Curvature.cpp

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/* SPDX-FileCopyrightText: 1999 Stephane Popinet
 * SPDX-FileCopyrightText: 2000-2003 `Bruno Levy <levy@loria.fr>`
 *
 * SPDX-License-Identifier: GPL-2.0-or-later
 *
 * The Original Code is:
 * - GTS - Library for the manipulation of triangulated surfaces.
 * - OGF/Graphite: Geometry and Graphics Programming Library + Utilities.
 */
 
#include <cassert>
#include <cstdlib>  // for malloc and free
#include <set>
#include <stack>
 
#include "Curvature.h"
#include "WEdge.h"
 
#include "BLI_math_base.h"
 
#include "../geometry/normal_cycle.h"
 
namespace Freestyle {
 
static bool angle_obtuse(WVertex *v, WFace *f)
{
  WOEdge *e;
  f->getOppositeEdge(v, e);
 
  Vec3r vec1(e->GetaVertex()->GetVertex() - v->GetVertex());
  Vec3r vec2(e->GetbVertex()->GetVertex() - v->GetVertex());
  return ((vec1 * vec2) < 0);
}
 
// FIXME
// WVvertex is useless but kept for history reasons
static bool triangle_obtuse(WVertex * /*v*/, WFace *f)
{
  bool b = false;
  for (int i = 0; i < 3; i++) {
    b = b || ((f->getEdgeList()[i]->GetVec() * f->getEdgeList()[(i + 1) % 3]->GetVec()) < 0);
  }
  return b;
}
 
static real cotan(WVertex *vo, WVertex *v1, WVertex *v2)
{
  /* cf. Appendix B of [Meyer et al 2002] */
  real udotv, denom;
 
  Vec3r u(v1->GetVertex() - vo->GetVertex());
  Vec3r v(v2->GetVertex() - vo->GetVertex());
 
  udotv = u * v;
  denom = sqrt(u.squareNorm() * v.squareNorm() - udotv * udotv);
 
  /* denom can be zero if u==v.  Returning 0 is acceptable, based on the callers of this function
   * below. */
  if (denom == 0.0) {
    return 0.0;
  }
  return (udotv / denom);
}
 
static real angle_from_cotan(WVertex *vo, WVertex *v1, WVertex *v2)
{
  /* cf. Appendix B and the caption of Table 1 from [Meyer et al 2002] */
  real udotv, denom;
 
  Vec3r u(v1->GetVertex() - vo->GetVertex());
  Vec3r v(v2->GetVertex() - vo->GetVertex());
 
  udotv = u * v;
  denom = sqrt(u.squareNorm() * v.squareNorm() - udotv * udotv);
 
  /* NOTE(Ray Jones): I assume this is what they mean by using #atan2. */
 
  /* tan = denom/udotv = y/x (see man page for atan2) */
  return fabs(atan2(denom, udotv));
}
 
bool gts_vertex_mean_curvature_normal(WVertex *v, Vec3r &Kh)
{
  real area = 0.0;
 
  if (!v) {
    return false;
  }
 
  /* this operator is not defined for boundary edges */
  if (v->isBoundary()) {
    return false;
  }
 
  WVertex::incoming_edge_iterator itE;
 
  for (itE = v->incoming_edges_begin(); itE != v->incoming_edges_end(); itE++) {
    area += (*itE)->GetaFace()->getArea();
  }
 
  Kh = Vec3r(0.0, 0.0, 0.0);
 
  for (itE = v->incoming_edges_begin(); itE != v->incoming_edges_end(); itE++) {
    WOEdge *e = (*itE)->getPrevOnFace();
#if 0
    if ((e->GetaVertex() == v) || (e->GetbVertex() == v)) {
      cerr << "BUG ";
    }
#endif
    WVertex *v1 = e->GetaVertex();
    WVertex *v2 = e->GetbVertex();
    real temp;
 
    temp = cotan(v1, v, v2);
    Kh = Vec3r(Kh + temp * (v2->GetVertex() - v->GetVertex()));
 
