/usr/include/libwildmagic/Wm5TangentsToCircles.h is in libwildmagic-dev 5.13-1+b2.
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// Copyright (c) 1998-2014
// Distributed under the Boost Software License, Version 1.0.
// http://www.boost.org/LICENSE_1_0.txt
// http://www.geometrictools.com/License/Boost/LICENSE_1_0.txt
//
// File Version: 5.0.1 (2010/10/01)
#ifndef WM5TANGENTSTOCIRCLES_H
#define WM5TANGENTSTOCIRCLES_H
// Given two circles |X-C0|^2 = R0^2 and |X-C1|^2 = R1^2, compute the
// lines that are tangent to both circles. Assumptions: The two circles
// do not intersect and one does not contain the other. This condition
// is guaranteed when |C1-C0| > R0+R1. Without loss of generality, assume
// that R1 >= R0.
//
// A line in parametric form is X(t) = P+t*D where D is a unit length
// vector. To intersect the first circle,
//
// R0^2 = |X(t)-C0|^2 = t^2 + 2*Dot(D,P-C0)*t + |P-C0|^2
//
// To be tangent at that intersect, we need
//
// 0 = Dot(D,X(t)-C0) = t + Dot(D,P-C0).
//
// Solve for t in the second equation and plug into the first to get
//
// R0^2 = |P-C0|^2 - [Dot(D,P-C0)]^2.
//
// Similarly for the second circle,
//
// R1^2 = |P-C1|^2 - [Dot(D,P-C1)]^2.
//
// It should be intuitive from the geometry that the point P can be
// chosen on the line containing the two centers C0 and C1. Let
// P = (1-s)*C0+s*C1 for unknown s. Define W = C1-C0. Then
// P-C0 = s*W and P-C1 = (s-1)*W. Plug these into the previous two
// displayed equations to get
//
// R0^2 = s^2*{|W|^2 - [Dot(D,W)]^2}
// R1^2 = (s-1)^2*{|W|^2 - [Dot(D,W)]^2}
//
// Thus, R0^2/s^2 = R1^2/(s-1)^2 or
//
// (R1^2-R0^2)*s^2 + 2*R0^2*s - R0^2 = 0.
//
// If R1 = R0, then s = 1/2. The point P is the midpoint of the line
// segment connecting the circle centers (expected from the geometric
// symmetry). Moreover, [Dot(D,W)]^2 = |W|^2 - 4*R0^2 = A^2 > 0, so
// Dot(D,W) = A (using -A just leads to a direction vector with
// opposite sign). If D = (d0,d1), Dot(D,W) = A is the equation for
// a line. The constraint |D|^2 = 1 corresponds to a circle. The two
// together represent intersection of line with circle. Either solution
// will do. Let W = (w0,w1). Then w0*d0+w1*d1 = A and d0^2+d1^2 = 1.
// If |w0| >= |w1|, then d0 = (A-w1*d1)/w0 and
//
// (w0^2+w1^2)*d1^2 - 2*A*w1*d1 + A^2-w0^2 = 0
//
// If |w1| >= |w0|, then d1 = (A-w0*d0)/w1 and
//
// (w0^2+w1^2)*d0^2 - 2*A*w0*d0 + A^2-w1^2 = 0.
//
// In either case, the two roots lead to two direction vectors for
// the tangent lines.
//
// If R1 > R0, the quadratic in s has two real-valued solutions. From
// the geometry, one of the value must satisfy 0 < s < 1 and produces
// the two tangents that intersect each other between the two circles.
// The other root cannot be s = 0 (otherwise P would be at a circle
// center, not possible).
//
// For each root s, the same idea as in the case R1 = R0 works. The
// quadratic to solve is [Dot(D,W)]^2 = |W|^2 - R0^2/s^2 = A^2 > 0.
// Also, [Dot(D,W)]^2 = |W|^2 - R^2/(s-1)^2 = A^2. The first equation
// should be used when s^2 >= (s-1)^2, otherwise use the second one.
// The same quadratics may be set up for d0 or d1 (A has a different
// value, though) and solved.
#include "Wm5MathematicsLIB.h"
#include "Wm5Circle2.h"
#include "Wm5Line2.h"
namespace Wm5
{
// The function returns 'true' if all four tangent lines have been computed
// successfully. The return value is 'false' if the circles are
// intersecting or nested, in which case the output array of lines is
// invalid.
template <typename Real> WM5_MATHEMATICS_ITEM
bool GetTangentsToCircles (const Circle2<Real>& circle0,
const Circle2<Real>& circle1, Line2<Real> line[4]);
}
#endif
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