Douglas-Peucker Polyline simplification

MIT-LICENSE

Copyright 2016 Mike Owens

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polygon-simplify.cc

/* You'll need a Point/Vec2 `dist_from_line` */

typedef std::vector<Point> Polygon;
typedef double Float;
  
/* Polyline Simplification Algorithm */
namespace DouglasPeucker {
  void simplify_section(const Polygon& pts,
                        Float tolerance,
                        size_t i, size_t j,
                        std::vector<bool>* mark_map,
                        size_t omitted = 0)
  {
    /* make sure we always return 2 points. */
    if (pts.size() - omitted <= 2)
      return;

    assert(mark_map && mark_map->size() == pts.size());

    if ((i + 1) == j) {
      return;
    }

    Float max_distance = -1.0f;
    size_t max_index = i;

    for (size_t k = i + 1; k < j; k++) {
      Float distance = abs(pts[k].dist_from_line(pts[i], pts[j]));

      if (distance > max_distance) {
        max_distance = distance;
        max_index = k;
      }
    }

    if (max_distance <= tolerance) {
      for (size_t k = i + 1; k < j; k++) {
        (*mark_map)[k] = false;
        ++omitted;
      }
    } else {
      simplify_section(pts, tolerance, i, max_index, mark_map, omitted);
      simplify_section(pts, tolerance, max_index, j, mark_map, omitted);
    }
  }


  Polygon simplify(const Polygon& vertices, Float tolerance)
  {
    std::vector<bool> mark_map(vertices.size(), true);

    simplify_section(vertices, tolerance, 0, vertices.size() - 1, &mark_map);

    Polygon result;
    for (size_t i = 0; i != vertices.size(); ++i) {
      if (mark_map[i]) {
        result.push_back(vertices[i]);
      }
    }

    return result;
  }
}

Nice thanks!