chiark / gitweb /
and the stop epsilon, from size hovering around 7.something e-4
[moebius2.git] / energy.c
index cca777f8cff22ded6570402e06cfbb3fe2fe5b59..09c54811520c45ca3a43a79464afa15ebe262619 100644 (file)
--- a/energy.c
+++ b/energy.c
 #include "common.h"
 #include "minimise.h"
 #include "mgraph.h"
+#include "parallel.h"
 
-double vertex_areas[N], vertex_mean_edge_lengths[N], edge_lengths[N][V6];
+double vertex_mean_edge_lengths[N];
+
+static double vertex_areas[N];
+static double edge_lengths[N][V6];
+static double rim_vertex_angles[N];
 
 static double best_energy= DBL_MAX;
 
 static void addcost(double *energy, double tweight, double tcost, int pr);
-#define COST(weight, compute) addcost(&energy, (weight), (compute), printing)
+
+/*---------- main energy computation, weights, etc. ----------*/
+
+typedef double CostComputation(const Vertices vertices, int section);
+typedef void PreComputation(const Vertices vertices, int section);
+
+typedef struct {
+  double weight;
+  CostComputation *fn;
+} CostContribution;
+
+#define NPRECOMPS ((sizeof(precomps)/sizeof(precomps[0])))
+#define NCOSTS ((sizeof(costs)/sizeof(costs[0])))
+#define COST(weight, compute) { (weight),(compute) },
+
+static PreComputation *const precomps[]= {
+  compute_edge_lengths,
+  compute_vertex_areas,
+  compute_rim_twist_angles
+};
+
+static const CostContribution costs[]= {
+
+#if XBITS==3
+#define STOP_EPSILON 1e-6
+    COST(  3e3,   vertex_displacement_cost)
+    COST( 0.4e3,  rim_proximity_cost)
+    COST(  1e7,   edge_angle_cost)
+                  #define EDGE_ANGLE_COST_CIRCCIRCRAT (0.2/1.7)
+    COST(  1e2,   small_triangles_cost)
+    COST(  1e12,   noncircular_rim_cost)
+#endif
+
+#if XBITS==4
+#define STOP_EPSILON 5e-3
+    COST(  3e4,   vertex_displacement_cost) // NB this is probably wrong now
+    COST(  3e4,   vertex_edgewise_displ_cost) // we have changed the power
+    COST( 0.2e3,  rim_proximity_cost)
+    COST( 1e4,  rim_twist_cost)
+    COST(  1e12,   noncircular_rim_cost)
+    COST(  10e1,   nonequilateral_triangles_cost)
+//    COST(  1e1,   small_triangles_cost)
+//    COST(  1e6,   edge_angle_cost)
+                  #define EDGE_ANGLE_COST_CIRCCIRCRAT (0.5/1.7)
+#endif
+
+#if XBITS==5
+#define STOP_EPSILON 1e-5
+    COST(  3e4,   vertex_displacement_cost)
+    COST(  3e4,   vertex_edgewise_displ_cost)
+    COST(  2e-1,  rim_proximity_cost)
+    COST(  3e3,  rim_twist_cost)
+    COST(  1e12,   noncircular_rim_cost)
+    COST(   3e2,   nonequilateral_triangles_cost)
+//    COST(  1e1,   small_triangles_cost)
+//    COST(  1e6,   edge_angle_cost)
+                  #define EDGE_ANGLE_COST_CIRCCIRCRAT (0.5/1.7)
+#endif
+
+#if XBITS==6
+#define STOP_EPSILON 8e-4
+    COST(  3e4,   vertex_displacement_cost)
+    COST(  3e4,   vertex_edgewise_displ_cost)
+    COST( 3e-2,  rim_proximity_cost)
+    COST(  1e4,  rim_twist_cost)
+    COST(  1e12,   noncircular_rim_cost)
+    COST(  10e1,   nonequilateral_triangles_cost)
+//    COST(  1e1,   small_triangles_cost)
+//    COST(  1e6,   edge_angle_cost)
+                  #define EDGE_ANGLE_COST_CIRCCIRCRAT (0.5/1.7)
+#endif
+
+#if XBITS>=7 /* nonsense follows but never mind */
+#define STOP_EPSILON 1e-6
+    COST(  3e5,   line_bending_cost)
+    COST( 10e2,   edge_length_variation_cost)
+    COST( 9.0e1,  rim_proximity_cost) // 5e1 is too much
+                                     // 2.5e1 is too little
+    // 0.2e1 grows compared to previous ?
+    // 0.6e0 shrinks compared to previous ?
+
+    COST(  1e12,   edge_angle_cost)
+                  #define EDGE_ANGLE_COST_CIRCCIRCRAT (0.5/1.3)
+    COST(  1e18,   noncircular_rim_cost)
+#endif
+
+};
+
+const double edge_angle_cost_circcircrat= EDGE_ANGLE_COST_CIRCCIRCRAT;
 
