2 * Copyright (c) 2006-2007 Erin Catto http://www.gphysics.com
4 * This software is provided 'as-is', without any express or implied
5 * warranty. In no event will the authors be held liable for any damages
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8 * including commercial applications, and to alter it and redistribute it
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12 * in a product, an acknowledgment in the product documentation would be
13 * appreciated but is not required.
14 * 2. Altered source versions must be plainly marked as such, and must not be
15 * misrepresented as being the original software.
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19 #include "b2Collision.h"
20 #include "Shapes/b2PolygonShape.h"
28 static int32 ClipSegmentToLine(ClipVertex vOut[2], ClipVertex vIn[2],
29 const b2Vec2& normal, float32 offset)
31 // Start with no output points
34 // Calculate the distance of end points to the line
35 float32 distance0 = b2Dot(normal, vIn[0].v) - offset;
36 float32 distance1 = b2Dot(normal, vIn[1].v) - offset;
38 // If the points are behind the plane
39 if (distance0 <= 0.0f) vOut[numOut++] = vIn[0];
40 if (distance1 <= 0.0f) vOut[numOut++] = vIn[1];
42 // If the points are on different sides of the plane
43 if (distance0 * distance1 < 0.0f)
45 // Find intersection point of edge and plane
46 float32 interp = distance0 / (distance0 - distance1);
47 vOut[numOut].v = vIn[0].v + interp * (vIn[1].v - vIn[0].v);
50 vOut[numOut].id = vIn[0].id;
54 vOut[numOut].id = vIn[1].id;
62 // Find the separation between poly1 and poly2 for a give edge normal on poly1.
63 static float32 EdgeSeparation(const b2PolygonShape* poly1, const b2XForm& xf1, int32 edge1,
64 const b2PolygonShape* poly2, const b2XForm& xf2)
66 int32 count1 = poly1->GetVertexCount();
67 const b2Vec2* vertices1 = poly1->GetVertices();
68 const b2Vec2* normals1 = poly1->GetNormals();
70 int32 count2 = poly2->GetVertexCount();
71 const b2Vec2* vertices2 = poly2->GetVertices();
73 b2Assert(0 <= edge1 && edge1 < count1);
75 // Convert normal from poly1's frame into poly2's frame.
76 b2Vec2 normal1World = b2Mul(xf1.R, normals1[edge1]);
77 b2Vec2 normal1 = b2MulT(xf2.R, normal1World);
79 // Find support vertex on poly2 for -normal.
81 float32 minDot = B2_FLT_MAX;
83 for (int32 i = 0; i < count2; ++i)
85 float32 dot = b2Dot(vertices2[i], normal1);
93 b2Vec2 v1 = b2Mul(xf1, vertices1[edge1]);
94 b2Vec2 v2 = b2Mul(xf2, vertices2[index]);
95 float32 separation = b2Dot(v2 - v1, normal1World);
99 // Find the max separation between poly1 and poly2 using edge normals from poly1.
100 static float32 FindMaxSeparation(int32* edgeIndex,
101 const b2PolygonShape* poly1, const b2XForm& xf1,
102 const b2PolygonShape* poly2, const b2XForm& xf2)
104 int32 count1 = poly1->GetVertexCount();
105 const b2Vec2* normals1 = poly1->GetNormals();
107 // Vector pointing from the centroid of poly1 to the centroid of poly2.
108 b2Vec2 d = b2Mul(xf2, poly2->GetCentroid()) - b2Mul(xf1, poly1->GetCentroid());
109 b2Vec2 dLocal1 = b2MulT(xf1.R, d);
111 // Find edge normal on poly1 that has the largest projection onto d.
113 float32 maxDot = -B2_FLT_MAX;
114 for (int32 i = 0; i < count1; ++i)
116 float32 dot = b2Dot(normals1[i], dLocal1);
124 // Get the separation for the edge normal.
125 float32 s = EdgeSeparation(poly1, xf1, edge, poly2, xf2);
131 // Check the separation for the previous edge normal.
132 int32 prevEdge = edge - 1 >= 0 ? edge - 1 : count1 - 1;
133 float32 sPrev = EdgeSeparation(poly1, xf1, prevEdge, poly2, xf2);
139 // Check the separation for the next edge normal.
140 int32 nextEdge = edge + 1 < count1 ? edge + 1 : 0;
141 float32 sNext = EdgeSeparation(poly1, xf1, nextEdge, poly2, xf2);
147 // Find the best edge and the search direction.
149 float32 bestSeparation;
151 if (sPrev > s && sPrev > sNext)
155 bestSeparation = sPrev;
161 bestSeparation = sNext;
169 // Perform a local search for the best edge normal.
