// Find the closest points between two shapes using the GJK algorithm.
static struct ClosestPoints
GJK(const struct SupportContext *ctx, cpCollisionID *id)
{
#if DRAW_GJK || DRAW_EPA
int count1 = 1;
int count2 = 1;
switch(ctx->shape1->klass->type) {
case CP_SEGMENT_SHAPE:
count1 = 2;
break;
case CP_POLY_SHAPE:
count1 = ((cpPolyShape *)ctx->shape1)->count;
break;
default:
break;
}
switch(ctx->shape2->klass->type) {
case CP_SEGMENT_SHAPE:
count1 = 2;
break;
case CP_POLY_SHAPE:
count2 = ((cpPolyShape *)ctx->shape2)->count;
break;
default:
break;
}
// draw the minkowski difference origin
cpVect origin = cpvzero;
ChipmunkDebugDrawDot(5.0, origin, RGBAColor(1,0,0,1));
int mdiffCount = count1*count2;
cpVect *mdiffVerts = alloca(mdiffCount*sizeof(cpVect));
for(int i=0; i<count1; i++) {
for(int j=0; j<count2; j++) {
cpVect v = cpvsub(ShapePoint(ctx->shape2, j).p, ShapePoint(ctx->shape1, i).p);
mdiffVerts[i*count2 + j] = v;
ChipmunkDebugDrawDot(2.0, v, RGBAColor(1, 0, 0, 1));
}
}
cpVect *hullVerts = alloca(mdiffCount*sizeof(cpVect));
int hullCount = cpConvexHull(mdiffCount, mdiffVerts, hullVerts, NULL, 0.0);
ChipmunkDebugDrawPolygon(hullCount, hullVerts, 0.0, RGBAColor(1, 0, 0, 1), RGBAColor(1, 0, 0, 0.25));
#endif
struct MinkowskiPoint v0, v1;
if(*id) {
// Use the minkowski points from the last frame as a starting point using the cached indexes.
v0 = MinkowskiPointNew(ShapePoint(ctx->shape1, (*id>>24)&0xFF), ShapePoint(ctx->shape2, (*id>>16)&0xFF));
v1 = MinkowskiPointNew(ShapePoint(ctx->shape1, (*id>> 8)&0xFF), ShapePoint(ctx->shape2, (*id )&0xFF));
} else {
// Recursive implementation of the EPA loop.
// Each recursion adds a point to the convex hull until it's known that we have the closest point on the surface.
static struct ClosestPoints
EPARecurse(const struct SupportContext *ctx, const int count, const struct MinkowskiPoint *hull, const int iteration)
{
int mini = 0;
cpFloat minDist = INFINITY;
// TODO: precalculate this when building the hull and save a step.
// Find the closest segment hull[i] and hull[i + 1] to (0, 0)
for(int j=0, i=count-1; j<count; i=j, j++) {
cpFloat d = ClosestDist(hull[i].ab, hull[j].ab);
if(d < minDist) {
minDist = d;
mini = i;
}
}
struct MinkowskiPoint v0 = hull[mini];
struct MinkowskiPoint v1 = hull[(mini + 1)%count];
cpAssertSoft(!cpveql(v0.ab, v1.ab), "Internal Error: EPA vertexes are the same (%d and %d)", mini, (mini + 1)%count);
// Check if there is a point on the minkowski difference beyond this edge.
struct MinkowskiPoint p = Support(ctx, cpvperp(cpvsub(v1.ab, v0.ab)));
#if DRAW_EPA
cpVect verts[count];
for(int i=0; i<count; i++) verts[i] = hull[i].ab;
ChipmunkDebugDrawPolygon(count, verts, 0.0, RGBAColor(1, 1, 0, 1), RGBAColor(1, 1, 0, 0.25));
ChipmunkDebugDrawSegment(v0.ab, v1.ab, RGBAColor(1, 0, 0, 1));
ChipmunkDebugDrawDot(5, p.ab, LAColor(1, 1));
#endif
if(CheckArea(cpvsub(v1.ab, v0.ab), cpvadd(cpvsub(p.ab, v0.ab), cpvsub(p.ab, v1.ab))) && iteration < MAX_EPA_ITERATIONS) {
// Rebuild the convex hull by inserting p.
