[SOLVED] bonus chapter: BVH

I've almost got the BVH optimization working in my renderer, but it's just a little... off.  Here's my teapot after calling root->divide(600)




And here's the teapot after only calling the first iteration of divide (so leaving out the recursive divisions):





Obviously I'm missing a step somewhere in the division (or I have an off-by-one that I'm not seeing), since it's leaving out chunks of the images


std::pair<ShapeVector, ShapeVector> Group::partition_children() {
   ShapeVector out_left;
   ShapeVector out_right;

   auto bounds = this->bounds_of();
   auto [left, right] = bounds->split();

   ShapeVector::iterator it;

   for (it = children_.begin(); it != children_.end(); /* no increment */) {
     auto child_bounds = (*it)->parent_space_bounds_of();

     if (left.contains(*child_bounds)) {
       out_left.push_back(*it);
       it = children_.erase(it);
       updated_ = true;
       continue;
     }
     if (right.contains(*child_bounds)) {
       out_right.push_back(*it);
       it = children_.erase(it);
       updated_ = true;
       continue;
     }
     ++it;
   }

   return { out_left, out_right };
 }

 void Group::make_subgroup(ShapeVector shapes) {
   auto g = std::make_shared<Group>();
   for (const auto& s : shapes) {
     g->add(s);
   }
   add(g);
 }

 void Group::divide(const size_t threshold) override {
   if (threshold <= children_.size()) {
     auto [left, right] = partition_children();
     if (! left.empty()) {
       make_subgroup(left);
     }
     if (! right.empty()) {
       make_subgroup(right);
     }
   }
   for (const auto& i : children_) {
     i->divide(threshold);
   }
 }

Any thoughts?