b2CollideCircle.cpp

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/*
* 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/b2Collision.h>
#include <Box2D/Collision/Shapes/b2CircleShape.h>
#include <Box2D/Collision/Shapes/b2PolygonShape.h>

void b2CollideCircles(
        b2Manifold* manifold,
        const b2CircleShape* circleA, const b2Transform& xfA,
        const b2CircleShape* circleB, const b2Transform& xfB)
{
        manifold->pointCount = 0;

        b2Vec2 pA = b2Mul(xfA, circleA->m_p);
        b2Vec2 pB = b2Mul(xfB, circleB->m_p);

        b2Vec2 d = pB - pA;
        float32 distSqr = b2Dot(d, d);
        float32 rA = circleA->m_radius, rB = circleB->m_radius;
        float32 radius = rA + rB;
        if (distSqr > radius * radius)
        {
                return;
        }

        manifold->type = b2Manifold::e_circles;
        manifold->localPoint = circleA->m_p;
        manifold->localNormal.SetZero();
        manifold->pointCount = 1;

        manifold->points[0].localPoint = circleB->m_p;
        manifold->points[0].id.key = 0;
}

void b2CollidePolygonAndCircle(
        b2Manifold* manifold,
        const b2PolygonShape* polygonA, const b2Transform& xfA,
        const b2CircleShape* circleB, const b2Transform& xfB)
{
        manifold->pointCount = 0;

        // Compute circle position in the frame of the polygon.
        b2Vec2 c = b2Mul(xfB, circleB->m_p);
        b2Vec2 cLocal = b2MulT(xfA, c);

        // Find the min separating edge.
        int32 normalIndex = 0;
        float32 separation = -b2_maxFloat;
        float32 radius = polygonA->m_radius + circleB->m_radius;
        int32 vertexCount = polygonA->m_count;
        const b2Vec2* vertices = polygonA->m_vertices;
        const b2Vec2* normals = polygonA->m_normals;

        for (int32 i = 0; i < vertexCount; ++i)
        {
                float32 s = b2Dot(normals[i], cLocal - vertices[i]);

                if (s > radius)
                {
                        // Early out.
                        return;
                }

                if (s > separation)
                {
                        separation = s;
                        normalIndex = i;
                }
        }

        // Vertices that subtend the incident face.
        int32 vertIndex1 = normalIndex;
        int32 vertIndex2 = vertIndex1 + 1 < vertexCount ? vertIndex1 + 1 : 0;
        b2Vec2 v1 = vertices[vertIndex1];
        b2Vec2 v2 = vertices[vertIndex2];

        // If the center is inside the polygon ...
        if (separation < b2_epsilon)
        {
                manifold->pointCount = 1;
                manifold->type = b2Manifold::e_faceA;
                manifold->localNormal = normals[normalIndex];
                manifold->localPoint = 0.5f * (v1 + v2);
                manifold->points[0].localPoint = circleB->m_p;
                manifold->points[0].id.key = 0;
                return;
        }

        // Compute barycentric coordinates
        float32 u1 = b2Dot(cLocal - v1, v2 - v1);
        float32 u2 = b2Dot(cLocal - v2, v1 - v2);
        if (u1 <= 0.0f)
        {
                if (b2DistanceSquared(cLocal, v1) > radius * radius)
                {
                        return;
                }

                manifold->pointCount = 1;
                manifold->type = b2Manifold::e_faceA;
                manifold->localNormal = cLocal - v1;
                manifold->localNormal.Normalize();
                manifold->localPoint = v1;
                manifold->points[0].localPoint = circleB->m_p;
                manifold->points[0].id.key = 0;
        }
        else if (u2 <= 0.0f)
        {
                if (b2DistanceSquared(cLocal, v2) > radius * radius)
                {
                        return;
                }

                manifold->pointCount = 1;
                manifold->type = b2Manifold::e_faceA;
                manifold->localNormal = cLocal - v2;
                manifold->localNormal.Normalize();
                manifold->localPoint = v2;
                manifold->points[0].localPoint = circleB->m_p;
                manifold->points[0].id.key = 0;
        }
        else
        {
                b2Vec2 faceCenter = 0.5f * (v1 + v2);
                float32 separation = b2Dot(cLocal - faceCenter, normals[vertIndex1]);
                if (separation > radius)
                {
                        return;
                }

                manifold->pointCount = 1;
                manifold->type = b2Manifold::e_faceA;
                manifold->localNormal = normals[vertIndex1];
                manifold->localPoint = faceCenter;
                manifold->points[0].localPoint = circleB->m_p;
                manifold->points[0].id.key = 0;
        }
}