 void energy_init(void) {
+  stop_epsilon= STOP_EPSILON;
 }
 
-/*---------- main energy computation and subroutines ----------*/
+/*---------- energy computation machinery ----------*/
+
+void compute_energy_separately(const struct Vertices *vs,
+                        int section, void *energies_v, void *totals_v) {
+  double *energies= energies_v;
+  int ci;
+  
+  for (ci=0; ci<NPRECOMPS; ci++) {
+    precomps[ci](vs->a, section);
+    inparallel_barrier();
+  }
+  for (ci=0; ci<NCOSTS; ci++)
+    energies[ci]= costs[ci].fn(vs->a, section);
+}
+
+void compute_energy_combine(const struct Vertices *vertices,
+                        int section, void *energies_v, void *totals_v) {
+  int ci;
+  double *energies= energies_v;
+  double *totals= totals_v;
+
+  for (ci=0; ci<NCOSTS; ci++)
+    totals[ci] += energies[ci];
+}
 
 double compute_energy(const struct Vertices *vs) {
   static int bests_unprinted;
-  
-  double energy;
-  int printing;
 
-  compute_edge_lengths(vs->a);
-  compute_vertex_areas(vs->a);
-  energy= 0;
+  double totals[NCOSTS], energy;
+  int ci, printing;
 
   printing= printing_check(pr_cost,0);
 
   if (printing) printf("%15lld c>e |", evaluations);
 
-  if (XBITS==3) {
-    COST(  3e2,   line_bending_cost(vs->a));
-    COST(  1e3,   edge_length_variation_cost(vs->a));
-    COST( 0.4e3,  rim_proximity_cost(vs->a));
-    COST(  1e6,   edge_angle_cost(vs->a));
-//    COST(  1e1,   small_triangles_cost(vs->a));
-    COST(  1e12,   noncircular_rim_cost(vs->a));
-    stop_epsilon= 1e-6;
-  } else if (XBITS==4) {
-    COST(  3e2,   line_bending_cost(vs->a));
-    COST(  3e3,   edge_length_variation_cost(vs->a));
-    COST( 9.0e1,  rim_proximity_cost(vs->a)); // 5e1 is too much
-                                                // 2.5e1 is too little
-    // 0.2e1 grows compared to previous ?
-    // 0.6e0 shrinks compared to previous ?
-    COST(  1e12,   edge_angle_cost(vs->a));
-    COST(  1e12,   noncircular_rim_cost(vs->a));
-    stop_epsilon= 1e-5;
-  } else {
-    abort();
-  }
+  for (ci=0; ci<NCOSTS; ci++)
+    totals[ci]= 0;
+
+  inparallel(vs,
+            compute_energy_separately,
+            compute_energy_combine,
+            sizeof(totals) /* really, size of energies */,
+            totals);
+
+  energy= 0;
+  for (ci=0; ci<NCOSTS; ci++)
+    addcost(&energy, costs[ci].weight, totals[ci], printing);
 
   if (printing) printf("| total %# e |", energy);
 
@@ -86,24 +190,24 @@ double compute_energy(const struct Vertices *vs) {
 
 static void addcost(double *energy, double tweight, double tcost, int pr) {
   double tenergy= tweight * tcost;
-  if (pr) printf(" %# e x %g > %# e* |", tcost, tweight, tenergy);
+  if (pr) printf(/*" %# e >"*/ " %# e* |", /*tcost,*/ tenergy);
   *energy += tenergy;
 }
 
 /*---------- Precomputations ----------*/
 
-void compute_edge_lengths(const Vertices vertices) {
+void compute_edge_lengths(const Vertices vertices, int section) {
   int v1,e,v2;
 
-  FOR_EDGE(v1,e,v2)
+  FOR_EDGE(v1,e,v2, OUTER)
     edge_lengths[v1][e]= hypotD(vertices[v1],vertices[v2]);
 }
 
