test/source/blender/bmesh/tools/bmesh_decimate_unsubdivide.cc

/* SPDX-FileCopyrightText: 2023 Blender Authors
 *
 * SPDX-License-Identifier: GPL-2.0-or-later */

/** \file
 * \ingroup bmesh
 *
 * BMesh decimator that uses a grid un-subdivide method.
 */

#include "MEM_guardedalloc.h"

#include "bmesh.h"
#include "bmesh_decimate.h" /* own include */

static bool bm_vert_dissolve_fan_test(BMVert *v)
{
  /* check if we should walk over these verts */
  BMIter iter;
  BMEdge *e;

  BMVert *varr[4];

  uint tot_edge = 0;
  uint tot_edge_boundary = 0;
  uint tot_edge_manifold = 0;
  uint tot_edge_wire = 0;

  BM_ITER_ELEM (e, &iter, v, BM_EDGES_OF_VERT) {
    if (BM_edge_is_boundary(e)) {
      tot_edge_boundary++;
    }
    else if (BM_edge_is_manifold(e)) {
      tot_edge_manifold++;
    }
    else if (BM_edge_is_wire(e)) {
      tot_edge_wire++;
    }

    /* bail out early */
    if (tot_edge == 4) {
      return false;
    }

    /* used to check overlapping faces */
    varr[tot_edge] = BM_edge_other_vert(e, v);

    tot_edge++;
  }

  if (((tot_edge == 4) && (tot_edge_boundary == 0) && (tot_edge_manifold == 4)) ||
      ((tot_edge == 3) && (tot_edge_boundary == 0) && (tot_edge_manifold == 3)) ||
      ((tot_edge == 3) && (tot_edge_boundary == 2) && (tot_edge_manifold == 1)))
  {
    if (!BM_face_exists(varr, tot_edge)) {
      return true;
    }
  }
  else if ((tot_edge == 2) && (tot_edge_wire == 2)) {
    return true;
  }
  return false;
}

static bool bm_vert_dissolve_fan(BMesh *bm, BMVert *v)
{
  /* Collapse under 2 conditions:
   * - vert connects to 4 manifold edges (and 4 faces).
   * - vert connects to 1 manifold edge, 2 boundary edges (and 2 faces).
   *
   * This covers boundary verts of a quad grid and center verts.
   * note that surrounding faces don't have to be quads.
   */

  BMIter iter;
  BMEdge *e;

  uint tot_loop = 0;
  uint tot_edge = 0;
  uint tot_edge_boundary = 0;
  uint tot_edge_manifold = 0;
  uint tot_edge_wire = 0;

  BM_ITER_ELEM (e, &iter, v, BM_EDGES_OF_VERT) {
    if (BM_edge_is_boundary(e)) {
      tot_edge_boundary++;
    }
    else if (BM_edge_is_manifold(e)) {
      tot_edge_manifold++;
    }
    else if (BM_edge_is_wire(e)) {
      tot_edge_wire++;
    }
    tot_edge++;
  }

  if (tot_edge == 2) {
    /* check for 2 wire verts only */
    if (tot_edge_wire == 2) {
      return (BM_vert_collapse_edge(bm, v->e, v, true, true, true) != nullptr);
    }
  }
  else if (tot_edge == 4) {
    /* check for 4 faces surrounding */
    if (tot_edge_boundary == 0 && tot_edge_manifold == 4) {
      /* good to go! */
      tot_loop = 4;
    }
  }
  else if (tot_edge == 3) {
    /* check for 2 faces surrounding at a boundary */
    if (tot_edge_boundary == 2 && tot_edge_manifold == 1) {
      /* good to go! */
      tot_loop = 2;
    }
    else if (tot_edge_boundary == 0 && tot_edge_manifold == 3) {
      /* good to go! */
      tot_loop = 3;
    }
  }

  if (tot_loop) {
    BMLoop *f_loop[4];
    uint i;