    temp = cotan(v2, v, v1);
    Kh = Vec3r(Kh + temp * (v1->GetVertex() - v->GetVertex()));
  }
  if (area > 0.0) {
    Kh[0] /= 2 * area;
    Kh[1] /= 2 * area;
    Kh[2] /= 2 * area;
  }
  else {
    return false;
  }
 
  return true;
}
 
bool gts_vertex_gaussian_curvature(WVertex *v, real *Kg)
{
  real area = 0.0;
  real angle_sum = 0.0;
 
  if (!v) {
    return false;
  }
  if (!Kg) {
    return false;
  }
 
  /* this operator is not defined for boundary edges */
  if (v->isBoundary()) {
    *Kg = 0.0;
    return false;
  }
 
  WVertex::incoming_edge_iterator itE;
  for (itE = v->incoming_edges_begin(); itE != v->incoming_edges_end(); itE++) {
    area += (*itE)->GetaFace()->getArea();
  }
 
  for (itE = v->incoming_edges_begin(); itE != v->incoming_edges_end(); itE++) {
    WOEdge *e = (*itE)->getPrevOnFace();
    WVertex *v1 = e->GetaVertex();
    WVertex *v2 = e->GetbVertex();
    angle_sum += angle_from_cotan(v, v1, v2);
  }
 
  *Kg = (2.0 * M_PI - angle_sum) / area;
 
  return true;
}
 
void gts_vertex_principal_curvatures(real Kh, real Kg, real *K1, real *K2)
{
  real temp = Kh * Kh - Kg;
 
  if (!K1 || !K2) {
    return;
  }
 
  if (temp < 0.0) {
    temp = 0.0;
  }
  temp = sqrt(temp);
  *K1 = Kh + temp;
  *K2 = Kh - temp;
}
 
/* from Maple */
static void linsolve(real m11, real m12, real b1, real m21, real m22, real b2, real *x1, real *x2)
{
  real temp;
 
  temp = 1.0 / (m21 * m12 - m11 * m22);
  *x1 = (m12 * b2 - m22 * b1) * temp;
  *x2 = (m11 * b2 - m21 * b1) * temp;
}
 
/* from Maple - largest eigenvector of [a b; b c] */
static void eigenvector(real a, real b, real c, Vec3r e)
{
  if (b == 0.0) {
    e[0] = 0.0;
  }
  else {
    e[0] = -(c - a - sqrt(c * c - 2 * a * c + a * a + 4 * b * b)) / (2 * b);
  }
  e[1] = 1.0;
  e[2] = 0.0;
}
 
void gts_vertex_principal_directions(WVertex *v, Vec3r Kh, real Kg, Vec3r &e1, Vec3r &e2)
{
  Vec3r N;
  real normKh;
 
  Vec3r basis1, basis2, d, eig;
  real ve2, vdotN;
  real aterm_da, bterm_da, cterm_da, const_da;
  real aterm_db, bterm_db, cterm_db, const_db;
  real a, b, c;
  real K1, K2;
  real *weights, *kappas, *d1s, *d2s;
  int edge_count;
  real err_e1, err_e2;
  int e;
  WVertex::incoming_edge_iterator itE;
 
  /* compute unit normal */
  normKh = Kh.norm();
 
  if (normKh > 0.0) {
    Kh.normalize();
  }
  else {
    /* This vertex is a point of zero mean curvature (flat or saddle point). Compute a normal by
     * averaging the adjacent triangles
     */
    N[0] = N[1] = N[2] = 0.0;
 
    for (itE = v->incoming_edges_begin(); itE != v->incoming_edges_end(); itE++) {
      N = Vec3r(N + (*itE)->GetaFace()->GetNormal());
    }
    real normN = N.norm();
    if (normN <= 0.0) {
      return;
    }
    N.normalize();
  }
 
