在 breadth_first_search 期间设置顶点的颜色

Question

我想在 BGL 图上进行区域增长。区域增长的想法是访问顶点，从指定的根顶点开始，收集并 return 一个子图或一个顶点列表，与它们的父节点相比，这些子图或顶点列表通过了一些标准函数。例如，假设我们有一个看起来像这样的简单图表：

A-B-C-D

边权重为：

AB = 4, BC = 10, CD = 3

现在我们要从 A 开始扩展一个区域。我们要执行以下操作：

发现A并将其添加到连通区域
发现B，并判断B是否"similar enough"到A。对于这个例子，假设标准是边缘权重的阈值：如果边缘权重> 5，那么我们不应该继续遍历到B。所以在这里，AB = 4所以我们应该增长到B，但是由于BC=10，我们永远不会到达C。
- 如果是这样，将 B 添加到连接区域并继续发现 C 并检查 C 是否与 B 足够相似，等等
- 如果没有，停止并return当前连接的区域

我可以在访客的tree_edge函数中查看这个判断函数。如果 A 和 B 太不相似，我尝试通过将传递的边的目标顶点设置为 tree_edge 来 "stop" BFS 继续（将 B 添加到队列然后稍后处理它等）变黑。然而，这似乎并没有停止遍历：

#include <iostream>

#include <boost/graph/graph_traits.hpp>
#include <boost/graph/adjacency_list.hpp>
#include <boost/property_map/property_map.hpp>
#include <boost/graph/breadth_first_search.hpp>

using EdgeWeightProperty = boost::property<boost::edge_weight_t, float>;

using ColorPropertyType = boost::property<boost::vertex_color_t, boost::default_color_type>;

using GraphType =  boost::adjacency_list<boost::setS, // out edge container
                                         boost::vecS, // vertex container
                                         boost::undirectedS, // directed or undirected
                                         ColorPropertyType, // vertex properites
                                         EdgeWeightProperty> // edge properties
                                         ;

template <typename TGraph>
void printColors(const TGraph& g)
{
    const auto& colorMapGraph = get(boost::vertex_color_t(), g);

    std::cout << "colors: ";
    for(unsigned int i = 0; i < num_vertices(g); ++i) {
        std::cout << get(colorMapGraph, vertex(i, g)) << " ";
    }

    std::cout << std::endl;
}

class BreadthFirstSearchVisitor : public boost::default_bfs_visitor
{
public:

    // We must provide a mutable version of the graph to the visitor since we want to change properties
    BreadthFirstSearchVisitor(GraphType& graph) : mGraph(graph) {}

    template < typename TEdge, typename TGraph>
    void tree_edge(TEdge e, const TGraph& g) const
    {
        std::cout << std::endl << "tree_edge: " << e << std::endl;
        printColors(g);
        const auto& colors = get(boost::vertex_color_t(), mGraph); // Though this is const&, you can still call put()

        const auto& edgeWeights = get(boost::edge_weight_t(), mGraph);

        boost::graph_traits<GraphType>::vertex_descriptor targetVertex = boost::target(e, g);
        std::cout << "targetVertex: " << targetVertex << std::endl;

        float edgeWeight = get(edgeWeights, e);
        std::cout << "edgeWeight: " << edgeWeight << std::endl;

        if(edgeWeight > 5.f) {
            std::cout << "Next vertex does not belong to the region!" << std::endl;
            put(colors, vertex(targetVertex, mGraph), boost::color_traits<GraphType>::black());
            printColors(g);
        }

    }

    // A very strange pattern, but this is (officially) recommended here: 
    GraphType& mGraph;
};

int main(int,char*[])
{
    // Create a graph object
    GraphType g(4);

    EdgeWeightProperty e0 = 4.f;
    add_edge(0, 1, e0, g);

    EdgeWeightProperty e1 = 10.f;
    add_edge(1, 2, e1, g);

    EdgeWeightProperty e2 = 3.f;
    add_edge(2, 3, e2, g);

    BreadthFirstSearchVisitor breadthFirstSearchVisitor(g);

    unsigned int startVertex = 0;

    // named argument signature
    breadth_first_search(g, vertex(startVertex, g), visitor(breadthFirstSearchVisitor).color_map(get(boost::vertex_color_t(), g)));

    return 0;
}

输出为：

tree_edge: (0,1)
colors: 1 0 0 0 
targetVertex: 1
edgeWeight: 4

tree_edge: (1,2)
colors: 4 1 0 0 
targetVertex: 2
edgeWeight: 10
Next vertex does not belong to the region!
colors: 4 1 4 0 

tree_edge: (2,3)
colors: 4 4 1 0 
targetVertex: 3
edgeWeight: 3

但我原以为它不会用边 (2,3) 调用 tree_edge 因为我们将顶点 2 标记为黑色。

谁能解释为什么这不像我预期的那样有效？

Answer 1

答案似乎只是将访问者中的处理 tree_edge 更改为 examine_edge。我想一旦 tree_edge 被调用，目标顶点已经被添加到队列中，所以它的颜色不再重要（因为颜色用于确定是否应将顶点添加到队列中）。

在 breadth_first_search 期间设置顶点的颜色

Setting the color of a vertex during breadth_first_search

c++

boost

boost-graph