add <initialize-to> to components

The tag <initialize-to> can be used to control the value
of the output when the component is first enabled. This
controls initialization of the output property and the current
value for internal computation
Valid values are

  <initialize-to>input</initialize-to>
set the output-property to the input value

  <initialize-to>output</initialize-to>
set the output-property to the output value

  <initialize-to>none</initialize-to>
ignore input and output value

src/Autopilot/digitalfilter.cxx

@@ -303,7 +303,8 @@ bool ExponentialFilterImplementation::configure( const std::string & nodeName, S
 /* --------------------------------------------------------------------------------- */
 
 DigitalFilter::DigitalFilter() :
-    AnalogComponent()
+    AnalogComponent(),
+    _initializeTo(INITIALIZE_INPUT)
 {
 }
 
@@ -335,6 +336,20 @@ bool DigitalFilter::configure(const string& nodeName, SGPropertyNode_ptr configN
     return true;
   }
 
+  if( nodeName == "initialize-to" ) {
+    string s( configNode->getStringValue() );
+    if( s == "input" ) {
+      _initializeTo = INITIALIZE_INPUT;
+    } else if( s == "output" ) {
+      _initializeTo = INITIALIZE_OUTPUT;
+    } else if( s == "none" ) {
+      _initializeTo = INITIALIZE_NONE;
+    } else {
+      SG_LOG( SG_AUTOPILOT, SG_WARN, "unhandled initialize-to value '" << s << "' ignored" );
+    }
+    return true;
+  }
+
   SG_LOG( SG_AUTOPILOT, SG_BULK, "DigitalFilter::configure(" << nodeName << ") [unhandled]" << endl );
   return false; // not handled by us, let the base class try
 }
@@ -344,8 +359,22 @@ void DigitalFilter::update( bool firstTime, double dt)
   if( _implementation == NULL ) return;
 
   if( firstTime ) {
+    switch( _initializeTo ) {
+
+      case INITIALIZE_INPUT:
         SG_LOG(SG_AUTOPILOT,SG_DEBUG, "First time initialization of " << get_name() << " to " << _valueInput.get_value() );
         _implementation->initialize( _valueInput.get_value() );
+        break;
+
+      case INITIALIZE_OUTPUT:
+        SG_LOG(SG_AUTOPILOT,SG_DEBUG, "First time initialization of " << get_name() << " to " << get_output_value() );
+        _implementation->initialize( get_output_value() );
+        break;
+
+      default:
+        SG_LOG(SG_AUTOPILOT,SG_DEBUG, "First time initialization of " << get_name() << " to (uninitialized)" );
+        break;
+    }
   }
 
   double input = _valueInput.get_value() - _referenceInput.get_value();

src/Autopilot/digitalfilter.hxx

@@ -53,6 +53,12 @@ class DigitalFilter : public AnalogComponent
 private:
     SGSharedPtr<DigitalFilterImplementation> _implementation;
 
+    enum InitializeTo {
+      INITIALIZE_OUTPUT,
+      INITIALIZE_INPUT,
+      INITIALIZE_NONE
+    };
+
 protected:
     bool configure( const std::string & nodeName, SGPropertyNode_ptr configNode);
     void update( bool firstTime, double dt);
@@ -61,6 +67,7 @@ protected:
     InputValueList _samples;
     InputValueList _rateOfChange;
     InputValueList _gain;
+    InitializeTo _initializeTo;
 
 public:
     DigitalFilter();