  /* construct a basis from N: */
  /* set basis1 to any component not the largest of N */
  basis1[0] = basis1[1] = basis1[2] = 0.0;
  if (fabs(N[0]) > fabs(N[1])) {
    basis1[1] = 1.0;
  }
  else {
    basis1[0] = 1.0;
  }
 
  /* make basis2 orthogonal to N */
  basis2 = (N ^ basis1);
  basis2.normalize();
 
  /* make basis1 orthogonal to N and basis2 */
  basis1 = (N ^ basis2);
  basis1.normalize();
 
  aterm_da = bterm_da = cterm_da = const_da = 0.0;
  aterm_db = bterm_db = cterm_db = const_db = 0.0;
  int nb_edges = v->GetEdges().size();
 
  weights = (real *)malloc(sizeof(real) * nb_edges);
  kappas = (real *)malloc(sizeof(real) * nb_edges);
  d1s = (real *)malloc(sizeof(real) * nb_edges);
  d2s = (real *)malloc(sizeof(real) * nb_edges);
  edge_count = 0;
 
  for (itE = v->incoming_edges_begin(); itE != v->incoming_edges_end(); itE++) {
    WOEdge *e;
    WFace *f1, *f2;
    real weight, kappa, d1, d2;
    Vec3r vec_edge;
    if (!*itE) {
      continue;
    }
    e = *itE;
 
    /* Since this vertex passed the tests in gts_vertex_mean_curvature_normal(),
     * this should be true. */
    // g_assert(gts_edge_face_number (e, s) == 2);
 
    /* Identify the two triangles bordering e in s. */
    f1 = e->GetaFace();
    f2 = e->GetbFace();
 
    /* We are solving for the values of the curvature tensor
     *     `B = [ a b ; b c ]`.
     * The computations here are from section 5 of [Meyer et al 2002].
     *
     * The first step is to calculate the linear equations governing the values of (a,b,c). These
     * can be computed by setting the derivatives of the error E to zero (section 5.3).
     *
     * Since a + c = norm(Kh), we only compute the linear equations for `dE/da` and `dE/db`.
     * (NOTE: [Meyer et al 2002] has the equation `a + b = norm(Kh)`,
     * but I'm almost positive this is incorrect).
     *
     * Note that the w_ij (defined in section 5.2) are all scaled by `(1/8*A_mixed)`. We drop this
     * uniform scale factor because the solution of the linear equations doesn't rely on it.
     *
     * The terms of the linear equations are xterm_dy with x in {a,b,c} and y in {a,b}. There are
     * also const_dy terms that are the constant factors in the equations.
     */
 
    /* find the vector from v along edge e */
    vec_edge = Vec3r(-1 * e->GetVec());
 
    ve2 = vec_edge.squareNorm();
    vdotN = vec_edge * N;
 
    /* section 5.2 - There is a typo in the computation of kappa. The edges should be x_j-x_i. */
    kappa = 2.0 * vdotN / ve2;
 
    /* section 5.2 */
 
    /* I don't like performing a minimization where some of the weights can be negative (as can be
     * the case if f1 or f2 are obtuse). To ensure all-positive weights, we check for obtuseness.
     */
    weight = 0.0;
    if (!triangle_obtuse(v, f1)) {
      weight += ve2 *
                cotan(
                    f1->GetNextOEdge(e->twin())->GetbVertex(), e->GetaVertex(), e->GetbVertex()) /
                8.0;
    }
    else {
      if (angle_obtuse(v, f1)) {
        weight += ve2 * f1->getArea() / 4.0;
      }
      else {
        weight += ve2 * f1->getArea() / 8.0;
      }
    }
 
    if (!triangle_obtuse(v, f2)) {
      weight += ve2 * cotan(f2->GetNextOEdge(e)->GetbVertex(), e->GetaVertex(), e->GetbVertex()) /
                8.0;
    }
    else {
      if (angle_obtuse(v, f2)) {
        weight += ve2 * f1->getArea() / 4.0;
      }
      else {
        weight += ve2 * f1->getArea() / 8.0;
      }
    }
 