-void compute_vertex_areas(const Vertices vertices) {
+void compute_vertex_areas(const Vertices vertices, int section) {
   int v0,v1,v2, e1,e2;
 //  int k;
 
-  FOR_VERTEX(v0) {
+  FOR_VERTEX(v0, OUTER) {
     double total= 0.0, edges_total=0;
     int count= 0;
 
@@ -129,7 +233,118 @@ void compute_vertex_areas(const Vertices vertices) {
   }
 }
 
-/*---------- Edgewise vertex displacement ----------*/
+/*---------- displacement of vertices across a midpoint ----------*/
+
+  /*
+   * Subroutine used where we have
+   *
+   *        R - - - - - - - M . -  -  -  -  R'
+   *                            ` .
+   *                                ` .
+   *                                    S
+   *
+   * and wish to say that the vector RM should be similar to MS
+   * or to put it another way S = M + RM
+   *
+   * NB this is not symmetric wrt R and S since it divides by
+   * |SM| but not |RM| so you probably want to call it twice.
+   *
+   * Details:
+   *
+   *   Let  R' = M + SM
+   *        D  = R' - R
+   *
+   * Then the (1/delta)th power of the cost is
+   *      proportional to |D|, and
+   *      inversely proportional to |SM|
+   * except that |D| is measured in a wierd way which counts
+   * distance in the same direction as SM 1/lambda times as much
+   * ie the equipotential surfaces are ellipsoids around R',
+   * lengthened by lambda in the direction of RM.
+   *
+   * So
+   *                                                               delta
+   *                [       -1                                    ]
+   *   cost      =  [ lambda  . ( D . SM/|SM| ) + | D x SM/|SM| | ]
+   *       R,S,M    [ ------------------------------------------- ]
+   *                [                      |SM|                   ]
+   *
+   */
+
+static double vertex_one_displ_cost(const double r[D3], const double s[D3],
+                                   const double midpoint[D3],
+                                   double delta, double inv_lambda) {
+  const double smlen2_epsilon= 1e-12;
+  double sm[D3], d[D3], ddot, dcross[D3];
+  int k;
+
+  K sm[k]= -s[k] + midpoint[k];
+  K d[k]= midpoint[k] + sm[k] - r[k];
+  ddot= dotprod(d,sm);
+  xprod(dcross, d,sm);
+  double smlen2= magnD2(sm);
+  double cost_basis= inv_lambda * ddot + magnD(dcross);
+  double cost= pow(cost_basis / (smlen2 + smlen2_epsilon), delta);
+
+  return cost;
+}
+
+/*---------- displacement of vertices opposite at a vertex ----------*/
+
+  /*
+   *   At vertex Q considering edge direction e to R
+   *   and corresponding opposite edge to S.
+   *
+   *   This is vertex displacement as above with M=Q
+   */
+
+double vertex_displacement_cost(const Vertices vertices, int section) {
+  const double inv_lambda= 1.0/1; //2;
+  const double delta= 6;
+
+  int si,e,qi,ri;
+  double total_cost= 0;
+
+  FOR_EDGE(qi,e,ri, OUTER) {
+    si= EDGE_END2(qi,(e+3)%V6); if (si<0) continue;
+
+    total_cost += vertex_one_displ_cost(vertices[ri], vertices[si], vertices[qi],
+                                       delta, inv_lambda);
+  }
+  return total_cost;
+}
+
+/*---------- displacement of vertices opposite at an edge ----------*/
+
+  /*
+   *   At edge PQ considering vertices R and S (see diagram
+   *   below for overly sharp edge cost).
+   *
+   *   Let  M  = midpoint of PQ
+   */
+
+double vertex_edgewise_displ_cost(const Vertices vertices, int section) {
+  const double inv_lambda= 1.0/1; //2;
+  const double delta= 6;
+
+  int pi,e,qi,ri,si, k;
+  double m[D3];
+  double total_cost= 0;
+
+  FOR_EDGE(pi,e,qi, OUTER) {
+    si= EDGE_END2(pi,(e+V6-1)%V6);  if (si<0) continue;
+    ri= EDGE_END2(pi,(e   +1)%V6);  if (ri<0) continue;
+
+    K m[k]= 0.5 * (vertices[pi][k] + vertices[qi][k]);
+    
+    total_cost += vertex_one_displ_cost(vertices[ri], vertices[si], m,
+                                       delta, inv_lambda);
+  }
+  return total_cost;
+}
+
+
+/*---------- at-vertex edge angles ----------*/
 
