Picking - Introduction to 3D Game Programming with DirectX 9.0 - page 264

float b = 2.0f * D3DXVec3Dot(&ray->_direction, &v);
float c = D3DXVec3Dot(&v, &v) - (sphere->_radius * sphere->
_radius);

// find the discriminant
float discriminant = (b * b) - (4.0f * c);

// test for imaginary number
if( discriminant < 0.0f )
return false;

discriminant = sqrtf(discriminant);

float s0 = (-b + discriminant) / 2.0f;
float s1 = (-b - discriminant) / 2.0f;

// if a solution is >= 0, then we intersected the sphere
if( s0 >= 0.0f || s1 >= 0.0f )
return true;

return false;

}
Of course, we have seen BoundingSphere already, but for conve-
nience we show its definition again here:
struct BoundingSphere
{

BoundingSphere();

D3DXVECTOR3 _center;
float _radius;

};

15.5 Sample Application: Picking

Figure 15.5:
Screen shot of this
chapter's sample

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