Controlling an A.R. Drone with the Kinect and Clojure

Yet another round of code dump, Clojure based gesture controller for AR Drone. I started coding without doing a google search to see if there is a simple-openni (OpenNI and NITE wrapper for Processing) wrapper for Clojure. I found out about bifocals, a kinect library for quil while uploading dependencies to Clojars. You can use that instead and skip directly to the core namespace.

(ns kinect-ardrone.class
  (:import (processing.core PApplet)))

(defonce kinect (atom nil))

 :name         com.nakkaya.SkelTracker
 :extends      processing.core.PApplet
 :constructors {[] []}
 :methods      [[onNewUser [int] void]
                [onEndCalibration [int boolean] void]
                [onStartPose [String int] void]])

(defn -onNewUser
  [this id]
  (.startPoseDetection @kinect "Psi" id))

(defn -onEndCalibration
  [this id successful]
  (if successful
    (.startTrackingSkeleton @kinect id)
    (.startPoseDetection @kinect "Psi" id)))

(defn -onStartPose [this pose id]
  (.stopPoseDetection @kinect id)
  (.requestCalibrationSkeleton @kinect id true))

For simple-openni to work correctly we need to implement a couple of callback functions. Problem is proxy can not add new methods to the class that are needed for simple-openni to work and gen-class can not overwrite methods (setup / draw) needed for Processing to work. So we generate SkelTracker class that extends PApplet with the callbacks implemented. Later we are going to proxy SkelTracker instead of PApplet to override setup and draw.

Several callback functions are necessary for skeleton tracking. onNewUser is called when a new user enters the field of view. We then call the startPoseDetection to tell the OpenNI to begin looking for calibration poses. Next we define onStartPose which is called when the library recognises that a user is beginning a calibration pose. We then tell the library to stop attempting to recognise a pose and tell it to calibrate the user's skeleton. onEndCalibration is called at the end of the calibration process. It is passed a boolean argument which indicates whether the calibration was successful or not. If the calibration was successful we start tracking the skeleton otherwise we restart pose detection.

(compile 'kinect-ardrone.class)

Compile it so we can proxy it.

(ns kinect-ardrone.skel-tracker
  (:use kinect-ardrone.class)
  (:import (processing.core PApplet PVector)
           (SimpleOpenNI SimpleOpenNI IntVector)))

(defn draw-skeleton [id]
  (.drawLimb @kinect id SimpleOpenNI/SKEL_HEAD SimpleOpenNI/SKEL_NECK)
  (.drawLimb @kinect id SimpleOpenNI/SKEL_NECK SimpleOpenNI/SKEL_LEFT_SHOULDER)
  (.drawLimb @kinect id SimpleOpenNI/SKEL_LEFT_SHOULDER SimpleOpenNI/SKEL_LEFT_ELBOW)
  (.drawLimb @kinect id SimpleOpenNI/SKEL_LEFT_ELBOW SimpleOpenNI/SKEL_LEFT_HAND)
  (.drawLimb @kinect id SimpleOpenNI/SKEL_NECK SimpleOpenNI/SKEL_RIGHT_SHOULDER)
  (.drawLimb @kinect id SimpleOpenNI/SKEL_RIGHT_SHOULDER SimpleOpenNI/SKEL_RIGHT_ELBOW)
  (.drawLimb @kinect id SimpleOpenNI/SKEL_RIGHT_SHOULDER SimpleOpenNI/SKEL_RIGHT_ELBOW)
  (.drawLimb @kinect id SimpleOpenNI/SKEL_RIGHT_ELBOW SimpleOpenNI/SKEL_RIGHT_HAND)
  (.drawLimb @kinect id SimpleOpenNI/SKEL_LEFT_SHOULDER SimpleOpenNI/SKEL_TORSO)
  (.drawLimb @kinect id SimpleOpenNI/SKEL_RIGHT_SHOULDER SimpleOpenNI/SKEL_TORSO)
  (.drawLimb @kinect id SimpleOpenNI/SKEL_TORSO SimpleOpenNI/SKEL_LEFT_HIP)
  (.drawLimb @kinect id SimpleOpenNI/SKEL_LEFT_HIP SimpleOpenNI/SKEL_LEFT_KNEE)
  (.drawLimb @kinect id SimpleOpenNI/SKEL_LEFT_KNEE SimpleOpenNI/SKEL_LEFT_FOOT)
  (.drawLimb @kinect id SimpleOpenNI/SKEL_TORSO SimpleOpenNI/SKEL_RIGHT_HIP)
  (.drawLimb @kinect id SimpleOpenNI/SKEL_RIGHT_HIP SimpleOpenNI/SKEL_RIGHT_KNEE)
  (.drawLimb @kinect id SimpleOpenNI/SKEL_RIGHT_KNEE SimpleOpenNI/SKEL_RIGHT_FOOT))

(let [font (.createFont (PApplet.) "Arial" 30 true)]
  (defn draw-text [applet text]
    (.textFont applet font 30)
    (.fill applet 204 102 0)
    (.text applet (str text) (float 10) (float 50))))

simple-openni has a function for drawing a line between the joints called drawLimb. It takes the id of, one of the skeletons being tracked and two joints, the joint to draw the line from and the joint to the line draw to. draw-text just puts a text to the top left corner of the image for debugging purposes.

(defn joint-position [id joint]
  (let [pvector (PVector.)
        conf (.getJointPositionSkeleton @kinect id joint pvector)]
    (when (> conf 0.5)

Joint positions are in millimeters. (0,0,0) is the position of the sensor. +X axis goes to the left, +Y goes to the top and +Z goes towards the Kinect sensor. We only return a joint position if the OpenNI's confidence for that joint is above 0.5.

(defn closest-skel-id []
  (->> (range 10)
       (filter #(true? (.isTrackingSkeleton @kinect %)))
       (map #(vector % (joint-position % SimpleOpenNI/SKEL_TORSO)))
       (remove #(nil? (second %)))
       (map first)
       (sort-by #(.dist (PVector. 0 0 0)
                        (joint-position % SimpleOpenNI/SKEL_TORSO)))

Kinect can track multiple people but we can't let them control the drone all at once so closest-skel-id checks the first 10 skeletons Kinect is tracking then sorts them according to their distances to the Kinect sensor, closest torso gets to control the drone.

(def joints (atom {}))
(def gesture (atom :none))

(defn applet []
  (proxy [com.nakkaya.SkelTracker] [] 
    (setup []
      (swap! kinect (fn [_] (SimpleOpenNI. this)))
      (.enableDepth @kinect)
      (.enableUser @kinect SimpleOpenNI/SKEL_PROFILE_ALL)
      (.stroke this 0 0 255)
      (.strokeWeight this 3)
      (.smooth this)
      (.size this (.depthWidth @kinect) (.depthHeight @kinect)))

    (draw []
      (.update @kinect)
      (.image this (.depthImage @kinect) 0 0)
      (draw-text this @gesture)
      (if-let [id (closest-skel-id)]
        (do (draw-skeleton id)
            (swap! joints
                     (fn [a b] (if (nil? b) a b))
                     {:left-hand (joint-position id SimpleOpenNI/SKEL_LEFT_HAND)
                      :right-hand (joint-position id SimpleOpenNI/SKEL_RIGHT_HAND)
                      :left-shoulder (joint-position id SimpleOpenNI/SKEL_LEFT_SHOULDER)
                      :right-shoulder (joint-position id SimpleOpenNI/SKEL_RIGHT_SHOULDER)
                      :neck (joint-position id SimpleOpenNI/SKEL_NECK)
                      :left-hip (joint-position id SimpleOpenNI/SKEL_LEFT_HIP)
                      :right-hip (joint-position id SimpleOpenNI/SKEL_RIGHT_HIP)
                      :torso (joint-position id SimpleOpenNI/SKEL_TORSO)})))))))

(defn skel-tracker []
  (let [applet (applet)
        frame (javax.swing.JFrame. "kinect-ardrone")]
    (.init applet)
    (doto frame
      (-> (.getContentPane) (.add applet))
      (.setSize 640 480)
      (.setDefaultCloseOperation javax.swing.JFrame/DO_NOTHING_ON_CLOSE)
      (.setVisible true))))

Proxy SkelTracker so we can overwrite setup and draw. setup sets up the atom holding a reference to OpenNI library then turns on the depth camera and tells the OpenNI to enable the skeleton tracking. Finally it applies some cosmetic changes such as stoke, smooth etc. Everytime draw is ticked, it will update the depth image on the camera (otherwise you get the same image over and over again), paint the image on to the canvas, paint any debug messages gesture atom holds. Finally we draw the closest skeleton to the Kinect and update joints map with the new joint positions.

sudo rmmod gspca_kinect

If you are on Linux and get this error: Failed to set USB interface!, remove the gspca\kinect module.

(ns kinect-ardrone.core
  (:refer-clojure :exclude [sequence])
  (:use kinect-ardrone.skel-tracker)
  (:use [alter-ego.core]

(defn gesture-takeoff? []
  (let [{:keys [left-hand right-hand right-hip torso]} @joints]
    (and (> (- (.y left-hand) (.y right-hand)) 500)
         (> (.y left-hand) (.y torso))
         (< (.y right-hand) (.y right-hip)))))

(defn gesture-land? []
  (let [{:keys [left-hand right-hand right-hip left-hip]} @joints]
    (and (< (.y right-hand) (.y right-hip))
         (< (.y left-hand) (.y left-hip)))))

First two gestures are self explanatory, you raise your left hand over your torso and you lower your right hand below your right hip for take off and when both hands are below your hips that is the gesture for land.