   /*
    * Definition:
@@ -165,17 +380,19 @@ void compute_vertex_areas(const Vertices vertices) {
    *          Q,e
    */
 
-double line_bending_cost(const Vertices vertices) {
+double line_bending_cost(const Vertices vertices, int section) {
   static const double axb_epsilon= 1e-6;
-  static const double exponent_r= 3;
+  static const double exponent_r= 4;
 
   int pi,e,qi,ri, k;
   double  a[D3], b[D3], axb[D3];
   double total_cost= 0;
 
-  FOR_EDGE(qi,e,ri) {
+  FOR_EDGE(qi,e,ri, OUTER) {
     pi= EDGE_END2(qi,(e+3)%V6); if (pi<0) continue;
 
+//if (!(qi&XMASK)) fprintf(stderr,"%02x-%02x-%02x (%d)\n",pi,qi,ri,e);
+
     K a[k]= -vertices[pi][k] + vertices[qi][k];
     K b[k]= -vertices[qi][k] + vertices[ri][k];
 
@@ -184,9 +401,6 @@ double line_bending_cost(const Vertices vertices) {
     double delta= atan2(magnD(axb) + axb_epsilon, dotprod(a,b));
     double cost= pow(delta,exponent_r);
 
-    if (!e && !(qi & ~XMASK))
-      cost *= 10;
-
     total_cost += cost;
   }
   return total_cost;
@@ -203,11 +417,11 @@ double line_bending_cost(const Vertices vertices) {
    *           Q,e
    */
 
-double edge_length_variation_cost(const Vertices vertices) {
+double edge_length_variation_cost(const Vertices vertices, int section) {
   double diff, cost= 0, exponent_r= 2;
   int q, e,r, eback;
 
-  FOR_EDGE(q,e,r) {
+  FOR_EDGE(q,e,r, OUTER) {
     eback= edge_reverse(q,e);
     diff= edge_lengths[q][e] - edge_lengths[q][eback];
     cost += pow(diff,exponent_r);
@@ -215,6 +429,34 @@ double edge_length_variation_cost(const Vertices vertices) {
   return cost;
 }
 
+/*---------- proportional edge length variation ----------*/
+
+  /*
+   * Definition:
+   *
+   *    See the diagram above.
+   *                                r
+   *       cost    = ( |PQ| - |QR| )
+   *           Q,e
+   */
+
+double prop_edge_length_variation_cost(const Vertices vertices, int section) {
+  const double num_epsilon= 1e-6;
+
+  double cost= 0, exponent_r= 2;
+  int q, e,r, eback;
+
+  FOR_EDGE(q,e,r, OUTER) {
+    eback= edge_reverse(q,e);
+    double le= edge_lengths[q][e];
+    double leback= edge_lengths[q][eback];
+    double diff= le - leback;
+    double num= MIN(le, leback);
+    cost += pow(diff / (num + num_epsilon), exponent_r);
+  }
+  return cost;
+}
+
 /*---------- rim proximity cost ----------*/
 
 static void find_nearest_oncircle(double oncircle[D3], const double p[D3]) {
@@ -228,11 +470,11 @@ static void find_nearest_oncircle(double oncircle[D3], const double p[D3]) {
   oncircle[1] *= mult;
 }
 
-double rim_proximity_cost(const Vertices vertices) {
-  double oncircle[3], cost=0;
+double rim_proximity_cost(const Vertices vertices, int section) {
+  double oncircle[D3], cost=0;
   int v;
 
-  FOR_VERTEX(v) {
+  FOR_VERTEX(v, OUTER) {
     int y= v >> YSHIFT;
     int nominal_edge_distance= y <= Y/2 ? y : Y-1-y;
     if (nominal_edge_distance==0) continue;
@@ -249,12 +491,12 @@ double rim_proximity_cost(const Vertices vertices) {
 
 /*---------- noncircular rim cost ----------*/
 
-double noncircular_rim_cost(const Vertices vertices) {
+double noncircular_rim_cost(const Vertices vertices, int section) {
   int vy,vx,v;
   double cost= 0.0;
   double oncircle[3];
 
-  FOR_RIM_VERTEX(vy,vx,v) {
+  FOR_RIM_VERTEX(vy,vx,v, OUTER) {
     find_nearest_oncircle(oncircle, vertices[v]);
 
     double d2= hypotD2(vertices[v], oncircle);
@@ -263,22 +505,58 @@ double noncircular_rim_cost(const Vertices vertices) {
   return cost;
 }
 