    /* ensure there are exactly tot_loop loops */
    BLI_assert(BM_iter_at_index(bm, BM_LOOPS_OF_VERT, v, tot_loop) == nullptr);
    BM_iter_as_array(bm, BM_LOOPS_OF_VERT, v, (void **)f_loop, tot_loop);

    for (i = 0; i < tot_loop; i++) {
      BMLoop *l = f_loop[i];
      if (l->f->len > 3) {
        BMLoop *l_new;
        BLI_assert(l->prev->v != l->next->v);
        BM_face_split(bm, l->f, l->prev, l->next, &l_new, nullptr, true);
        BM_elem_flag_merge_into(l_new->e, l->e, l->prev->e);
      }
    }

    return BM_vert_dissolve(bm, v);
  }

  return false;
}

enum {
  VERT_INDEX_DO_COLLAPSE = -1,
  VERT_INDEX_INIT = 0,
  VERT_INDEX_IGNORE = 1,
};

// #define USE_WALKER  /* gives uneven results, disable for now */

/* - BMVert.flag & BM_ELEM_TAG:  shows we touched this vert
 * - BMVert.index == -1:         shows we will remove this vert
 */

void BM_mesh_decimate_unsubdivide_ex(BMesh *bm, const int iterations, const bool tag_only)
{
#ifdef USE_WALKER
#  define ELE_VERT_TAG 1
#else
  BMVert **vert_seek_a = static_cast<BMVert **>(
      MEM_mallocN(sizeof(BMVert *) * bm->totvert, __func__));
  BMVert **vert_seek_b = static_cast<BMVert **>(
      MEM_mallocN(sizeof(BMVert *) * bm->totvert, __func__));
  uint vert_seek_a_tot = 0;
  uint vert_seek_b_tot = 0;
#endif

  BMIter iter;

  const uint offset = 0;
  const uint nth = 2;

  int iter_step;

  /* if tag_only is set, we assume the caller knows what verts to tag
   * needed for the operator */
  if (tag_only == false) {
    BMVert *v;
    BM_ITER_MESH (v, &iter, bm, BM_VERTS_OF_MESH) {
      BM_elem_flag_enable(v, BM_ELEM_TAG);
    }
  }

  for (iter_step = 0; iter_step < iterations; iter_step++) {
    BMVert *v, *v_next;
    bool iter_done;

    BM_ITER_MESH (v, &iter, bm, BM_VERTS_OF_MESH) {
      if (BM_elem_flag_test(v, BM_ELEM_TAG) && bm_vert_dissolve_fan_test(v)) {
#ifdef USE_WALKER
        BMO_vert_flag_enable(bm, v, ELE_VERT_TAG);
#endif
        BM_elem_index_set(v, VERT_INDEX_INIT); /* set_dirty! */
      }
      else {
        BM_elem_index_set(v, VERT_INDEX_IGNORE); /* set_dirty! */
      }
    }
    /* done with selecting tagged verts */

    /* main loop, keep tagging until we can't tag any more islands */
    while (true) {
#ifdef USE_WALKER
      BMWalker walker;
#else
      uint depth = 1;
      uint i;
#endif
      BMVert *v_first = nullptr;

      /* we could avoid iterating from the start each time */
      BM_ITER_MESH (v, &iter, bm, BM_VERTS_OF_MESH) {
        if (v->e && (BM_elem_index_get(v) == VERT_INDEX_INIT)) {
#ifdef USE_WALKER
          if (BMO_vert_flag_test(bm, v, ELE_VERT_TAG))
#endif
          {
            /* Check again in case the topology changed. */
            if (bm_vert_dissolve_fan_test(v)) {
              v_first = v;
            }
            break;
          }
        }
      }
      if (v_first == nullptr) {
        break;
      }