struct MinkowskiPoint *hull2 = (struct MinkowskiPoint *)alloca((count + 1)*sizeof(struct MinkowskiPoint));
int count2 = 1;
hull2[0] = p;
for(int i=0; i<count; i++) {
int index = (mini + 1 + i)%count;
cpVect h0 = hull2[count2 - 1].ab;
cpVect h1 = hull[index].ab;
cpVect h2 = (i + 1 < count ? hull[(index + 1)%count] : p).ab;
if(CheckArea(cpvsub(h2, h0), cpvadd(cpvsub(h1, h0), cpvsub(h1, h2)))) {
hull2[count2] = hull[index];
count2++;
}
}
return EPARecurse(ctx, count2, hull2, iteration + 1);
} else {
// Could not find a new point to insert, so we have found the closest edge of the minkowski difference.
cpAssertWarn(iteration < WARN_EPA_ITERATIONS, "High EPA iterations: %d", iteration);
return ClosestPointsNew(v0, v1);
}
}
static struct ClosestPoints
EPARecurse(const struct SupportContext *ctx, const int count, const struct MinkowskiPoint *hull, const int iteration)
{
int mini = 0;
cpFloat minDist = INFINITY;
// TODO: precalculate this when building the hull and save a step.
for(int j=0, i=count-1; j<count; i=j, j++){
cpFloat d = ClosestDist(hull[i].ab, hull[j].ab);
if(d < minDist){
minDist = d;
mini = i;
}
}
struct MinkowskiPoint v0 = hull[mini];
struct MinkowskiPoint v1 = hull[(mini + 1)%count];
cpAssertSoft(!cpveql(v0.ab, v1.ab), "Internal Error: EPA vertexes are the same (%d and %d)", mini, (mini + 1)%count);
struct MinkowskiPoint p = Support(ctx, cpvperp(cpvsub(v1.ab, v0.ab)));
#if DRAW_EPA
cpVect verts[count];
for(int i=0; i<count; i++) verts[i] = hull[i].ab;
ChipmunkDebugDrawPolygon(count, verts, 0.0, RGBAColor(1, 1, 0, 1), RGBAColor(1, 1, 0, 0.25));
ChipmunkDebugDrawSegment(v0.ab, v1.ab, RGBAColor(1, 0, 0, 1));
ChipmunkDebugDrawDot(5, p.ab, LAColor(1, 1));
#endif
cpFloat area2x = cpvcross(cpvsub(v1.ab, v0.ab), cpvadd(cpvsub(p.ab, v0.ab), cpvsub(p.ab, v1.ab)));
if(area2x > 0.0f && iteration < MAX_EPA_ITERATIONS){
int count2 = 1;
struct MinkowskiPoint *hull2 = (struct MinkowskiPoint *)alloca((count + 1)*sizeof(struct MinkowskiPoint));
hull2[0] = p;
for(int i=0; i<count; i++){
int index = (mini + 1 + i)%count;
cpVect h0 = hull2[count2 - 1].ab;
cpVect h1 = hull[index].ab;
cpVect h2 = (i + 1 < count ? hull[(index + 1)%count] : p).ab;
// TODO: Should this be changed to an area2x check?
if(cpvcross(cpvsub(h2, h0), cpvsub(h1, h0)) > 0.0f){
hull2[count2] = hull[index];
count2++;
}
}
return EPARecurse(ctx, count2, hull2, iteration + 1);
} else {
cpAssertWarn(iteration < WARN_EPA_ITERATIONS, "High EPA iterations: %d", iteration);
return ClosestPointsNew(v0, v1);
}
}
// Recursive implementatino of the GJK loop.
static inline struct ClosestPoints
GJKRecurse(const struct SupportContext *ctx, const struct MinkowskiPoint v0, const struct MinkowskiPoint v1, const int iteration)
{
if(iteration > MAX_GJK_ITERATIONS) {
cpAssertWarn(iteration < WARN_GJK_ITERATIONS, "High GJK iterations: %d", iteration);
return ClosestPointsNew(v0, v1);
}
cpVect delta = cpvsub(v1.ab, v0.ab);
// TODO: should this be an area2x check?
if(cpvcross(delta, cpvadd(v0.ab, v1.ab)) > 0.0f) {
// Origin is behind axis. Flip and try again.