-/*---------- triangle bad normals cost ----------*/
+/*---------- rim contact angle rotation ----------*/
+
+void compute_rim_twist_angles(const Vertices vertices, int section) {
+  double oncircle[D3], distance[D3];
+  int vpy,vpx,v,k;
+    
+  FOR_NEAR_RIM_VERTEX(vpy,vpx,v, 1,OUTER) {
+    find_nearest_oncircle(oncircle, vertices[v]);
+    /* we are interested in the angle subtended at the rim, from the
+     * rim's point of view. */
+    K distance[k]= vertices[v][k] - oncircle[k];
+    double distance_positive_z= distance[3];
+    double distance_radial_outwards= dotprod(distance, oncircle);
+    rim_vertex_angles[v]= atan2(distance_positive_z, distance_radial_outwards);
+  }
+}
+
+double rim_twist_cost(const Vertices vertices, int section) {
+  double total_cost= 0;
+  int vpy,vpx,v0,v1;
+  
+  FOR_NEAR_RIM_VERTEX(vpy,vpx,v0, 1,OUTER) {
+    v1= EDGE_END2(v0,0);  assert(v1!=0);
+    double delta= rim_vertex_angles[v0] - rim_vertex_angles[v1];
+    if (delta < M_PI) delta += 2*M_PI;
+    if (delta > M_PI) delta -= 2*M_PI;
+
+    double cost= pow(delta, 4);
+    total_cost += cost;
+  }
+
+  return total_cost;
+}
+
+/*---------- overly sharp edge cost ----------*/
 
   /*
    *
    *                       Q `-_
-   *              / |    `-_
-   *             /  |       `-.
-   *            /   |           S
-   *           /    |      _,-'
-   *          /     |  _,-'
-   *         /    , P '
-   *        /  ,-'
-   *       /,-'
+   *              / |    `-_                          P'Q' ------ S'
+   *             /  |       `-.                             _,' `.       .
+   *            /   |           S                _,'     :      .
+   *           /    |      _,-'                _,'        :r    .r
+   *          /     |  _,-'               R' '           `.   .
+   *         /    , P '                               ` .   r     :  .
+   *        /  ,-'                                 `  .   : 
+   *       /,-'                                                 ` C'
    *      /'
    *            R
    *
+   *
+   *
    *  Let delta =  angle between two triangles' normals
    *
    *  Giving energy contribution:
@@ -288,80 +566,66 @@ double noncircular_rim_cost(const Vertices vertices) {
    *     vd, edge PQ      vd
    */
 
-double edge_angle_cost(const Vertices vertices) {
-  double sq[D3], pq[D3], qr[D3], sqp[D3], pqr[D3], rs[D3];
-  double x[D3], y[D3];
+double edge_angle_cost(const Vertices vertices, int section) {
+  double pq1[D3], rp[D3], ps[D3], rp_2d[D3], ps_2d[D3], rs_2d[D3];
+  double a,b,c,s,r;
+  const double minradius_base= 0.2;
+
   int pi,e,qi,ri,si, k;
 //  double our_epsilon=1e-6;
   double total_cost= 0;
-  
-  FOR_EDGE(pi,e,qi) {
+
+  FOR_EDGE(pi,e,qi, OUTER) {
 //    if (!(RIM_VERTEX_P(pi) || RIM_VERTEX_P(qi))) continue;
 
     si= EDGE_END2(pi,(e+V6-1)%V6);  if (si<0) continue;
     ri= EDGE_END2(pi,(e   +1)%V6);  if (ri<0) continue;
 
     K {
-      sq[k]= vertices[si][k] - vertices[qi][k];
-      pq[k]= vertices[pi][k] - vertices[qi][k];
-      qr[k]= vertices[qi][k] - vertices[ri][k];
+      pq1[k]= -vertices[pi][k] + vertices[qi][k];
+      rp[k]=  -vertices[ri][k] + vertices[pi][k];
+      ps[k]=  -vertices[pi][k] + vertices[si][k];
     }
-    xprod(sqp, pq,sq);
-    xprod(pqr, pq,qr);
-
-    double dot= dotprod(sqp,pqr);
-    xprod(x,sqp,pqr);
-
-    K rs[k]= -vertices[ri][k] + vertices[si][k];
-    xprod(y, pq,rs);
 
