inline const SGVec4f& sky_color () const { return _sky_color; }
inline const SGVec4f& cloud_color () const { return _cloud_color; }
inline const SGVec4f& adj_fog_color () const { return _adj_fog_color; }
inline const SGVec4f& adj_sky_color () const { return _adj_sky_color; }
inline const SGVec4f& sky_color () const { return _sky_color; }
inline const SGVec4f& cloud_color () const { return _cloud_color; }
inline const SGVec4f& adj_fog_color () const { return _adj_fog_color; }
inline const SGVec4f& adj_sky_color () const { return _adj_sky_color; }