Addition of comments for ClusterCullingCallback

include/osg/ClusterCullingCallback

@@ -19,9 +19,33 @@
 
 namespace osg {
 
-/** Implements cluster culling to cull back facing drawables. Derived from
+///
-  * Drawable::CullCallback.
+/// @class ClusterCullingCallback
-*/
+/// @brief Implements cluster culling to cull back facing drawables. Derived from Drawable::CullCallback.
+///
+/// This culling callback is intended to be attached to a node using the setCullCallback method. If the
+/// node is a drawable cull(osg::NodeVisitor*, osg::Drawable*, osg::State*) otherwise
+/// operator()(Node*, NodeVisitor*) will be called during the cull traversal.
+///
+/// To decide whether the node (in case of a drawable) or its children (in case of any other node type) are
+/// to be culled depends on four parameters:
+///  - a control point,
+///  - a normal specified at the control point,
+///  - a deviation value representing the cosinus of an enclosed angle and
+///  - a radius describing a sphere around the control point.
+///
+/// The node is culled if the following two conditions are fulfilled:
+///  - the distance between the current eye/view point to the control point is larger or equal to radius,
+///  - the cosinus of the enclosed angle between the normal and the vector from the control point to the eye/view point
+///    is smaller(!) than the specified deviation value (normally this value is negative meaning that the enclosed angle
+///    between the control point and the eye/view point is larger than the angle indirectly specified by the
+///    deviation value).
+///
+/// @remark As the deviation is representing the cosinus of an enclosed angle its value should be within the
+///         the interval [-1; 1]. A value of one will cull all nodes while a value of -1 will never cull a node.
+///         The deviation will normally have negative values because then the enclosed angle between the normal and the
+///         eye/view point is larger than 90 degrees (and therefore the eye sees the "back" from the control point).
+///
 class OSG_EXPORT ClusterCullingCallback : public DrawableCullCallback, public NodeCallback
 {
     public: