axmol/external/Box2D/Collision/b2Distance.cpp

/*
* Copyright (c) 2007-2009 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/b2Distance.h>
#include <Box2D/Collision/Shapes/b2CircleShape.h>
#include <Box2D/Collision/Shapes/b2EdgeShape.h>
#include <Box2D/Collision/Shapes/b2ChainShape.h>
#include <Box2D/Collision/Shapes/b2PolygonShape.h>

// GJK using Voronoi regions (Christer Ericson) and Barycentric coordinates.
int32 b2_gjkCalls, b2_gjkIters, b2_gjkMaxIters;

void b2DistanceProxy::Set(const b2Shape* shape, int32 index)
{
    switch (shape->GetType())
    {
    case b2Shape::e_circle:
        {
            const b2CircleShape* circle = (b2CircleShape*)shape;
            m_vertices = &circle->m_p;
            m_count = 1;
            m_radius = circle->m_radius;
        }
        break;

    case b2Shape::e_polygon:
        {
            const b2PolygonShape* polygon = (b2PolygonShape*)shape;
            m_vertices = polygon->m_vertices;
            m_count = polygon->m_vertexCount;
            m_radius = polygon->m_radius;
        }
        break;

    case b2Shape::e_chain:
        {
            const b2ChainShape* chain = (b2ChainShape*)shape;
            b2Assert(0 <= index && index < chain->m_count);

            m_buffer[0] = chain->m_vertices[index];
            if (index + 1 < chain->m_count)
            {
                m_buffer[1] = chain->m_vertices[index + 1];
            }
            else
            {
                m_buffer[1] = chain->m_vertices[0];
            }

            m_vertices = m_buffer;
            m_count = 2;
            m_radius = chain->m_radius;
        }
        break;

    case b2Shape::e_edge:
        {
            const b2EdgeShape* edge = (b2EdgeShape*)shape;
            m_vertices = &edge->m_vertex1;
            m_count = 2;
            m_radius = edge->m_radius;
        }
        break;

    default:
        b2Assert(false);
    }
}


struct b2SimplexVertex
{
    b2Vec2 wA;        // support point in proxyA
    b2Vec2 wB;        // support point in proxyB
    b2Vec2 w;        // wB - wA
    float32 a;        // barycentric coordinate for closest point
    int32 indexA;    // wA index
    int32 indexB;    // wB index
};

struct b2Simplex
{
    void ReadCache(    const b2SimplexCache* cache,
                    const b2DistanceProxy* proxyA, const b2Transform& transformA,
                    const b2DistanceProxy* proxyB, const b2Transform& transformB)
    {
        b2Assert(cache->count <= 3);
        
        // Copy data from cache.
        m_count = cache->count;
        b2SimplexVertex* vertices = &m_v1;
        for (int32 i = 0; i < m_count; ++i)
        {
            b2SimplexVertex* v = vertices + i;
            v->indexA = cache->indexA[i];
            v->indexB = cache->indexB[i];
            b2Vec2 wALocal = proxyA->GetVertex(v->indexA);
            b2Vec2 wBLocal = proxyB->GetVertex(v->indexB);
            v->wA = b2Mul(transformA, wALocal);
            v->wB = b2Mul(transformB, wBLocal);
            v->w = v->wB - v->wA;
            v->a = 0.0f;
        }

        // Compute the new simplex metric, if it is substantially different than
        // old metric then flush the simplex.
        if (m_count > 1)
        {
            float32 metric1 = cache->metric;
            float32 metric2 = GetMetric();
            if (metric2 < 0.5f * metric1 || 2.0f * metric1 < metric2 || metric2 < b2_epsilon)
            {
                // Reset the simplex.
                m_count = 0;
            }
        }

