourke.net/geometry/circlesphere/.一旦你得到圆的方程,你可以简单地把它放在一个涉及H和K的形式.点(h,k)将是中心.

(在链接上向下滚动一点以获得方程式)

Answer 3

很抱歉我的回答迟了。当两个点形成一条垂直线时，任何使用“斜率”的解决方案都会失败，因为斜率将是无限的。

这是 2019 年的一个简单、稳健的解决方案，始终能正常工作：

public static boolean circleCenter(double[] p1, double[] p2, double[] p3, double[] center) {
    double ax = (p1[0] + p2[0]) / 2;
    double ay = (p1[1] + p2[1]) / 2;
    double ux = (p1[1] - p2[1]);
    double uy = (p2[0] - p1[0]);
    double bx = (p2[0] + p3[0]) / 2;
    double by = (p2[1] + p3[1]) / 2;
    double vx = (p2[1] - p3[1]);
    double vy = (p3[0] - p2[0]);
    double dx = ax - bx;
    double dy = ay - by;
    double vu = vx * uy - vy * ux;
    if (vu == 0)
        return false; // Points are collinear, so no unique solution
    double g = (dx * uy - dy * ux) / vu;
    center[0] = bx + g * vx;
    center[1] = by + g * vy;
    return true;
}

当且仅当 3 个点共线时，上面的代码才会返回“false”。

Answer 4

当我将鼠标悬停在这个问题上时，我正在寻找一种类似的算法。用了您的代码，但是发现在两个斜率之一为0或无穷大的情况下这将不起作用（当xDelta_a或xDelta_b为0时可能为真）。

我更正了算法，这是我的代码。注意：我使用了Objective-C编程语言，并且只是更改了用于点值初始化的代码，因此，如果那是错误的，我确信使用Java的程序员可以纠正它。但是，所有逻辑都是相同的（上帝保佑算法！:)）

就我自己的功能测试而言，它工作得很好。请随时告诉我逻辑是否错误。

pt circleCenter(pt A, pt B, pt C) {

float yDelta_a = B.y - A.y;
float xDelta_a = B.x - A.x;
float yDelta_b = C.y - B.y;
float xDelta_b = C.x - B.x;
pt center = P(0,0);

float aSlope = yDelta_a/xDelta_a;
float bSlope = yDelta_b/xDelta_b;

pt AB_Mid = P((A.x+B.x)/2, (A.y+B.y)/2);
pt BC_Mid = P((B.x+C.x)/2, (B.y+C.y)/2);

if(yDelta_a == 0)         //aSlope == 0
{
    center.x = AB_Mid.x;
    if (xDelta_b == 0)         //bSlope == INFINITY
    {
        center.y = BC_Mid.y;
    }
    else
    {
        center.y = BC_Mid.y + (BC_Mid.x-center.x)/bSlope;
    }
}
else if (yDelta_b == 0)               //bSlope == 0
{
    center.x = BC_Mid.x;
    if (xDelta_a == 0)             //aSlope == INFINITY
    {
        center.y = AB_Mid.y;
    }
    else
    {
        center.y = AB_Mid.y + (AB_Mid.x-center.x)/aSlope;
    }
}
else if (xDelta_a == 0)        //aSlope == INFINITY
{
    center.y = AB_Mid.y;
    center.x = bSlope*(BC_Mid.y-center.y) + BC_Mid.x;
}
else if (xDelta_b == 0)        //bSlope == INFINITY
{
    center.y = BC_Mid.y;
    center.x = aSlope*(AB_Mid.y-center.y) + AB_Mid.x;
}
else
{
    center.x = (aSlope*bSlope*(AB_Mid.y-BC_Mid.y) - aSlope*BC_Mid.x + bSlope*AB_Mid.x)/(bSlope-aSlope);
    center.y = AB_Mid.y - (center.x - AB_Mid.x)/aSlope;
}

return center;
}