Search code examples
javascriptmathgeometrycomputational-geometry

Given a point outside of an arc, how can one find the point on the arc which extends to that point?

Given a point outside of an arc, how can one find the point on the arc which extends to that point?

For example, the radius of the circle (R) is 10cm, it's center point is [0,0].
The origin (o) of the line (8) is at [-3, 10]
How can we find the point (p) (p8) were the tangent at that point continues to the origin of the line?

A brute force solution would not be acceptable.

Extensions of arc

Solution

  • Let point coordinates are px, py, and circle center is at (0,0) (if not - subtract center cx, cy from all coordinates to simplify equations, add them back at the end).

    You can write two equations for unknown x,y. Circle equation and perpendicularity one - tangent is perpendicular to radius-vector, their dot product is zero.

    (x - px) * x + (y - py) * y = 0
x^2 + y^2 = r^2

x^2 - px * x + y^2 - py * y = 0
r^2 - px * x = py * y
y = (r^2 - px * x) / py
y^2  = r^4 / py ^2 - x * 2 * r^2 * px / py^2 + x^2 * px^2 / py^2    

x^2 * (1 + px^2 / py^2) - x * 2 * r^2 * px / py^2 +  (r^4 / py^2 - r^2) = 0
x^2 * (py^2 + px^2) - x * 2 * r^2 * px  +  (r^4 - r^2 * py^2) = 0

    Solve the last quadratic equation for x, then calculate y.

    enter image description here

    Delphi function for reference (note: py=0 case is treated separately)

    function GetTangentPts(px, py, r: Double): TArray<Double>;
var
  px2, py2, pxpy, r2, Dis, x, y: Double;
begin
  px2 := px * px;
  py2 := py * py;
  r2 := r * r;
  pxpy := px2 + py2;
  Dis := pxpy - r2;
  if Dis < 0 then    //point is inside
    Exit(nil)
  else if Dis = 0 then begin    //point is at circumference
    SetLength(Result, 2);
    if py = 0 then begin
      x := px;
      y := 0;
    end else begin
      x := px * r2 / pxpy;
      y := (r2 - px * x) / py;
    end;
    Result[0] := x;
    Result[1] := y;
  end else begin       //general case, two tangents
    SetLength(Result, 4);
    if py = 0 then begin
       y := - r * Sqrt(Dis) / px;
       x := px / Abs(px) * r * Sqrt(1 - Dis/px2);
       Result[0] := x;
       Result[1] := y;
       y := r * Sqrt(Dis) / px;
       Result[2] := x;
       Result[3] := y;
    end else begin
      x := (px * r2 - r * Sqrt(py2 * Dis)) / pxpy;
      y := (r2 - px * x) / py;
      Result[0] := x;
      Result[1] := y;
      x := (px * r2 + r * Sqrt(py2 * Dis)) / pxpy;
      y := (r2 - px * x) / py;
      Result[2] := x;
      Result[3] := y;
    end;
  end;
end;

    some results:

    10.00 10.00 10.00 //two perpendicular tangents
 0.00
10.00
10.00
 0.00

-10.00 10.00 10.00
-10.00
 0.00
 0.00
10.00

 1.00  1.00 10.00
 //inside

 0.00 10.00 10.00 //horizontal tangent
 0.00
10.00

10.00  0.00 10.00 //vertical tangent
10.00
 0.00

-14.14  0.00 10.00  //two tangents from OX-axis
-7.07
 7.07
-7.07
-7.07