        // If the cache is empty or invalid ...
        if (m_count == 0)
        {
            b2SimplexVertex* v = vertices + 0;
            v->indexA = 0;
            v->indexB = 0;
            b2Vec2 wALocal = proxyA->GetVertex(0);
            b2Vec2 wBLocal = proxyB->GetVertex(0);
            v->wA = b2Mul(transformA, wALocal);
            v->wB = b2Mul(transformB, wBLocal);
            v->w = v->wB - v->wA;
            m_count = 1;
        }
    }

    void WriteCache(b2SimplexCache* cache) const
    {
        cache->metric = GetMetric();
        cache->count = uint16(m_count);
        const b2SimplexVertex* vertices = &m_v1;
        for (int32 i = 0; i < m_count; ++i)
        {
            cache->indexA[i] = uint8(vertices[i].indexA);
            cache->indexB[i] = uint8(vertices[i].indexB);
        }
    }

    b2Vec2 GetSearchDirection() const
    {
        switch (m_count)
        {
        case 1:
            return -m_v1.w;

        case 2:
            {
                b2Vec2 e12 = m_v2.w - m_v1.w;
                float32 sgn = b2Cross(e12, -m_v1.w);
                if (sgn > 0.0f)
                {
                    // Origin is left of e12.
                    return b2Cross(1.0f, e12);
                }
                else
                {
                    // Origin is right of e12.
                    return b2Cross(e12, 1.0f);
                }
            }

        default:
            b2Assert(false);
            return b2Vec2_zero;
        }
    }

    b2Vec2 GetClosestPoint() const
    {
        switch (m_count)
        {
        case 0:
            b2Assert(false);
            return b2Vec2_zero;

        case 1:
            return m_v1.w;

        case 2:
            return m_v1.a * m_v1.w + m_v2.a * m_v2.w;

        case 3:
            return b2Vec2_zero;

        default:
            b2Assert(false);
            return b2Vec2_zero;
        }
    }

    void GetWitnessPoints(b2Vec2* pA, b2Vec2* pB) const
    {
        switch (m_count)
        {
        case 0:
            b2Assert(false);
            break;

        case 1:
            *pA = m_v1.wA;
            *pB = m_v1.wB;
            break;

        case 2:
            *pA = m_v1.a * m_v1.wA + m_v2.a * m_v2.wA;
            *pB = m_v1.a * m_v1.wB + m_v2.a * m_v2.wB;
            break;

        case 3:
            *pA = m_v1.a * m_v1.wA + m_v2.a * m_v2.wA + m_v3.a * m_v3.wA;
            *pB = *pA;
            break;

        default:
            b2Assert(false);
            break;
        }
    }

    float32 GetMetric() const
    {
        switch (m_count)
        {
        case 0:
            b2Assert(false);
            return 0.0;

        case 1:
            return 0.0f;

        case 2:
            return b2Distance(m_v1.w, m_v2.w);

        case 3:
            return b2Cross(m_v2.w - m_v1.w, m_v3.w - m_v1.w);

        default:
            b2Assert(false);
            return 0.0f;
        }
    }

    void Solve2();
    void Solve3();

    b2SimplexVertex m_v1, m_v2, m_v3;
    int32 m_count;
};


// Solve a line segment using barycentric coordinates.
//
// p = a1 * w1 + a2 * w2
// a1 + a2 = 1
//
// The vector from the origin to the closest point on the line is
// perpendicular to the line.
// e12 = w2 - w1
// dot(p, e) = 0
// a1 * dot(w1, e) + a2 * dot(w2, e) = 0
//
// 2-by-2 linear system
// [1      1     ][a1] = [1]
// [w1.e12 w2.e12][a2] = [0]
//
// Define
// d12_1 =  dot(w2, e12)
// d12_2 = -dot(w1, e12)
// d12 = d12_1 + d12_2
//
// Solution
// a1 = d12_1 / d12
// a2 = d12_2 / d12
void b2Simplex::Solve2()
{
    b2Vec2 w1 = m_v1.w;
    b2Vec2 w2 = m_v2.w;
    b2Vec2 e12 = w2 - w1;

    // w1 region
    float32 d12_2 = -b2Dot(w1, e12);
    if (d12_2 <= 0.0f)
    {
        // a2 <= 0, so we clamp it to 0
        m_v1.a = 1.0f;
        m_count = 1;
        return;
    }

    // w2 region
    float32 d12_1 = b2Dot(w2, e12);
    if (d12_1 <= 0.0f)
    {
        // a1 <= 0, so we clamp it to 0
        m_v2.a = 1.0f;
        m_count = 1;
        m_v1 = m_v2;
        return;
    }

    // Must be in e12 region.
    float32 inv_d12 = 1.0f / (d12_1 + d12_2);
    m_v1.a = d12_1 * inv_d12;
    m_v2.a = d12_2 * inv_d12;
    m_count = 2;
}

