b2CircleShape.cpp

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/*
* Copyright (c) 2006-2009 Erin Catto http://www.box2d.org
*
* This software is provided 'as-is', without any express or implied
* warranty.  In no event will the authors be held liable for any damages
* arising from the use of this software.
* Permission is granted to anyone to use this software for any purpose,
* including commercial applications, and to alter it and redistribute it
* freely, subject to the following restrictions:
* 1. The origin of this software must not be misrepresented; you must not
* claim that you wrote the original software. If you use this software
* in a product, an acknowledgment in the product documentation would be
* appreciated but is not required.
* 2. Altered source versions must be plainly marked as such, and must not be
* misrepresented as being the original software.
* 3. This notice may not be removed or altered from any source distribution.
*/

#include <Box2D/Collision/Shapes/b2CircleShape.h>
#include <new>

b2Shape* b2CircleShape::Clone(b2BlockAllocator* allocator) const
{
        void* mem = allocator->Allocate(sizeof(b2CircleShape));
        b2CircleShape* clone = new (mem) b2CircleShape;
        *clone = *this;
        return clone;
}

int32 b2CircleShape::GetChildCount() const
{
        return 1;
}

bool b2CircleShape::TestPoint(const b2Transform& transform, const b2Vec2& p) const
{
        b2Vec2 center = transform.p + b2Mul(transform.q, m_p);
        b2Vec2 d = p - center;
        return b2Dot(d, d) <= m_radius * m_radius;
}

// Collision Detection in Interactive 3D Environments by Gino van den Bergen
// From Section 3.1.2
// x = s + a * r
// norm(x) = radius
bool b2CircleShape::RayCast(b2RayCastOutput* output, const b2RayCastInput& input,
                                                        const b2Transform& transform, int32 childIndex) const
{
        B2_NOT_USED(childIndex);

        b2Vec2 position = transform.p + b2Mul(transform.q, m_p);
        b2Vec2 s = input.p1 - position;
        float32 b = b2Dot(s, s) - m_radius * m_radius;

        // Solve quadratic equation.
        b2Vec2 r = input.p2 - input.p1;
        float32 c =  b2Dot(s, r);
        float32 rr = b2Dot(r, r);
        float32 sigma = c * c - rr * b;

        // Check for negative discriminant and short segment.
        if (sigma < 0.0f || rr < b2_epsilon)
        {
                return false;
        }

        // Find the point of intersection of the line with the circle.
        float32 a = -(c + b2Sqrt(sigma));

        // Is the intersection point on the segment?
        if (0.0f <= a && a <= input.maxFraction * rr)
        {
                a /= rr;
                output->fraction = a;
                output->normal = s + a * r;
                output->normal.Normalize();
                return true;
        }

        return false;
}

void b2CircleShape::ComputeAABB(b2AABB* aabb, const b2Transform& transform, int32 childIndex) const
{
        B2_NOT_USED(childIndex);

        b2Vec2 p = transform.p + b2Mul(transform.q, m_p);
        aabb->lowerBound.Set(p.x - m_radius, p.y - m_radius);
        aabb->upperBound.Set(p.x + m_radius, p.y + m_radius);
}

void b2CircleShape::ComputeMass(b2MassData* massData, float32 density) const
{
        massData->mass = density * b2_pi * m_radius * m_radius;
        massData->center = m_p;

        // inertia about the local origin
        massData->I = massData->mass * (0.5f * m_radius * m_radius + b2Dot(m_p, m_p));
}