b2_collision.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_distance.h"

void b2WorldManifold::Initialize(const b2Manifold* manifold,
                                                  const b2Transform& xfA, float radiusA,
                                                  const b2Transform& xfB, float radiusB)
{
        if (manifold->pointCount == 0)
        {
                return;
        }

        switch (manifold->type)
        {
        case b2Manifold::e_circles:
                {
                        normal.Set(1.0f, 0.0f);
                        b2Vec2 pointA = b2Mul(xfA, manifold->localPoint);
                        b2Vec2 pointB = b2Mul(xfB, manifold->points[0].localPoint);
                        if (b2DistanceSquared(pointA, pointB) > b2_epsilon * b2_epsilon)
                        {
                                normal = pointB - pointA;
                                normal.Normalize();
                        }

                        b2Vec2 cA = pointA + radiusA * normal;
                        b2Vec2 cB = pointB - radiusB * normal;
                        points[0] = 0.5f * (cA + cB);
                        separations[0] = b2Dot(cB - cA, normal);
                }
                break;

        case b2Manifold::e_faceA:
                {
                        normal = b2Mul(xfA.q, manifold->localNormal);
                        b2Vec2 planePoint = b2Mul(xfA, manifold->localPoint);
                        
                        for (int32 i = 0; i < manifold->pointCount; ++i)
                        {
                                b2Vec2 clipPoint = b2Mul(xfB, manifold->points[i].localPoint);
                                b2Vec2 cA = clipPoint + (radiusA - b2Dot(clipPoint - planePoint, normal)) * normal;
                                b2Vec2 cB = clipPoint - radiusB * normal;
                                points[i] = 0.5f * (cA + cB);
                                separations[i] = b2Dot(cB - cA, normal);
                        }
                }
                break;

        case b2Manifold::e_faceB:
                {
                        normal = b2Mul(xfB.q, manifold->localNormal);
                        b2Vec2 planePoint = b2Mul(xfB, manifold->localPoint);

                        for (int32 i = 0; i < manifold->pointCount; ++i)
                        {
                                b2Vec2 clipPoint = b2Mul(xfA, manifold->points[i].localPoint);
                                b2Vec2 cB = clipPoint + (radiusB - b2Dot(clipPoint - planePoint, normal)) * normal;
                                b2Vec2 cA = clipPoint - radiusA * normal;
                                points[i] = 0.5f * (cA + cB);
                                separations[i] = b2Dot(cA - cB, normal);
                        }

                        // Ensure normal points from A to B.
                        normal = -normal;
                }
                break;
        }
}

void b2GetPointStates(b2PointState state1[b2_maxManifoldPoints], b2PointState state2[b2_maxManifoldPoints],
                                          const b2Manifold* manifold1, const b2Manifold* manifold2)
{
        for (int32 i = 0; i < b2_maxManifoldPoints; ++i)
        {
                state1[i] = b2_nullState;
                state2[i] = b2_nullState;
        }

        // Detect persists and removes.
        for (int32 i = 0; i < manifold1->pointCount; ++i)
        {
                b2ContactID id = manifold1->points[i].id;

                state1[i] = b2_removeState;

                for (int32 j = 0; j < manifold2->pointCount; ++j)
                {
                        if (manifold2->points[j].id.key == id.key)
                        {
                                state1[i] = b2_persistState;
                                break;
                        }
                }
        }

        // Detect persists and adds.
        for (int32 i = 0; i < manifold2->pointCount; ++i)
        {
                b2ContactID id = manifold2->points[i].id;

                state2[i] = b2_addState;

                for (int32 j = 0; j < manifold1->pointCount; ++j)
                {
                        if (manifold1->points[j].id.key == id.key)
                        {
                                state2[i] = b2_persistState;
                                break;
                        }
                }
        }
}

// From Real-time Collision Detection, p179.
bool b2AABB::RayCast(b2RayCastOutput* output, const b2RayCastInput& input) const
{
        float tmin = -b2_maxFloat;
        float tmax = b2_maxFloat;

        b2Vec2 p = input.p1;
        b2Vec2 d = input.p2 - input.p1;
        b2Vec2 absD = b2Abs(d);

        b2Vec2 normal;

        for (int32 i = 0; i < 2; ++i)
        {
                if (absD(i) < b2_epsilon)
                {
                        // Parallel.
                        if (p(i) < lowerBound(i) || upperBound(i) < p(i))
                        {
                                return false;
                        }
                }
                else
                {
                        float inv_d = 1.0f / d(i);
                        float t1 = (lowerBound(i) - p(i)) * inv_d;
                        float t2 = (upperBound(i) - p(i)) * inv_d;

                        // Sign of the normal vector.
                        float s = -1.0f;

                        if (t1 > t2)
                        {
                                b2Swap(t1, t2);
                                s = 1.0f;
                        }

                        // Push the min up
                        if (t1 > tmin)
                        {
                                normal.SetZero();
                                normal(i) = s;
                                tmin = t1;
                        }

                        // Pull the max down
                        tmax = b2Min(tmax, t2);

                        if (tmin > tmax)
                        {
                                return false;
                        }
                }
        }

        // Does the ray start inside the box?
        // Does the ray intersect beyond the max fraction?
        if (tmin < 0.0f || input.maxFraction < tmin)
        {
                return false;
        }

        // Intersection.
        output->fraction = tmin;
        output->normal = normal;
        return true;
}

// Sutherland-Hodgman clipping.
int32 b2ClipSegmentToLine(b2ClipVertex vOut[2], const b2ClipVertex vIn[2],
                                                const b2Vec2& normal, float offset, int32 vertexIndexA)
{
        // Start with no output points
        int32 count = 0;

        // Calculate the distance of end points to the line
        float distance0 = b2Dot(normal, vIn[0].v) - offset;
        float distance1 = b2Dot(normal, vIn[1].v) - offset;

        // If the points are behind the plane
        if (distance0 <= 0.0f) vOut[count++] = vIn[0];
        if (distance1 <= 0.0f) vOut[count++] = vIn[1];

        // If the points are on different sides of the plane
        if (distance0 * distance1 < 0.0f)
        {
                // Find intersection point of edge and plane
                float interp = distance0 / (distance0 - distance1);
                vOut[count].v = vIn[0].v + interp * (vIn[1].v - vIn[0].v);

                // VertexA is hitting edgeB.
                vOut[count].id.cf.indexA = static_cast<uint8>(vertexIndexA);
                vOut[count].id.cf.indexB = vIn[0].id.cf.indexB;
                vOut[count].id.cf.typeA = b2ContactFeature::e_vertex;
                vOut[count].id.cf.typeB = b2ContactFeature::e_face;
                ++count;

                b2Assert(count == 2);
        }

        return count;
}

bool b2TestOverlap(     const b2Shape* shapeA, int32 indexA,
                                        const b2Shape* shapeB, int32 indexB,
                                        const b2Transform& xfA, const b2Transform& xfB)
{
        b2DistanceInput input;
        input.proxyA.Set(shapeA, indexA);
        input.proxyB.Set(shapeB, indexB);
        input.transformA = xfA;
        input.transformB = xfB;
        input.useRadii = true;

        b2SimplexCache cache;
        cache.count = 0;

        b2DistanceOutput output;

        b2Distance(&output, &cache, &input);

        return output.distance < 10.0f * b2_epsilon;
}