// Possible regions:
// - points[2]
// - edge points[0]-points[2]
// - edge points[1]-points[2]
// - inside the triangle
void b2Simplex::Solve3()
{
    b2Vec2 w1 = m_v1.w;
    b2Vec2 w2 = m_v2.w;
    b2Vec2 w3 = m_v3.w;

    // Edge12
    // [1      1     ][a1] = [1]
    // [w1.e12 w2.e12][a2] = [0]
    // a3 = 0
    b2Vec2 e12 = w2 - w1;
    float32 w1e12 = b2Dot(w1, e12);
    float32 w2e12 = b2Dot(w2, e12);
    float32 d12_1 = w2e12;
    float32 d12_2 = -w1e12;

    // Edge13
    // [1      1     ][a1] = [1]
    // [w1.e13 w3.e13][a3] = [0]
    // a2 = 0
    b2Vec2 e13 = w3 - w1;
    float32 w1e13 = b2Dot(w1, e13);
    float32 w3e13 = b2Dot(w3, e13);
    float32 d13_1 = w3e13;
    float32 d13_2 = -w1e13;

    // Edge23
    // [1      1     ][a2] = [1]
    // [w2.e23 w3.e23][a3] = [0]
    // a1 = 0
    b2Vec2 e23 = w3 - w2;
    float32 w2e23 = b2Dot(w2, e23);
    float32 w3e23 = b2Dot(w3, e23);
    float32 d23_1 = w3e23;
    float32 d23_2 = -w2e23;
    
    // Triangle123
    float32 n123 = b2Cross(e12, e13);

    float32 d123_1 = n123 * b2Cross(w2, w3);
    float32 d123_2 = n123 * b2Cross(w3, w1);
    float32 d123_3 = n123 * b2Cross(w1, w2);

    // w1 region
    if (d12_2 <= 0.0f && d13_2 <= 0.0f)
    {
        m_v1.a = 1.0f;
        m_count = 1;
        return;
    }

    // e12
    if (d12_1 > 0.0f && d12_2 > 0.0f && d123_3 <= 0.0f)
    {
        float32 inv_d12 = 1.0f / (d12_1 + d12_2);
        m_v1.a = d12_1 * inv_d12;
        m_v2.a = d12_2 * inv_d12;
        m_count = 2;
        return;
    }

    // e13
    if (d13_1 > 0.0f && d13_2 > 0.0f && d123_2 <= 0.0f)
    {
        float32 inv_d13 = 1.0f / (d13_1 + d13_2);
        m_v1.a = d13_1 * inv_d13;
        m_v3.a = d13_2 * inv_d13;
        m_count = 2;
        m_v2 = m_v3;
        return;
    }

    // w2 region
    if (d12_1 <= 0.0f && d23_2 <= 0.0f)
    {
        m_v2.a = 1.0f;
        m_count = 1;
        m_v1 = m_v2;
        return;
    }

    // w3 region
    if (d13_1 <= 0.0f && d23_1 <= 0.0f)
    {
        m_v3.a = 1.0f;
        m_count = 1;
        m_v1 = m_v3;
        return;
    }

    // e23
    if (d23_1 > 0.0f && d23_2 > 0.0f && d123_1 <= 0.0f)
    {
        float32 inv_d23 = 1.0f / (d23_1 + d23_2);
        m_v2.a = d23_1 * inv_d23;
        m_v3.a = d23_2 * inv_d23;
        m_count = 2;
        m_v1 = m_v3;
        return;
    }

    // Must be in triangle123
    float32 inv_d123 = 1.0f / (d123_1 + d123_2 + d123_3);
    m_v1.a = d123_1 * inv_d123;
    m_v2.a = d123_2 * inv_d123;
    m_v3.a = d123_3 * inv_d123;
    m_count = 3;
}

void b2Distance(b2DistanceOutput* output,
                b2SimplexCache* cache,
                const b2DistanceInput* input)
{
    ++b2_gjkCalls;

    const b2DistanceProxy* proxyA = &input->proxyA;
    const b2DistanceProxy* proxyB = &input->proxyB;

    b2Transform transformA = input->transformA;
    b2Transform transformB = input->transformB;

    // Initialize the simplex.
    b2Simplex simplex;
    simplex.ReadCache(cache, proxyA, transformA, proxyB, transformB);

