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Garrett Brown
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// MIT License
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// Copyright (c) 2019 Erin Catto
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// Permission is hereby granted, free of charge, to any person obtaining a copy
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// of this software and associated documentation files (the "Software"), to deal
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// in the Software without restriction, including without limitation the rights
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// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
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// copies of the Software, and to permit persons to whom the Software is
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// furnished to do so, subject to the following conditions:
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// The above copyright notice and this permission notice shall be included in all
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// copies or substantial portions of the Software.
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// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
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// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
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// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
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// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
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// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
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// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
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// SOFTWARE.
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#include "box2d/b2_collision.h"
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#include "box2d/b2_polygon_shape.h"
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// Find the max separation between poly1 and poly2 using edge normals from poly1.
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static float b2FindMaxSeparation(int32* edgeIndex,
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const b2PolygonShape* poly1, const b2Transform& xf1,
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const b2PolygonShape* poly2, const b2Transform& xf2)
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{
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int32 count1 = poly1->m_count;
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int32 count2 = poly2->m_count;
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const b2Vec2* n1s = poly1->m_normals;
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const b2Vec2* v1s = poly1->m_vertices;
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const b2Vec2* v2s = poly2->m_vertices;
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b2Transform xf = b2MulT(xf2, xf1);
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int32 bestIndex = 0;
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float maxSeparation = -b2_maxFloat;
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for (int32 i = 0; i < count1; ++i)
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{
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// Get poly1 normal in frame2.
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b2Vec2 n = b2Mul(xf.q, n1s[i]);
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b2Vec2 v1 = b2Mul(xf, v1s[i]);
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// Find deepest point for normal i.
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float si = b2_maxFloat;
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for (int32 j = 0; j < count2; ++j)
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{
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float sij = b2Dot(n, v2s[j] - v1);
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if (sij < si)
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{
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si = sij;
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}
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}
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if (si > maxSeparation)
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{
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maxSeparation = si;
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bestIndex = i;
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}
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}
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*edgeIndex = bestIndex;
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return maxSeparation;
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}
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static void b2FindIncidentEdge(b2ClipVertex c[2],
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const b2PolygonShape* poly1, const b2Transform& xf1, int32 edge1,
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const b2PolygonShape* poly2, const b2Transform& xf2)
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{
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const b2Vec2* normals1 = poly1->m_normals;
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int32 count2 = poly2->m_count;
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const b2Vec2* vertices2 = poly2->m_vertices;
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const b2Vec2* normals2 = poly2->m_normals;
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b2Assert(0 <= edge1 && edge1 < poly1->m_count);
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// Get the normal of the reference edge in poly2's frame.
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b2Vec2 normal1 = b2MulT(xf2.q, b2Mul(xf1.q, normals1[edge1]));
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// Find the incident edge on poly2.
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int32 index = 0;
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float minDot = b2_maxFloat;
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for (int32 i = 0; i < count2; ++i)
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{
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float dot = b2Dot(normal1, normals2[i]);
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if (dot < minDot)
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{
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minDot = dot;
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index = i;
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}
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}
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// Build the clip vertices for the incident edge.