173 edge = bestEdge - 1 >= 0 ? bestEdge - 1 : count1 - 1;
175 edge = bestEdge + 1 < count1 ? bestEdge + 1 : 0;
177 s = EdgeSeparation(poly1, xf1, edge, poly2, xf2);
183 if (s > bestSeparation)
194 *edgeIndex = bestEdge;
195 return bestSeparation;
198 static void FindIncidentEdge(ClipVertex c[2],
199 const b2PolygonShape* poly1, const b2XForm& xf1, int32 edge1,
200 const b2PolygonShape* poly2, const b2XForm& xf2)
202 int32 count1 = poly1->GetVertexCount();
203 const b2Vec2* normals1 = poly1->GetNormals();
205 int32 count2 = poly2->GetVertexCount();
206 const b2Vec2* vertices2 = poly2->GetVertices();
207 const b2Vec2* normals2 = poly2->GetNormals();
209 b2Assert(0 <= edge1 && edge1 < count1);
211 // Get the normal of the reference edge in poly2's frame.
212 b2Vec2 normal1 = b2MulT(xf2.R, b2Mul(xf1.R, normals1[edge1]));
214 // Find the incident edge on poly2.
216 float32 minDot = B2_FLT_MAX;
217 for (int32 i = 0; i < count2; ++i)
219 float32 dot = b2Dot(normal1, normals2[i]);
227 // Build the clip vertices for the incident edge.
229 int32 i2 = i1 + 1 < count2 ? i1 + 1 : 0;
231 c[0].v = b2Mul(xf2, vertices2[i1]);
232 c[0].id.features.referenceEdge = (uint8)edge1;
233 c[0].id.features.incidentEdge = (uint8)i1;
234 c[0].id.features.incidentVertex = 0;
236 c[1].v = b2Mul(xf2, vertices2[i2]);
237 c[1].id.features.referenceEdge = (uint8)edge1;
238 c[1].id.features.incidentEdge = (uint8)i2;
239 c[1].id.features.incidentVertex = 1;
242 // Find edge normal of max separation on A - return if separating axis is found
243 // Find edge normal of max separation on B - return if separation axis is found
244 // Choose reference edge as min(minA, minB)
245 // Find incident edge
248 // The normal points from 1 to 2
249 void b2CollidePolygons(b2Manifold* manifold,
250 const b2PolygonShape* polyA, const b2XForm& xfA,
251 const b2PolygonShape* polyB, const b2XForm& xfB)
253 manifold->pointCount = 0;
256 float32 separationA = FindMaxSeparation(&edgeA, polyA, xfA, polyB, xfB);
257 if (separationA > 0.0f)
261 float32 separationB = FindMaxSeparation(&edgeB, polyB, xfB, polyA, xfA);
262 if (separationB > 0.0f)
265 const b2PolygonShape* poly1; // reference poly
266 const b2PolygonShape* poly2; // incident poly
268 int32 edge1; // reference edge
270 const float32 k_relativeTol = 0.98f;
271 const float32 k_absoluteTol = 0.001f;
273 // TODO_ERIN use "radius" of poly for absolute tolerance.
274 if (separationB > k_relativeTol * separationA + k_absoluteTol)
293 ClipVertex incidentEdge[2];
294 FindIncidentEdge(incidentEdge, poly1, xf1, edge1, poly2, xf2);
296 int32 count1 = poly1->GetVertexCount();
297 const b2Vec2* vertices1 = poly1->GetVertices();
299 b2Vec2 v11 = vertices1[edge1];
300 b2Vec2 v12 = edge1 + 1 < count1 ? vertices1[edge1+1] : vertices1[0];
302 b2Vec2 dv = v12 - v11;
303 b2Vec2 sideNormal = b2Mul(xf1.R, v12 - v11);
304 sideNormal.Normalize();
305 b2Vec2 frontNormal = b2Cross(sideNormal, 1.0f);
307 v11 = b2Mul(xf1, v11);
308 v12 = b2Mul(xf1, v12);
310 float32 frontOffset = b2Dot(frontNormal, v11);
311 float32 sideOffset1 = -b2Dot(sideNormal, v11);
312 float32 sideOffset2 = b2Dot(sideNormal, v12);
314 // Clip incident edge against extruded edge1 side edges.
315 ClipVertex clipPoints1[2];
316 ClipVertex clipPoints2[2];
319 // Clip to box side 1
320 np = ClipSegmentToLine(clipPoints1, incidentEdge, -sideNormal, sideOffset1);
325 // Clip to negative box side 1
326 np = ClipSegmentToLine(clipPoints2, clipPoints1, sideNormal, sideOffset2);
331 // Now clipPoints2 contains the clipped points.
332 manifold->normal = flip ? -frontNormal : frontNormal;
334 int32 pointCount = 0;
335 for (int32 i = 0; i < b2_maxManifoldPoints; ++i)
337 float32 separation = b2Dot(frontNormal, clipPoints2[i].v) - frontOffset;
339 if (separation <= 0.0f)
341 b2ManifoldPoint* cp = manifold->points + pointCount;
342 cp->separation = separation;
343 cp->localPoint1 = b2MulT(xfA, clipPoints2[i].v);
344 cp->localPoint2 = b2MulT(xfB, clipPoints2[i].v);
345 cp->id = clipPoints2[i].id;
346 cp->id.features.flip = flip;
351 manifold->pointCount = pointCount;