    // Get simplex vertices as an array.
    b2SimplexVertex* vertices = &simplex.m_v1;
    const int32 k_maxIters = 20;

    // These store the vertices of the last simplex so that we
    // can check for duplicates and prevent cycling.
    int32 saveA[3], saveB[3];
    int32 saveCount = 0;

    b2Vec2 closestPoint = simplex.GetClosestPoint();
    float32 distanceSqr1 = closestPoint.LengthSquared();
    float32 distanceSqr2 = distanceSqr1;

    // Main iteration loop.
    int32 iter = 0;
    while (iter < k_maxIters)
    {
        // Copy simplex so we can identify duplicates.
        saveCount = simplex.m_count;
        for (int32 i = 0; i < saveCount; ++i)
        {
            saveA[i] = vertices[i].indexA;
            saveB[i] = vertices[i].indexB;
        }

        switch (simplex.m_count)
        {
        case 1:
            break;

        case 2:
            simplex.Solve2();
            break;

        case 3:
            simplex.Solve3();
            break;

        default:
            b2Assert(false);
        }

        // If we have 3 points, then the origin is in the corresponding triangle.
        if (simplex.m_count == 3)
        {
            break;
        }

        // Compute closest point.
        b2Vec2 p = simplex.GetClosestPoint();
        distanceSqr2 = p.LengthSquared();

        // Ensure progress
        if (distanceSqr2 >= distanceSqr1)
        {
            //break;
        }
        distanceSqr1 = distanceSqr2;

        // Get search direction.
        b2Vec2 d = simplex.GetSearchDirection();

        // Ensure the search direction is numerically fit.
        if (d.LengthSquared() < b2_epsilon * b2_epsilon)
        {
            // The origin is probably contained by a line segment
            // or triangle. Thus the shapes are overlapped.

            // We can't return zero here even though there may be overlap.
            // In case the simplex is a point, segment, or triangle it is difficult
            // to determine if the origin is contained in the CSO or very close to it.
            break;
        }

        // Compute a tentative new simplex vertex using support points.
        b2SimplexVertex* vertex = vertices + simplex.m_count;
        vertex->indexA = proxyA->GetSupport(b2MulT(transformA.q, -d));
        vertex->wA = b2Mul(transformA, proxyA->GetVertex(vertex->indexA));
        b2Vec2 wBLocal;
        vertex->indexB = proxyB->GetSupport(b2MulT(transformB.q, d));
        vertex->wB = b2Mul(transformB, proxyB->GetVertex(vertex->indexB));
        vertex->w = vertex->wB - vertex->wA;

        // Iteration count is equated to the number of support point calls.
        ++iter;
        ++b2_gjkIters;

        // Check for duplicate support points. This is the main termination criteria.
        bool duplicate = false;
        for (int32 i = 0; i < saveCount; ++i)
        {
            if (vertex->indexA == saveA[i] && vertex->indexB == saveB[i])
            {
                duplicate = true;
                break;
            }
        }

        // If we found a duplicate support point we must exit to avoid cycling.
        if (duplicate)
        {
            break;
        }

        // New vertex is ok and needed.
        ++simplex.m_count;
    }

    b2_gjkMaxIters = b2Max(b2_gjkMaxIters, iter);

    // Prepare output.
    simplex.GetWitnessPoints(&output->pointA, &output->pointB);
    output->distance = b2Distance(output->pointA, output->pointB);
    output->iterations = iter;

    // Cache the simplex.
    simplex.WriteCache(cache);

    // Apply radii if requested.
    if (input->useRadii)
    {
        float32 rA = proxyA->m_radius;
        float32 rB = proxyB->m_radius;

        if (output->distance > rA + rB && output->distance > b2_epsilon)
        {
            // Shapes are still no overlapped.
            // Move the witness points to the outer surface.
            output->distance -= rA + rB;
            b2Vec2 normal = output->pointB - output->pointA;
            normal.Normalize();
            output->pointA += rA * normal;
            output->pointB -= rB * normal;
        }
        else
        {
            // Shapes are overlapped when radii are considered.
            // Move the witness points to the middle.
            b2Vec2 p = 0.5f * (output->pointA + output->pointB);
            output->pointA = p;
            output->pointB = p;
            output->distance = 0.0f;
        }
    }
}