Event-input: axis interpolater support

Expand the range of filtering options on axis values to include the
optional use of an interpolation table, which gives many more options
when dealing with some devices.

src/Input/FGEventInput.cxx

@@ -30,6 +30,7 @@
 #include <simgear/io/sg_file.hxx>
 #include <simgear/props/props_io.hxx>
 #include <simgear/math/SGMath.hxx>
+#include <simgear/math/interpolater.hxx>
 #include <Scripting/NasalSys.hxx>
 
 using simgear::PropertyList;
@@ -153,6 +154,17 @@ FGAxisEvent::FGAxisEvent( FGInputDevice * device, SGPropertyNode_ptr node ) :
   lowThreshold = node->getDoubleValue("low-threshold", -0.9);
   highThreshold = node->getDoubleValue("high-threshold", 0.9);
   lastValue = 9999999;
+    
+// interpolation of values
+  if (node->hasChild("interpolater")) {
+    interpolater.reset(new SGInterpTable{node->getChild("interpolater")});
+    mirrorInterpolater = node->getBoolValue("interpolater/mirrored", false);
+  }
+}
+
+FGAxisEvent::~FGAxisEvent()
+{
+    
 }
 
 void FGAxisEvent::fire( FGEventData & eventData )
@@ -170,6 +182,15 @@ void FGAxisEvent::fire( FGEventData & eventData )
   if( minRange != maxRange )
     ed.value = 2.0*(eventData.value-minRange)/(maxRange-minRange)-1.0;
 
+  if (interpolater) {
+    if ((ed.value < 0.0) && mirrorInterpolater) {
+      // mirror the positive interpolation for negative values
+      ed.value = -interpolater->interpolate(fabs(ed.value));
+    } else {
+      ed.value = interpolater->interpolate(ed.value);
+    }
+  }
+    
   FGInputEvent::fire( ed );
 }
 

src/Input/FGEventInput.hxx

@@ -26,11 +26,15 @@
 #include "FGCommonInput.hxx"
 
 #include <vector>
+#include <memory>
 
 #include "FGButton.hxx"
 #include "FGDeviceConfigurationMap.hxx"
 #include <simgear/structure/subsystem_mgr.hxx>
 
+// forward decls
+class SGInterpTable;
+
 /*
  * A base structure for event data. 
  * To be extended for O/S specific implementation data
@@ -179,6 +183,8 @@ protected:
 class FGAxisEvent : public FGInputEvent {
 public:
   FGAxisEvent( FGInputDevice * device, SGPropertyNode_ptr node );
+  ~FGAxisEvent();
+    
   void SetMaxRange( double value ) { maxRange = value; }
   void SetMinRange( double value ) { minRange = value; }
   void SetRange( double min, double max ) { minRange = min; maxRange = max; }
@@ -192,6 +198,8 @@ protected:
   double lowThreshold;
   double highThreshold;
   double lastValue;
+  std::unique_ptr<SGInterpTable> interpolater;
+  bool mirrorInterpolater = false;
 };
 
 class FGRelAxisEvent : public FGAxisEvent {