Use ground-network to improve MP start location.

Search for the closest ground-net node near but /not/ on, the requested
runway. This works fairly well, although for some airports the selected
node is surprisingly far from the runway.

src/Airports/groundnetwork.cxx

@@ -36,6 +36,9 @@
 #include <simgear/scene/util/OsgMath.hxx>
 #include <simgear/structure/exception.hxx>
 #include <simgear/timing/timestamp.hxx>
+#include <simgear/math/SGLineSegment.hxx>
+#include <simgear/math/SGGeometryFwd.hxx>
+#include <simgear/math/SGIntersect.hxx>
 
 #include <Airports/airport.hxx>
 #include <Airports/runways.hxx>
@@ -222,18 +225,28 @@ FGTaxiNodeRef FGGroundNetwork::findNearestNode(const SGGeod & aGeod) const
     return result;
 }
 
-FGTaxiNodeRef FGGroundNetwork::findNearestNodeOffRunway(const SGGeod& aGeod) const
+FGTaxiNodeRef FGGroundNetwork::findNearestNodeOffRunway(const SGGeod& aGeod, FGRunway* rwy, double marginM) const
 {
-    SGVec3d cartPos = SGVec3d::fromGeod(aGeod);
+    FGTaxiNodeVector nodes;
-    auto node = std::min_element(m_nodes.begin(), m_nodes.end(),
+    const SGLineSegmentd runwayLine(rwy->cart(), SGVec3d::fromGeod(rwy->end()));
-                                 [cartPos](const FGTaxiNodeRef& a, const FGTaxiNodeRef& b)
+    const double marginMSqr = marginM * marginM;
-                                 {
+    const SGVec3d cartPos = SGVec3d::fromGeod(aGeod);
-                                     double aDist = a->getIsOnRunway() ? DBL_MAX : distSqr(cartPos, a->cart());
-                                     double bDist = b->getIsOnRunway() ? DBL_MAX : distSqr(cartPos, b->cart());
-                                     return  aDist < bDist;
-                                 });
 
-    if (node == m_nodes.end()) {
+    std::copy_if(m_nodes.begin(), m_nodes.end(), std::back_inserter(nodes),
+                 [runwayLine, cartPos, marginMSqr] (const FGTaxiNodeRef& a)
+    {
+        if (a->getIsOnRunway()) return false;
+        return (distSqr(runwayLine, a->cart()) >= marginMSqr);
+    });
+
+
+
+    // find closest of matching nodes
+    auto node = std::min_element(nodes.begin(), nodes.end(),
+                                 [cartPos](const FGTaxiNodeRef& a, const FGTaxiNodeRef& b)
+                                 { return distSqr(cartPos, a->cart()) < distSqr(cartPos, b->cart()); });
+
+    if (node == nodes.end()) {
         return FGTaxiNodeRef();
     }
 

src/Airports/groundnetwork.hxx

@@ -249,7 +249,7 @@ public:
     FGTaxiNodeRef findNearestNode(const SGGeod& aGeod) const;
     FGTaxiNodeRef findNearestNodeOnRunway(const SGGeod& aGeod, FGRunway* aRunway = NULL) const;
 
-    FGTaxiNodeRef findNearestNodeOffRunway(const SGGeod& aGeod) const;
+    FGTaxiNodeRef findNearestNodeOffRunway(const SGGeod& aGeod, FGRunway* aRunway, double distanceM) const;
 
     FGTaxiSegment *findSegment(unsigned int idx) const;
 

src/Main/positioninit.cxx

@@ -36,6 +36,7 @@
 #include <Airports/runways.hxx>
 #include <Airports/airport.hxx>
 #include <Airports/dynamics.hxx>
+#include <Airports/groundnetwork.hxx>
 #include <AIModel/AIManager.hxx>
 #include <GUI/MessageBox.hxx>
 
@@ -166,21 +167,28 @@ boost::tuple<SGGeod, double> runwayStartPos(FGRunwayRef runway)
 {
     fgSetString("/sim/atc/runway", runway->ident().c_str());
     double offsetNm = fgGetDouble("/sim/presets/offset-distance-nm");
+    double startOffset = fgGetDouble("/sim/airport/runways/start-offset-m", 5.0);
+    SGGeod pos = runway->pointOnCenterline(startOffset);
 
     if (isMPEnabled() && (fabs(offsetNm) <0.1)) {
         SG_LOG( SG_GENERAL, SG_WARN, "Requested to start on " << runway->airport()->ident() << "/" <<
                runway->ident() << ", MP is enabled so computing hold short position to avoid runway incursion");
 
-        // set this so multiplayer.nas can inform the user
+        FGGroundNetwork* groundNet = runway->airport()->groundNetwork();
-        fgSetBool("/sim/presets/avoided-mp-runway", true);
+        // add a margin, try to keep the entire aeroplane comfortable off the
+        // runway. 
+        double margin = startOffset + (runway->widthM() * 1.5);
+        FGTaxiNodeRef taxiNode = groundNet ? groundNet->findNearestNodeOffRunway(pos, runway, margin) : 0;
+        if (taxiNode) {
+            // set this so multiplayer.nas can inform the user
+            fgSetBool("/sim/presets/avoided-mp-runway", true);
+            return boost::make_tuple(taxiNode->geod(), SGGeodesy::courseDeg(taxiNode->geod(), pos));
+        }
 
-        double width = runway->widthM();
+        // if we couldn't find a suitable taxi-node, give up. Guessing a position
-        double offset = fgGetDouble("/sim/airport/runways/start-offset-m", 5.0) + width;
+        // causes too much pain (starting in the water or similar bad things)
-        SGGeod pos = runway->pointOffCenterline(width * 0.5, -offset);
-        return boost::make_tuple(pos, runway->headingDeg() + 90.0);
     }
 
-    SGGeod pos = runway->pointOnCenterline(fgGetDouble("/sim/airport/runways/start-offset-m", 5.0));
     return boost::make_tuple(pos, runway->headingDeg());
 }