Add infrastructure in preparation for tilting the view.

src/Main/fg_props.cxx

@@ -356,6 +356,48 @@ setGoalViewOffset (double offset)
 	->set_goal_view_offset(offset * SGD_DEGREES_TO_RADIANS);
 }
 
+/**
+ * Get the current view tilt in degrees.
+ */
+static double
+getViewTilt ()
+{
+  return (globals->get_current_view()
+	  ->get_view_tilt() * SGD_RADIANS_TO_DEGREES);
+}
+
+
+static void
+setViewTilt (double tilt)
+{
+  globals->get_current_view()->set_view_tilt(tilt * SGD_DEGREES_TO_RADIANS);
+}
+
+static double
+getGoalViewTilt ()
+{
+  return (globals->get_current_view()
+	  ->get_goal_view_tilt() * SGD_RADIANS_TO_DEGREES);
+}
+
+static void
+setGoalViewTilt (double tilt)
+{
+    while ( tilt < 0 ) {
+	tilt += 360.0;
+    }
+    while ( tilt > 360.0 ) {
+	tilt -= 360.0;
+    }
+    // Snap to center if we are close
+    if ( fabs(tilt) < 1.0 ||  fabs(tilt) > 359.0 ) {
+	tilt = 0.0;
+    }
+
+    globals->get_current_view()
+	->set_goal_view_tilt(tilt * SGD_DEGREES_TO_RADIANS);
+}
+
 
 /**
  * Pilot position offset from CG.
@@ -1051,6 +1093,9 @@ fgInitProps ()
   fgTie("/sim/view/offset-deg", getViewOffset, setViewOffset, false);
   fgSetArchivable("/sim/view/offset-deg");
   fgTie("/sim/view/goal-offset-deg", getGoalViewOffset, setGoalViewOffset, false);
+  fgTie("/sim/view/tilt-deg", getViewTilt, setViewTilt, false);
+  fgSetArchivable("/sim/view/tilt-deg");
+  fgTie("/sim/view/goal-tilt-deg", getGoalViewTilt, setGoalViewTilt, false);
   fgSetArchivable("/sim/view/goal-offset-deg");
   fgTie("/sim/view/pilot/x-offset-m",
 	getPilotPositionXOffset, setPilotPositionXOffset);

src/Main/viewer.cxx

@@ -38,7 +38,9 @@
 FGViewer::FGViewer( void ):
     fov(55.0),
     view_offset(0.0),
-    goal_view_offset(0.0)
+    goal_view_offset(0.0),
+    view_tilt(0.0),
+    goal_view_tilt(0.0)
 {
     sgSetVec3( pilot_offset, 0.0, 0.0, 0.0 );
     sgdZeroVec3(geod_view_pos);
@@ -104,4 +106,33 @@ FGViewer::update (int dt)
       }
     }
   }
+
+  for ( int i = 0; i < dt; i++ ) {
+    if ( fabs(get_goal_view_tilt() - get_view_tilt()) < 0.05 ) {
+      set_view_tilt( get_goal_view_tilt() );
+      break;
+    } else {
+      // move current_view.view_tilt towards
+      // current_view.goal_view_tilt
+      if ( get_goal_view_tilt() > get_view_tilt() )
+	{
+	  if ( get_goal_view_tilt() - get_view_tilt() < SGD_PI ){
+	    inc_view_tilt( 0.01 );
+	  } else {
+	    inc_view_tilt( -0.01 );
+	  }
+	} else {
+	  if ( get_view_tilt() - get_goal_view_tilt() < SGD_PI ){
+	    inc_view_tilt( -0.01 );
+	  } else {
+	    inc_view_tilt( 0.01 );
+	  }
+	}
+      if ( get_view_tilt() > SGD_2PI ) {
+	inc_view_tilt( -SGD_2PI );
+      } else if ( get_view_tilt() < 0 ) {
+	inc_view_tilt( SGD_2PI );
+      }
+    }
+  }
 }

src/Main/viewer.hxx

@@ -78,6 +78,10 @@ protected:
     // the goal view offset angle  (used for smooth view changes)
     double goal_view_offset;
 
+    // the view tilt angles
+    double view_tilt;
+    double goal_view_tilt;
+
     // geodetic view position
     sgdVec3 geod_view_pos;
 
@@ -168,6 +172,24 @@ public:
     inline void set_reverse_view_offset( bool val ) {
 	reverse_view_offset = val;
     }
+    inline void set_view_tilt( double a ) {
+	set_dirty();
+	view_tilt = a;
+    }
+    inline void inc_view_tilt( double amt ) {
+	set_dirty();
+	view_tilt += amt;
+    }
+    inline void set_goal_view_tilt( double a) {
+	set_dirty();
+	goal_view_tilt = a;
+	while ( goal_view_tilt < 0 ) {
+	    goal_view_tilt += 360.0;
+	}
+	while ( goal_view_tilt > 360.0 ) {
+	    goal_view_tilt -= 360.0;
+	}
+    }
     inline void set_geod_view_pos( double lon, double lat, double alt ) {
 	// data should be in radians and meters asl
 	set_dirty();
@@ -197,6 +219,8 @@ public:
     inline double get_view_offset() const { return view_offset; }
     inline bool get_reverse_view_offset() const { return reverse_view_offset; }
     inline double get_goal_view_offset() const { return goal_view_offset; }
+    inline double get_view_tilt() const { return view_tilt; }
+    inline double get_goal_view_tilt() const { return goal_view_tilt; }
     inline double *get_geod_view_pos() { return geod_view_pos; }
     inline float *get_pilot_offset() { return pilot_offset; }
     inline double get_sea_level_radius() const { return sea_level_radius; }