Make star cutoff controllable at runtime.

When a new property /environment/star-magnitude-cutoff is defined,
use this to set an upper bound on the magnitude of stars displayed.

simgear/scene/sky/sky.cxx

@@ -94,7 +94,7 @@ void SGSky::build( double h_radius_m,
     planets = new SGStars;
     _ephTransform->addChild( planets->build(eph.getNumPlanets(), eph.getPlanets(), h_radius_m) );
 
-    stars = new SGStars;
+    stars = new SGStars(property_tree_node);
     _ephTransform->addChild( stars->build(eph.getNumStars(), eph.getStars(), h_radius_m) );
     
     moon = new SGMoon;

simgear/scene/sky/stars.cxx

@@ -30,9 +30,11 @@
 #include <simgear/compiler.h>
 #include <simgear/constants.h>
 #include <simgear/debug/logstream.hxx>
+#include <simgear/props/props.hxx>
 
 #include <stdio.h>
 #include <iostream>
+#include <algorithm>
 
 #include <osg/AlphaFunc>
 #include <osg/BlendFunc>
@@ -47,9 +49,14 @@
 #include "stars.hxx"
 
 // Constructor
-SGStars::SGStars( void ) :
-old_phase(-1)
+SGStars::SGStars( SGPropertyNode* props ) :
+    old_phase(-1)
 {
+    if (props) {
+        // don't create here - if it's not defined, we won't use the cutoff
+        // from a property
+        _cutoffProperty = props->getNode("star-magnitude-cutoff");
+    }
 }
 
 
@@ -161,10 +168,17 @@ bool SGStars::repaint( double sun_angle, int num, const SGVec3d star_data[] ) {
         cutoff = 0.0;
         phase = 7;
     }
-
-    if( phase != old_phase ) {
+    
+    if (_cutoffProperty) {
+        double propCutoff = _cutoffProperty->getDoubleValue();
+        cutoff = std::min(propCutoff, cutoff);
+    }
+    
+    if ((phase != old_phase) || (cutoff != _cachedCutoff)) {
 	// cout << "  phase change, repainting stars, num = " << num << endl;
         old_phase = phase;
+        _cachedCutoff = cutoff;
+
         for ( int i = 0; i < num; ++i ) {
             // if ( star_data[i][2] < min ) { min = star_data[i][2]; }
             // if ( star_data[i][2] > max ) { max = star_data[i][2]; }
@@ -190,11 +204,8 @@ bool SGStars::repaint( double sun_angle, int num, const SGVec3d star_data[] ) {
             if (alpha < 0.0) { alpha = 0.0; }
 
             (*cl)[i] = osg::Vec4(1, 1, 1, alpha);
-            // cout << "alpha[" << i << "] = " << alpha << endl;
         }
         cl->dirty();
-    } else {
-	// cout << "  no phase change, skipping" << endl;
     }
 
     // cout << "min = " << min << " max = " << max << " count = " << num 

simgear/scene/sky/stars.hxx

@@ -33,18 +33,20 @@
 
 #include <simgear/math/SGMath.hxx>
 #include <simgear/structure/SGReferenced.hxx>
-
+#include <simgear/props/propsfwd.hxx>
 
 class SGStars : public SGReferenced {
 
     osg::ref_ptr<osg::Vec4Array> cl;
 
     int old_phase;		// data for optimization
-
+    
+    double _cachedCutoff = 0.0;
+    SGPropertyNode_ptr _cutoffProperty;
 public:
 
     // Constructor
-    SGStars( void );
+    SGStars( SGPropertyNode* props = nullptr);
 
     // Destructor
     ~SGStars( void );