    /* projection of edge perpendicular to N (section 5.3) */
    d[0] = vec_edge[0] - vdotN * N[0];
    d[1] = vec_edge[1] - vdotN * N[1];
    d[2] = vec_edge[2] - vdotN * N[2];
    d.normalize();
 
    /* not explicit in the paper, but necessary. Move d to 2D basis. */
    d1 = d * basis1;
    d2 = d * basis2;
 
    /* store off the curvature, direction of edge, and weights for later use */
    weights[edge_count] = weight;
    kappas[edge_count] = kappa;
    d1s[edge_count] = d1;
    d2s[edge_count] = d2;
    edge_count++;
 
    /* Finally, update the linear equations */
    aterm_da += weight * d1 * d1 * d1 * d1;
    bterm_da += weight * d1 * d1 * 2 * d1 * d2;
    cterm_da += weight * d1 * d1 * d2 * d2;
    const_da += weight * d1 * d1 * (-kappa);
 
    aterm_db += weight * d1 * d2 * d1 * d1;
    bterm_db += weight * d1 * d2 * 2 * d1 * d2;
    cterm_db += weight * d1 * d2 * d2 * d2;
    const_db += weight * d1 * d2 * (-kappa);
  }
 
  /* now use the identity (Section 5.3) a + c = |Kh| = 2 * kappa_h */
  aterm_da -= cterm_da;
  const_da += cterm_da * normKh;
 
  aterm_db -= cterm_db;
  const_db += cterm_db * normKh;
 
  /* check for solvability of the linear system */
  if (((aterm_da * bterm_db - aterm_db * bterm_da) != 0.0) &&
      ((const_da != 0.0) || (const_db != 0.0)))
  {
    linsolve(aterm_da, bterm_da, -const_da, aterm_db, bterm_db, -const_db, &a, &b);
 
    c = normKh - a;
 
    eigenvector(a, b, c, eig);
  }
  else {
    /* region of v is planar */
    eig[0] = 1.0;
    eig[1] = 0.0;
  }
 
  /* Although the eigenvectors of B are good estimates of the principal directions, it seems that
   * which one is attached to which curvature direction is a bit arbitrary. This may be a bug in my
   * implementation, or just a side-effect of the inaccuracy of B due to the discrete nature of the
   * sampling.
   *
   * To overcome this behavior, we'll evaluate which assignment best matches the given eigenvectors
   * by comparing the curvature estimates computed above and the curvatures calculated from the
   * discrete differential operators.
   */
 
  gts_vertex_principal_curvatures(0.5 * normKh, Kg, &K1, &K2);
 
  err_e1 = err_e2 = 0.0;
  /* loop through the values previously saved */
  for (e = 0; e < edge_count; e++) {
    real weight, kappa, d1, d2;
    real temp1, temp2;
    real delta;
 
    weight = weights[e];
    kappa = kappas[e];
    d1 = d1s[e];
    d2 = d2s[e];
 
    temp1 = fabs(eig[0] * d1 + eig[1] * d2);
    temp1 = temp1 * temp1;
    temp2 = fabs(eig[1] * d1 - eig[0] * d2);
    temp2 = temp2 * temp2;
 
    /* err_e1 is for K1 associated with e1 */
    delta = K1 * temp1 + K2 * temp2 - kappa;
    err_e1 += weight * delta * delta;
 
    /* err_e2 is for K1 associated with e2 */
    delta = K2 * temp1 + K1 * temp2 - kappa;
    err_e2 += weight * delta * delta;
  }
  free(weights);
  free(kappas);
  free(d1s);
  free(d2s);
 
  /* rotate eig by a right angle if that would decrease the error */
  if (err_e2 < err_e1) {
    real temp = eig[0];
 
    eig[0] = eig[1];
    eig[1] = -temp;
  }
 
  e1[0] = eig[0] * basis1[0] + eig[1] * basis2[0];
  e1[1] = eig[0] * basis1[1] + eig[1] * basis2[1];
  e1[2] = eig[0] * basis1[2] + eig[1] * basis2[2];
  e1.normalize();
 