return GJKRecurse(ctx, v1, v0, iteration);
} else {
cpFloat t = ClosestT(v0.ab, v1.ab);
cpVect n = (-1.0f < t && t < 1.0f ? cpvperp(delta) : cpvneg(LerpT(v0.ab, v1.ab, t)));
struct MinkowskiPoint p = Support(ctx, n);
#if DRAW_GJK
ChipmunkDebugDrawSegment(v0.ab, v1.ab, RGBAColor(1, 1, 1, 1));
cpVect c = cpvlerp(v0.ab, v1.ab, 0.5);
ChipmunkDebugDrawSegment(c, cpvadd(c, cpvmult(cpvnormalize(n), 5.0)), RGBAColor(1, 0, 0, 1));
ChipmunkDebugDrawDot(5.0, p.ab, LAColor(1, 1));
#endif
if(
cpvcross(cpvsub(v1.ab, p.ab), cpvadd(v1.ab, p.ab)) > 0.0f &&
cpvcross(cpvsub(v0.ab, p.ab), cpvadd(v0.ab, p.ab)) < 0.0f
) {
// The triangle v0, p, v1 contains the origin. Use EPA to find the MSA.
cpAssertWarn(iteration < WARN_GJK_ITERATIONS, "High GJK->EPA iterations: %d", iteration);
return EPA(ctx, v0, p, v1);
} else {
if(cpvdot(p.ab, n) <= cpfmax(cpvdot(v0.ab, n), cpvdot(v1.ab, n))) {
// The edge v0, v1 that we already have is the closest to (0, 0) since p was not closer.
cpAssertWarn(iteration < WARN_GJK_ITERATIONS, "High GJK iterations: %d", iteration);
return ClosestPointsNew(v0, v1);
} else {
// p was closer to the origin than our existing edge.
// Need to figure out which existing point to drop.
if(ClosestDist(v0.ab, p.ab) < ClosestDist(p.ab, v1.ab)) {
return GJKRecurse(ctx, v0, p, iteration + 1);
} else {
return GJKRecurse(ctx, p, v1, iteration + 1);
}
}
}
}
}
static inline struct ClosestPoints
GJKRecurse(const struct SupportContext *ctx, const struct MinkowskiPoint v0, const struct MinkowskiPoint v1, const int iteration)
{
if(iteration > MAX_GJK_ITERATIONS){
cpAssertWarn(iteration < WARN_GJK_ITERATIONS, "High GJK iterations: %d", iteration);
return ClosestPointsNew(v0, v1);
}
cpVect delta = cpvsub(v1.ab, v0.ab);
if(cpvcross(delta, cpvadd(v0.ab, v1.ab)) > 0.0f){
// Origin is behind axis. Flip and try again.
return GJKRecurse(ctx, v1, v0, iteration + 1);
} else {
cpFloat t = ClosestT(v0.ab, v1.ab);
cpVect n = (-1.0f < t && t < 1.0f ? cpvperp(delta) : cpvneg(LerpT(v0.ab, v1.ab, t)));
struct MinkowskiPoint p = Support(ctx, n);
#if DRAW_GJK
ChipmunkDebugDrawSegment(v0.ab, v1.ab, RGBAColor(1, 1, 1, 1));
cpVect c = cpvlerp(v0.ab, v1.ab, 0.5);
ChipmunkDebugDrawSegment(c, cpvadd(c, cpvmult(cpvnormalize(n), 5.0)), RGBAColor(1, 0, 0, 1));
ChipmunkDebugDrawDot(5.0, p.ab, LAColor(1, 1));
#endif
if(
cpvcross(cpvsub(v1.ab, p.ab), cpvadd(v1.ab, p.ab)) > 0.0f &&
cpvcross(cpvsub(v0.ab, p.ab), cpvadd(v0.ab, p.ab)) < 0.0f
){
cpAssertWarn(iteration < WARN_GJK_ITERATIONS, "High GJK->EPA iterations: %d", iteration);
// The triangle v0, p, v1 contains the origin. Use EPA to find the MSA.
return EPA(ctx, v0, p, v1);
} else {
// The new point must be farther along the normal than the existing points.
if(cpvdot(p.ab, n) <= cpfmax(cpvdot(v0.ab, n), cpvdot(v1.ab, n))){
cpAssertWarn(iteration < WARN_GJK_ITERATIONS, "High GJK iterations: %d", iteration);
return ClosestPointsNew(v0, v1);
} else {
if(ClosestDist(v0.ab, p.ab) < ClosestDist(p.ab, v1.ab)){
return GJKRecurse(ctx, v0, p, iteration + 1);
} else {
return GJKRecurse(ctx, p, v1, iteration + 1);
}
}
}
}
}
