b2_collide_circle.cpp

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// MIT License

// Copyright (c) 2019 Erin Catto

// Permission is hereby granted, free of charge, to any person obtaining a copy
// of this software and associated documentation files (the "Software"), to deal
// in the Software without restriction, including without limitation the rights
// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
// copies of the Software, and to permit persons to whom the Software is
// furnished to do so, subject to the following conditions:

// The above copyright notice and this permission notice shall be included in all
// copies or substantial portions of the Software.

// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
// SOFTWARE.

#include "box2d/b2_collision.h"
#include "box2d/b2_circle_shape.h"
#include "box2d/b2_polygon_shape.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;
        float distSqr = b2Dot(d, d);
        float rA = circleA->m_radius, rB = circleB->m_radius;
        float 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;
        float separation = -b2_maxFloat;
        float 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)
        {
                float 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
        float u1 = b2Dot(cLocal - v1, v2 - v1);
        float 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);
                float s = b2Dot(cLocal - faceCenter, normals[vertIndex1]);
                if (s > 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;
        }
}