-    double delta=
-      pow(atan2(magnD(x), dot), 2.0) * pow(magnD2(pq), 2.0) /
-      (pow(magnD(y), 0.3) + 1e-6);
-    double cost= pow(delta, 2.0);
+    normalise(pq1,1,1e-6);
+    xprod(rp_2d, rp,pq1); /* projects RP into plane normal to PQ */
+    xprod(ps_2d, ps,pq1); /* likewise PS */
+    K rs_2d[k]= rp_2d[k] + ps_2d[k];
+    /* radius of circumcircle of R'P'S' from Wikipedia
+     * `Circumscribed circle' */
+    a= magnD(rp_2d);
+    b= magnD(ps_2d);
+    c= magnD(rs_2d);
+    s= 0.5*(a+b+c);
+    r= a*b*c / sqrt((a+b+c)*(a-b+c)*(b-c+a)*(c-a+b) + 1e-6);
+
+    double minradius= minradius_base + edge_angle_cost_circcircrat*(a+b);
+    double deficit= minradius - r;
+    if (deficit < 0) continue;
+    double cost= deficit*deficit;
 
-//double cost= pow(magnD(spqxpqr), 3);
-//assert(dot>=-1 && dot <=1);
-//double cost= 1-dot;
     total_cost += cost;
   }
-  
+
   return total_cost;
 }
 
 /*---------- small triangles cost ----------*/
 
   /*
+   * Consider a triangle PQS
    *
-   *                       Q `-_
-   *              / |    `-_
-   *             /  |       `-.
-   *            /   |           S
-   *           /    |      _,-'
-   *          /     |  _,-'
-   *         /    , P '
-   *        /  ,-'
-   *       /,-'
-   *      /'
-   *            R
-   *
-   *  Let delta =  angle between two triangles' normals
-   *
-   *  Giving energy contribution:
-   *
-   *                                   2
-   *    E             =  F   .  delta
-   *     vd, edge PQ      vd
+   * Cost is 1/( area^2 )
    */
 
-double small_triangles_cost(const Vertices vertices) {
+double small_triangles_cost(const Vertices vertices, int section) {
   double pq[D3], ps[D3];
   double x[D3];
   int pi,e,qi,si, k;
 //  double our_epsilon=1e-6;
   double total_cost= 0;
-  
-  FOR_EDGE(pi,e,qi) {
+
+  FOR_EDGE(pi,e,qi, OUTER) {
 //    if (!(RIM_VERTEX_P(pi) || RIM_VERTEX_P(qi))) continue;
 
     si= EDGE_END2(pi,(e+V6-1)%V6);  if (si<0) continue;
@@ -379,6 +643,51 @@ double small_triangles_cost(const Vertices vertices) {
 //double cost= 1-dot;
     total_cost += cost;
   }
-  
+
+  return total_cost;
+}
+
+/*---------- nonequilateral triangles cost ----------*/
+
+  /*
+   * Consider a triangle PQR
+   *
+   * let edge lengths a=|PQ| b=|QR| c=|RP|
+   *
+   * predicted edge length p = 1/3 * (a+b+c)
+   *
+   * compute cost for each x in {a,b,c}
+   *
+   *
+   *      cost      = (x-p)^2 / p^2
+   *          PQR,x
+   */
+
+double nonequilateral_triangles_cost(const Vertices vertices, int section) {
+  double pr[D3], abc[3];
+  int pi,e0,e1,qi,ri, k,i;
+  double our_epsilon=1e-6;
+  double total_cost= 0;
+
+  FOR_EDGE(pi,e0,qi, OUTER) {
+    e1= (e0+V6-1)%V6;
+    ri= EDGE_END2(pi,e1);  if (ri<0) continue;
+
+    K pr[k]= -vertices[pi][k] + vertices[ri][k];
+
+    abc[0]= edge_lengths[pi][e0]; /* PQ */
+    abc[1]= edge_lengths[qi][e1]; /* QR */
+    abc[2]= magnD(pr);
+
+    double p= (1/3.0) * (abc[0]+abc[1]+abc[2]);
+    double p_inv2= 1/(p*p + our_epsilon);
+
+    for (i=0; i<3; i++) {
+      double diff= (abc[i] - p);
+      double cost= diff*diff * p_inv2;
+      total_cost += cost;
+    }
+  }
+
   return total_cost;
 }