From Martin Beckett, added get/setScrollWheelZoomDelta() support for controlling the mouse scroll wheel zoom delta.

include/osgGA/TrackballManipulator

@@ -78,6 +78,13 @@ class OSGGA_EXPORT TrackballManipulator : public MatrixManipulator
         /** get the minimum distance (as ratio) the eye point can be zoomed in */
         double getMinimumZoomScale() const { return _minimumZoomScale; }
 
+        /** set the mouse scroll wheel zoom delta.
+          * Range -1.0 to +1.0,  -ve value inverts wheel direction and zero switches off scroll wheel. */
+        void setScroolWheelZoomDelta(double zoomDelta) { _zoomDelta = zoomDelta; }
+
+        /** get the mouse scroll wheel zoom delta. */
+        double getScroolWheelZoomDelta() const { return _zoomDelta; }
+
         /** Set the center of the trackball. */
         void setCenter(const osg::Vec3d& center) { _center = center; }
 
@@ -139,6 +146,7 @@ class OSGGA_EXPORT TrackballManipulator : public MatrixManipulator
         osg::Quat    _rotation;
         double       _distance;
         float        _trackballSize;
+        float        _zoomDelta;
 
 };
 

src/osgGA/TrackballManipulator.cpp

@@ -14,6 +14,7 @@ TrackballManipulator::TrackballManipulator()
 
     _distance = 1.0f;
     _trackballSize = 0.8f;
+    _zoomDelta = 0.1f;
 }
 
 
@@ -245,7 +246,7 @@ void TrackballManipulator::computePosition(const osg::Vec3& eye,const osg::Vec3&
 
 bool TrackballManipulator::calcMovement()
 {
-    // mosue scroll is only a single event
+    // mouse scroll is only a single event
     if (_ga_t0.get()==NULL) return false;
 
     float dx=0.0f;
@@ -254,7 +255,7 @@ bool TrackballManipulator::calcMovement()
 
     if (_ga_t0->getEventType()==GUIEventAdapter::SCROLL)
     {
-        dy = _ga_t0->getScrollingMotion() == osgGA::GUIEventAdapter::SCROLL_DOWN ? -0.1 : 0.1;
+        dy = _ga_t0->getScrollingMotion() == osgGA::GUIEventAdapter::SCROLL_UP ? _zoomDelta : -_zoomDelta;
         buttonMask=GUIEventAdapter::SCROLL;
     } 
     else