(defn scale-force [a b]
  (scale (clamp (- a b) 0 100) 0 100 0 0.3))

(defn roll-input []
  (let [{:keys [left-hand neck left-shoulder]} @joints]
     (< (.x left-hand) (.x left-shoulder))
     (* -1 (scale-force (.x left-shoulder) (.x left-hand)))
     (> (.x left-hand) (.x neck))
     (scale-force (.x left-hand) (.x neck))
     ;;dead zone
     :default 0)))

(defn pitch-input []
  (let [{:keys [right-hand left-hand torso]} @joints
        torso-z (- (.z torso) 200)]
     (> (.z right-hand) torso-z)
     (* -1 (scale-force (.z right-hand) torso-z))
     (< (.z right-hand) (- torso-z 200))
     (scale-force (- torso-z 200) (.z right-hand))
     ;;dead zone
     :default 0)))

Gestures only controls the roll and pitch angles. Altitude and yaw are controlled by PID controllers. With roll and pitch it is actually fun to fly when you map all angles to gestures it just turns into a workout session.

For roll, the area between your left shoulder and neck is the dead zone the further you move your hand away from your left shoulder the harder it will try to steer right. Same applies for steering left, the further you move your hand from your neck the harder it will try to steer left.

Same idea applies for pitch but instead of x values we check z values for the torso so moving your right hand towards Kinect, away from your torso moves the drone backwards, moving your hand away from Kinect moves it forward, area around your torso is the dead zone.

How hard/fast it steers in one direction is determined by scale-force. It takes two numbers (i.e. x values for the left hand and neck) the bigger the difference between these values the stronger the force. Basically it is a P controller that maps 0 - 100 to 0 - 0.3.

(defpid alt-hold
  :kp 0.50
  :ki 1/400
  :kd 1/10
  :set-point 1.5
  :bounds [0 3 -1 1])

(defpid yaw-hold
  :kp 2
  :ki 1/20
  :kd 0
  :set-point -13
  :bounds [-180 180 -1 1])

Above mentioned PID controllers for altitude and yaw. yaw set point is set to -13 because gestures assume the drone's nose is pointing towards the location of Kinect.

(defn fly []
   (action (swap! gesture (fn [_] :user-control)))

      (Thread/sleep 250)
      (let [{:keys [alt battery yaw]} (nav-data)]
        (println :alt alt :battery battery :yaw yaw)

      (Thread/sleep 50)
      (let [roll-input (roll-input)
            pitch-input (pitch-input)]
        (attitude pitch-input roll-input nil nil))))

      (Thread/sleep 10)
      (attitude nil nil
                (yaw-hold (:yaw (nav-data)))
                (alt-hold (:alt (nav-data)))))))))

For flying we use 3 threads, first thread prints out debug information (altitude, battery, yaw) every 250 milliseconds, second thread reads roll and pitch input every 50 milliseconds and send it to the drone, third thread sends yaw and altitude corrections to the drone every 10 milliseconds.

(defn land-seq []
  (action (hover)
          (dotimes [_ 10] (land))

(defn take-off-seq []
  (action (reset-comm-watchdog)

(defn wait-for [f msg]
    (action (Thread/sleep 20)
   (action (swap! gesture (fn [_] msg)))))

(defn control []
   (wait-for #(> (count (keys @joints)) 7) :user-calibrated)
    (sequence (action (swap! gesture (fn [_] :waiting)))
              (wait-for gesture-takeoff? :takeoff)
              (wait-for #(> (:alt (nav-data)) 0.5) :alt-wait)
               (wait-for gesture-land? :landing)

control puts all of the above together. It will first wait for a user to be calibrated after that in an infinite loop it will wait for a takeoff gesture, execute takeoff sequence wait until the drones is half a meter in the air and let the user take control of it. interrupter will execute fly sequence while checking for a land gesture as soon as it detect a land gesture it will interrupt fly sequence and execute land-seq.

  (exec-repl (control) (land-seq))


(defproject kinect-ardrone "0.1.0-SNAPSHOT"
  :dependencies [[org.clojure/clojure "1.4.0"]
                 [alter-ego "0.0.5-SNAPSHOT"]
                 [pid "0.1.0-SNAPSHOT"]
                 [ardrone "0.1.0-SNAPSHOT"]
                 [org.clojars.processing-core/org.processing.core "1.5.1"]
                 [simple-open-ni "0.27.0"]]
  :jvm-opts ["-server"])