#ifdef USE_WALKER
      /* Walk over selected elements starting at active */
      BMW_init(
          &walker,
          bm,
          BMW_CONNECTED_VERTEX,
          ELE_VERT_TAG,
          BMW_MASK_NOP,
          BMW_MASK_NOP,
          BMW_FLAG_NOP, /* don't use #BMW_FLAG_TEST_HIDDEN here since we want to deselect all. */
          BMW_NIL_LAY);

      BLI_assert(walker.order == BMW_BREADTH_FIRST);
      for (v = BMW_begin(&walker, v_first); v != nullptr; v = BMW_step(&walker)) {
        /* Deselect elements that aren't at "nth" depth from active */
        if (BM_elem_index_get(v) == VERT_INDEX_INIT) {
          if ((offset + BMW_current_depth(&walker)) % nth) {
            /* tag for removal */
            BM_elem_index_set(v, VERT_INDEX_DO_COLLAPSE); /* set_dirty! */
          }
          else {
            /* works better to allow these verts to be checked again */
            // BM_elem_index_set(v, VERT_INDEX_IGNORE);  /* set_dirty! */
          }
        }
      }
      BMW_end(&walker);
#else

      BM_elem_index_set(v_first,
                        ((offset + depth) % nth) ? VERT_INDEX_IGNORE :
                                                   VERT_INDEX_DO_COLLAPSE); /* set_dirty! */

      vert_seek_b_tot = 0;
      vert_seek_b[vert_seek_b_tot++] = v_first;

      while (true) {
        BMEdge *e;

        if ((offset + depth) % nth) {
          vert_seek_a_tot = 0;
          for (i = 0; i < vert_seek_b_tot; i++) {
            v = vert_seek_b[i];
            BLI_assert(BM_elem_index_get(v) == VERT_INDEX_IGNORE);
            BM_ITER_ELEM (e, &iter, v, BM_EDGES_OF_VERT) {
              BMVert *v_other = BM_edge_other_vert(e, v);
              if (BM_elem_index_get(v_other) == VERT_INDEX_INIT) {
                BM_elem_index_set(v_other, VERT_INDEX_DO_COLLAPSE); /* set_dirty! */
                vert_seek_a[vert_seek_a_tot++] = v_other;
              }
            }
          }
          if (vert_seek_a_tot == 0) {
            break;
          }
        }
        else {
          vert_seek_b_tot = 0;
          for (i = 0; i < vert_seek_a_tot; i++) {
            v = vert_seek_a[i];
            BLI_assert(BM_elem_index_get(v) == VERT_INDEX_DO_COLLAPSE);
            BM_ITER_ELEM (e, &iter, v, BM_EDGES_OF_VERT) {
              BMVert *v_other = BM_edge_other_vert(e, v);
              if (BM_elem_index_get(v_other) == VERT_INDEX_INIT) {
                BM_elem_index_set(v_other, VERT_INDEX_IGNORE); /* set_dirty! */
                vert_seek_b[vert_seek_b_tot++] = v_other;
              }
            }
          }
          if (vert_seek_b_tot == 0) {
            break;
          }
        }

        depth++;
      }
#endif /* USE_WALKER */
    }

    /* now we tagged all verts -1 for removal, lets loop over and rebuild faces */
    iter_done = false;
    BM_ITER_MESH_MUTABLE (v, v_next, &iter, bm, BM_VERTS_OF_MESH) {
      if (BM_elem_index_get(v) == VERT_INDEX_DO_COLLAPSE) {
        if (bm_vert_dissolve_fan(bm, v)) {
          iter_done = true;
        }
      }
    }

    if (iter_done == false) {
      break;
    }
  }

  bm->elem_index_dirty |= BM_VERT;

#ifndef USE_WALKER
  MEM_freeN(vert_seek_a);
  MEM_freeN(vert_seek_b);
#endif
}

void BM_mesh_decimate_unsubdivide(BMesh *bm, const int iterations)
{
  BM_mesh_decimate_unsubdivide_ex(bm, iterations, false);
}