Osrm-backend: what is the purpose of Phantom_node ?

[bsnyh]

Hi, I found this PhantomNode struct really interesting. Yet, I was confused about the following piece of code except from the file phantom_node.hpp.

EdgeDistance GetForwardDistance() const
    {
        // .....                  <-- forward_distance
        //      ....              <-- offset
        // .........              <-- desired distance
        //         x              <-- this is PhantomNode.location
        // 0----1----2----3----4  <-- EdgeBasedGraph Node segments
        BOOST_ASSERT(forward_segment_id.enabled);
        return forward_distance + forward_distance_offset;
    }

    EdgeDistance GetReverseDistance() const
    {
        //            ..........  <-- reverse_distance
        //         ...            <-- offset
        //         .............  <-- desired distance
        //         x              <-- this is PhantomNode.location
        // 0----1----2----3----4  <-- EdgeBasedGraph Node segments
        BOOST_ASSERT(reverse_segment_id.enabled);
        return reverse_distance + reverse_distance_offset;
    }

I was wondering, where to find any explanations for this PhantomNode?

[danpat]

The road network is built up of nodes and edges, as people draw them in OpenStreetmap.

The routing algorithm implemented in OSRM assumes you'll start on a "node".

However, when you supply a longitude/latitude to OSRM, it's very rarely exactly on an intersection point on the road network - it's usually off to the side of the road.

The PhantomNode class represents a temporary node that's inserted into the graph at the position where your input longitude/latitude is. This acts as the starting and ending "node"s for the routing algorithm, and allows the algorithm to not have a special case for the initial/last iteration.