  /* make N,e1,e2 a right handed coordinate system */
  e2 = N ^ e1;
  e2.normalize();
}
 
namespace OGF {
 
#if 0
inline static real angle(WOEdge *h)
{
  const Vec3r &n1 = h->GetbFace()->GetNormal();
  const Vec3r &n2 = h->GetaFace()->GetNormal();
  const Vec3r v = h->GetVec();
  real sine = (n1 ^ n2) * v / v.norm();
  if (sine >= 1.0) {
    return M_PI_2;
  }
  if (sine <= -1.0) {
    return -M_PI_2;
  }
  return ::asin(sine);
}
#endif
 
// precondition1: P is inside the sphere
// precondition2: P,V points to the outside of the sphere (i.e. OP.V > 0)
static bool sphere_clip_vector(const Vec3r &O, real r, const Vec3r &P, Vec3r &V)
{
  Vec3r W = P - O;
  real a = V.squareNorm();
  real b = 2.0 * V * W;
  real c = W.squareNorm() - r * r;
  real delta = b * b - 4 * a * c;
  if (delta < 0) {
    // Should not happen, but happens sometimes (numerical precision)
    return true;
  }
  real t = -b + ::sqrt(delta) / (2.0 * a);
  if (t < 0.0) {
    // Should not happen, but happens sometimes (numerical precision)
    return true;
  }
  if (t >= 1.0) {
    // Inside the sphere
    return false;
  }
 
  V[0] = (t * V.x());
  V[1] = (t * V.y());
  V[2] = (t * V.z());
 
  return true;
}
 
/* TODO: check optimizations:
 * use marking ? (measure *timings* ...). */
void compute_curvature_tensor(WVertex *start, real radius, NormalCycle &nc)
{
  // in case we have a non-manifold vertex, skip it...
  if (start->isBoundary()) {
    return;
  }
 
  std::set<WVertex *> vertices;
  const Vec3r &O = start->GetVertex();
  std::stack<WVertex *> S;
  S.push(start);
  vertices.insert(start);
  while (!S.empty()) {
    WVertex *v = S.top();
    S.pop();
    if (v->isBoundary()) {
      continue;
    }
    const Vec3r &P = v->GetVertex();
    WVertex::incoming_edge_iterator woeit = v->incoming_edges_begin();
    WVertex::incoming_edge_iterator woeitend = v->incoming_edges_end();
    for (; woeit != woeitend; ++woeit) {
      WOEdge *h = *woeit;
      if ((v == start) || h->GetVec() * (O - P) > 0.0) {
        Vec3r V(-1 * h->GetVec());
        bool isect = sphere_clip_vector(O, radius, P, V);
        assert(h->GetOwner()->GetNumberOfOEdges() ==
               2);  // Because otherwise v->isBoundary() would be true
        nc.accumulate_dihedral_angle(V, h->GetAngle());
 
        if (!isect) {
          WVertex *w = h->GetaVertex();
          if (vertices.find(w) == vertices.end()) {
            vertices.insert(w);
            S.push(w);
          }
        }
      }
    }
  }
}
 
void compute_curvature_tensor_one_ring(WVertex *start, NormalCycle &nc)
{
  // in case we have a non-manifold vertex, skip it...
  if (start->isBoundary()) {
    return;
  }
 
  WVertex::incoming_edge_iterator woeit = start->incoming_edges_begin();
  WVertex::incoming_edge_iterator woeitend = start->incoming_edges_end();
  for (; woeit != woeitend; ++woeit) {
    WOEdge *h = (*woeit)->twin();
    nc.accumulate_dihedral_angle(h->GetVec(), h->GetAngle());
    WOEdge *hprev = h->getPrevOnFace();
    nc.accumulate_dihedral_angle(hprev->GetVec(), hprev->GetAngle());
  }
}
 
}  // namespace OGF
 
} /* namespace Freestyle */