SGLight: Add support for dimming through a SGExpression ala Rembrandt

simgear/scene/model/SGLight.cxx

@@ -27,6 +27,8 @@
 #include <simgear/math/SGMath.hxx>
 #include <simgear/scene/tgdb/userdata.hxx>
 
+#include "animation.hxx"
+
 class SGLightDebugListener : public SGPropertyChangeListener {
 public:
     SGLightDebugListener(osg::Switch *sw) : _sw(sw) {}
@@ -109,6 +111,17 @@ SGLight::appendLight(const SGPropertyNode *configNode,
     }
     align->setMatrix(r * t);
 
+    const SGPropertyNode *dim_factor = configNode->getChild("dim-factor");
+    if (dim_factor) {
+        const SGExpressiond *expression =
+            read_value(dim_factor, modelRoot, "", 0, 1);
+        light->setUpdateCallback(
+            new SGLight::UpdateCallback(expression,
+                                        light->getAmbient(),
+                                        light->getDiffuse(),
+                                        light->getSpecular()));
+    }
+
     osg::Shape *debug_shape = nullptr;
     if (light->getType() == SGLight::Type::POINT) {
         debug_shape = new osg::Sphere(osg::Vec3(0, 0, 0), light->getRange());

simgear/scene/model/SGLight.hxx

@@ -21,6 +21,7 @@
 #include <osg/Group>
 
 #include <simgear/props/props.hxx>
+#include <simgear/structure/SGExpression.hxx>
 
 class SGLight : public osg::Node {
 public:
@@ -29,6 +30,29 @@ public:
         SPOT
     };
 
+    class UpdateCallback : public osg::NodeCallback {
+    public:
+        UpdateCallback(const SGExpressiond *expression,
+                       osg::Vec4 ambient, osg::Vec4 diffuse, osg::Vec4 specular) :
+            _expression(expression),
+            _ambient(ambient), _diffuse(diffuse), _specular(specular) {}
+        virtual void operator()(osg::Node *node, osg::NodeVisitor *nv) {
+            double value = _expression->getValue();
+            if (value != _prev_value) {
+                SGLight *light = dynamic_cast<SGLight *>(node);
+                if (light) {
+                    light->setAmbient(_ambient * value);
+                    light->setDiffuse(_diffuse * value);
+                    light->setSpecular(_specular * value);
+                }
+            }
+        }
+    private:
+        SGSharedPtr<SGExpressiond const> _expression;
+        osg::Vec4 _ambient, _diffuse, _specular;
+        double _prev_value = -1.0;
+    };
+
     SGLight();
 
     SGLight(const SGLight& l,