diff --git a/simgear/scene/viewer/ClusteredShading.cxx b/simgear/scene/viewer/ClusteredShading.cxx
index cb325544..25d15247 100644
--- a/simgear/scene/viewer/ClusteredShading.cxx
+++ b/simgear/scene/viewer/ClusteredShading.cxx
@@ -194,10 +194,9 @@ ClusteredShading::update(const SGLightList &light_list)
     _slice_bias->set(-_depth_slices * log2(_zNear) / log2(_zFar / _zNear));
 
     const osg::Viewport *vp = _camera->getViewport();
-    static int old_width = 0, old_height = 0;
     int width = vp->width(); int height = vp->height();
-    if (width != old_width || height != old_height) {
-        old_width = width; old_height = height;
+    if (width != _old_width || height != _old_height) {
+        _old_width = width; _old_height = height;
 
         _n_htiles = (width  + _tile_size - 1) / _tile_size;
         _n_vtiles = (height + _tile_size - 1) / _tile_size;
diff --git a/simgear/scene/viewer/ClusteredShading.hxx b/simgear/scene/viewer/ClusteredShading.hxx
index 1fa6366a..1bf3b9f8 100644
--- a/simgear/scene/viewer/ClusteredShading.hxx
+++ b/simgear/scene/viewer/ClusteredShading.hxx
@@ -79,6 +79,9 @@ protected:
     float                           _zNear = 0.0f;
     float                           _zFar = 0.0f;
 
+    int                             _old_width = 0;
+    int                             _old_height = 0;
+
     int                             _n_htiles = 0;
     int                             _n_vtiles = 0;