请解释下这段代码namespace cros { // FaceTracker takes a set of face data produced by FaceDetector as input, // filters the input, and produces the bounding rectangle that encloses the // filtered input. class FaceTracker { public: struct Options { // The dimension of the active sensory array in pixels. Used for normalizing // the input face coordinates. Size active_array_dimension; // The dimension of the active stream that will be cropped. Used for // translating the ROI coordinates in the active array space. Size active_stream_dimension; // The threshold in ms for including a newly detected face for tracking. int face_phase_in_threshold_ms = 3000; // The threshold in ms for excluding a face that's no longer detected for // tracking. int face_phase_out_threshold_ms = 2000; // The angle range [|pan_angle_range|, -|pan_angle_range|] in degrees used // to determine if a face is looking at the camera. float pan_angle_range = 30.0f; }; explicit FaceTracker(const Options& options); ~FaceTracker() = default; FaceTracker(FaceTracker& other) = delete; FaceTracker& operator=(FaceTracker& other) = delete; // Callback for when new face data are ready. void OnNewFaceData(const std::vector<human_sensing::CrosFace>& faces); // The all the rectangles of all the detected faces. std::vector<Rect<float>> GetActiveFaceRectangles() const; // Gets the rectangle than encloses all the detected faces. Returns a // normalized rectangle in [0.0, 1.0] x [0.0, 1.0] with respect to the active // stream dimension. Rect<float> GetActiveBoundingRectangleOnActiveStream() const; void OnOptionsUpdated(const base::Value& json_values); private: struct FaceState { Rect<float> normalized_bounding_box = {0.0f, 0.0f, 0.0f, 0.0f}; base::TimeTicks first_detected_ticks; base::TimeTicks last_detected_ticks; bool has_attention = false; }; Options options_; std::vector<FaceState> faces_; }; } // namespace cros