**A
Sphere-Sphere Sweep Test**

Figure 2 shows two spheres that collided between frames. If these spheres experienced acceleration during the frame, their trajectories will be second or higher order curves; however, usually their paths can be accurately approximated as linear segments according to the equations

Since
both spheres traveled for the same amount of time, *u* is the same
for both trajectories. The square of the distance between the lines
is

and
to calculate when they first make contact, we must solve for *u*
such that

This leads to the quadratic equation

The
vector **v _{ba}** can be thought of as the displacement of
B observed by A. This equation is quadratic in

*Listing
2. The sphere-sphere sweep test.*

#include "vector.h"

template< class T >

inline
void SWAP( T& a, T& b )

//swap the values of a and b

{

const T temp = a;

a = b;

b = temp;

}

// Return
true if r1 and r2 are real

inline bool
QuadraticFormula

(

const SCALAR
a,

const SCALAR
b,

const SCALAR
c,

SCALAR&
r1, //first

SCALAR&
r2 //and second roots

)

{

const SCALAR q = b*b - 4*a*c;

if( q >= 0 ){

const SCALAR sq = sqrt(q);

const SCALAR d = 1 / (2*a);

r1 = ( -b + sq ) * d;

r2 = ( -b - sq ) * d;

return true;//real roots}

else

{

return false;//complex roots

}

}

const
bool SphereSphereSweep

(

const SCALAR ra, //radius
of sphere A

const VECTOR& A0, //previous
position of sphere A

const VECTOR& A1, //current
position of sphere A

const SCALAR rb, //radius
of sphere B

const VECTOR& B0, //previous
position of sphere B

const VECTOR& B1, //current
position of sphere B

SCALAR& u0, //normalized
time of first collision

SCALAR& u1 //normalized
time of second collision

)

{

const VECTOR va = A1 - A0;

//vector from A0 to A1

const VECTOR vb = B1 - B0;

//vector from B0 to B1

const VECTOR AB = B0 - A0;

//vector from A0 to B0

const VECTOR vab = vb - va;

//relative velocity (in normalized time)

const SCALAR rab = ra + rb;

const SCALAR a = vab.dot(vab);

//u*u coefficient

const SCALAR b = 2*vab.dot(AB);

//u coefficient

const SCALAR c = AB.dot(AB) - rab*rab;

//constant term

//check if they're currently overlapping

if( AB.dot(AB) <= rab*rab ){

u0 = 0;

u1 = 0;

return true;}

//check if they hit each other

// during the frame

if( QuadraticFormula( a, b, c, u0, u1 ) )

{

if( u0 > u1 )

SWAP( u0, u1 );

return true;}

return false;

}