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int32 i1 = index;
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int32 i2 = i1 + 1 < count2 ? i1 + 1 : 0;
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c[0].v = b2Mul(xf2, vertices2[i1]);
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c[0].id.cf.indexA = (uint8)edge1;
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c[0].id.cf.indexB = (uint8)i1;
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c[0].id.cf.typeA = b2ContactFeature::e_face;
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c[0].id.cf.typeB = b2ContactFeature::e_vertex;
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c[1].v = b2Mul(xf2, vertices2[i2]);
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c[1].id.cf.indexA = (uint8)edge1;
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c[1].id.cf.indexB = (uint8)i2;
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c[1].id.cf.typeA = b2ContactFeature::e_face;
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c[1].id.cf.typeB = b2ContactFeature::e_vertex;
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}
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// Find edge normal of max separation on A - return if separating axis is found
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// Find edge normal of max separation on B - return if separation axis is found
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// Choose reference edge as min(minA, minB)
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// Find incident edge
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// Clip
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// The normal points from 1 to 2
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void b2CollidePolygons(b2Manifold* manifold,
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const b2PolygonShape* polyA, const b2Transform& xfA,
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const b2PolygonShape* polyB, const b2Transform& xfB)
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{
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manifold->pointCount = 0;
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float totalRadius = polyA->m_radius + polyB->m_radius;
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int32 edgeA = 0;
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float separationA = b2FindMaxSeparation(&edgeA, polyA, xfA, polyB, xfB);
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if (separationA > totalRadius)
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return;
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int32 edgeB = 0;
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float separationB = b2FindMaxSeparation(&edgeB, polyB, xfB, polyA, xfA);
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if (separationB > totalRadius)
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return;
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const b2PolygonShape* poly1; // reference polygon
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const b2PolygonShape* poly2; // incident polygon
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b2Transform xf1, xf2;
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int32 edge1; // reference edge
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uint8 flip;
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const float k_tol = 0.1f * b2_linearSlop;
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if (separationB > separationA + k_tol)
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{
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poly1 = polyB;
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poly2 = polyA;
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xf1 = xfB;
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xf2 = xfA;
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edge1 = edgeB;
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manifold->type = b2Manifold::e_faceB;
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flip = 1;
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}
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else
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{
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poly1 = polyA;
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poly2 = polyB;
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xf1 = xfA;
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xf2 = xfB;
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edge1 = edgeA;
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manifold->type = b2Manifold::e_faceA;
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flip = 0;
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}
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b2ClipVertex incidentEdge[2];
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b2FindIncidentEdge(incidentEdge, poly1, xf1, edge1, poly2, xf2);
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int32 count1 = poly1->m_count;
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const b2Vec2* vertices1 = poly1->m_vertices;
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int32 iv1 = edge1;
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int32 iv2 = edge1 + 1 < count1 ? edge1 + 1 : 0;
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b2Vec2 v11 = vertices1[iv1];
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b2Vec2 v12 = vertices1[iv2];
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b2Vec2 localTangent = v12 - v11;
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localTangent.Normalize();
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b2Vec2 localNormal = b2Cross(localTangent, 1.0f);
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b2Vec2 planePoint = 0.5f * (v11 + v12);
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b2Vec2 tangent = b2Mul(xf1.q, localTangent);
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b2Vec2 normal = b2Cross(tangent, 1.0f);
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v11 = b2Mul(xf1, v11);
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v12 = b2Mul(xf1, v12);
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// Face offset.
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float frontOffset = b2Dot(normal, v11);
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// Side offsets, extended by polytope skin thickness.
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float sideOffset1 = -b2Dot(tangent, v11) + totalRadius;
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float sideOffset2 = b2Dot(tangent, v12) + totalRadius;
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// Clip incident edge against extruded edge1 side edges.
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b2ClipVertex clipPoints1[2];
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b2ClipVertex clipPoints2[2];
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int np;
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// Clip to box side 1
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np = b2ClipSegmentToLine(clipPoints1, incidentEdge, -tangent, sideOffset1, iv1);
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if (np < 2)
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return;
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// Clip to negative box side 1
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np = b2ClipSegmentToLine(clipPoints2, clipPoints1, tangent, sideOffset2, iv2);
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if (np < 2)
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{
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return;
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}
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// Now clipPoints2 contains the clipped points.
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manifold->localNormal = localNormal;
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manifold->localPoint = planePoint;
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int32 pointCount = 0;
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for (int32 i = 0; i < b2_maxManifoldPoints; ++i)
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{
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float separation = b2Dot(normal, clipPoints2[i].v) - frontOffset;
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if (separation <= totalRadius)
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{
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b2ManifoldPoint* cp = manifold->points + pointCount;
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cp->localPoint = b2MulT(xf2, clipPoints2[i].v);
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cp->id = clipPoints2[i].id;
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if (flip)
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{
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// Swap features
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b2ContactFeature cf = cp->id.cf;
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cp->id.cf.indexA = cf.indexB;
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cp->id.cf.indexB = cf.indexA;
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cp->id.cf.typeA = cf.typeB;
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cp->id.cf.typeB = cf.typeA;
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}
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++pointCount;
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}
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}
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manifold->pointCount = pointCount;
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}
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