OpenCV / Python : multi-threading for live facial recognition

Question

I'm using OpenCv and Dlib to execute facial recognition w/ landmarks, live from the webcam stream. The language is Python. It works fine on my macbook laptop, but I need it to run from a desktop computer 24/7. The computer is a PC Intel® Core™2 Quad CPU Q6600 @ 2.40GHz 32bit running Debian Jessie. The drop in performance is drastic : there is a 10 seconds delay due to processing !

I therefore looked into multi-threading to gain performance :

1. I first tried the sample code by OpenCv, and the result is great! All four cores hit 100%, and the performance is much better.
2. I then replaced the frame processing code with my code, and it doesn't improve performance at all ! Only one core hits the 100%, the other ones stay very low. I even think it's worse with multi-threading on.

I got the facial landmark code from the dlib sample code. I know it can probably be optimized, but I want to understand why am I not able to use my (old) computer's full power with multi-threading ?

I'll drop my code below, thanks a lot for reading :)

from __future__ import print_function

import numpy as np
import cv2
import dlib

from multiprocessing.pool import ThreadPool
from collections import deque

from common import clock, draw_str, StatValue
import video

class DummyTask:
    def __init__(self, data):
        self.data = data
    def ready(self):
        return True
    def get(self):
        return self.data

if __name__ == '__main__':
    import sys

    print(__doc__)

    try:
        fn = sys.argv[1]
    except:
        fn = 0
    cap = video.create_capture(fn)
    
    #Face detector
    detector = dlib.get_frontal_face_detector()

    #Landmarks shape predictor 
    predictor = dlib.shape_predictor("landmarks/shape_predictor_68_face_landmarks.dat")

    # This is where the facial detection takes place
    def process_frame(frame, t0, detector, predictor):
        # some intensive computation...
        gray = cv2.cvtColor(frame, cv2.COLOR_BGR2GRAY)
        clahe = cv2.createCLAHE(clipLimit=2.0, tileGridSize=(8,8))
        clahe_image = clahe.apply(gray)
        detections = detector(clahe_image, 1)
        for k,d in enumerate(detections): 
            shape = predictor(clahe_image, d) 
            for i in range(1,68): #There are 68 landmark points on each face
               cv2.circle(frame, (shape.part(i).x, shape.part(i).y), 1, (0,0,255), thickness=2)
        return frame, t0

    threadn = cv2.getNumberOfCPUs()
    pool = ThreadPool(processes = threadn)
    pending = deque()

    threaded_mode = True

    latency = StatValue()
    frame_interval = StatValue()
    last_frame_time = clock()
    while True:
        while len(pending) > 0 and pending[0].ready():
            res, t0 = pending.popleft().get()
            latency.update(clock() - t0)
            draw_str(res, (20, 20), "threaded      :  " + str(threaded_mode))
            draw_str(res, (20, 40), "latency        :  %.1f ms" % (latency.value*1000))
            draw_str(res, (20, 60), "frame interval :  %.1f ms" % (frame_interval.value*1000))
            cv2.imshow('threaded video', res)
        if len(pending) < threadn:
            ret, frame = cap.read()
            t = clock()
            frame_interval.update(t - last_frame_time)
            last_frame_time = t
            if threaded_mode:
                task = pool.apply_async(process_frame, (frame.copy(), t, detector, predictor))
            else:
                task = DummyTask(process_frame(frame, t, detector, predictor))
            pending.append(task)
        ch = cv2.waitKey(1)
        if ch == ord(' '):
            threaded_mode = not threaded_mode
        if ch == 27:
            break
cv2.destroyAllWindows()

Answer 1

Performance issue was due to a bad compilation of dlib. Do not use pip install dlib which runs very very slowly for some reason compared to the proper compilation. I went from almost 10 seconds lag to about 2 seconds this way. So finally I didn't need multi-threading/processing, but I'm working on it to enhance the speed even more. Thanks for the help :)

Answer 2

i tried a simplified approach like P.Ro mentioned in his answer with processes writing to an output queue but somehow the queue got locked most of the time because all the processes wrote to it at the same time. (just my guess) i probably did something wrong.

in the end i ended up using pipes.

the code is nasty. but if i was me a few hours ago. i would still be glad to find an example that actually runs without effort.

from multiprocessing import Process, Queue, Manager,Pipe
import multiprocessing
import face_recognition as fik
import cv2
import time


video_input = 0

obama_image = fik.load_image_file("obama.png")
obama_face_encoding = fik.face_encodings(obama_image)[0]



quality = 0.7


def f(id,fi,fl):
    import face_recognition as fok

    while True:
        small_frame = fi.get()
        print("running thread"+str(id))
        face_locations = fok.face_locations(small_frame)

        if(len(face_locations)>0):
            print(face_locations)
            for (top7, right7, bottom7, left7) in face_locations:

                small_frame_c = small_frame[top7:bottom7, left7:right7]
                fl.send(small_frame_c)

fps_var =0
if __name__ == '__main__':
        multiprocessing.set_start_method('spawn')


        # global megaman
        with Manager() as manager:

            video_capture = cv2.VideoCapture(video_input)

            fi = Queue(maxsize=14)

            threads = 8
            proc = []

            parent_p = []
            thread_p = []
            # procids = range(0,threads)
            for t in range(0,threads):
                p_t,c_t = Pipe()
                parent_p.append(p_t)
                thread_p.append(c_t)
                print(t)
                proc.append(Process(target=f, args=(t,fi,thread_p[t])))
                proc[t].start()


            useframe = False

            frame_id = 0
            while True:
                # Grab a single frame of video
                ret, frame = video_capture.read()
                effheight, effwidth = frame.shape[:2]
                if effwidth < 20:
                    break
                # Resize frame of video to 1/4 size for faster face recognition processing
                xxx = 930
                yyy = 10/16 #0.4234375
                small_frame = cv2.resize(frame, (xxx, int(xxx*yyy)))
                if frame_id%2 == 0:
                    if not fi.full():


                        fi.put(small_frame)

                        print(frame_id)

                        cv2.imshow('Video', small_frame)


                        print("FPS: ", int(1.0 / (time.time() - fps_var)))
                        fps_var = time.time()


                #GET ALL DETECTIONS
                for t in range(0,threads):
                    if parent_p[t].poll():
                        small_frame_c = parent_p[t].recv()
                        cv2.imshow('recc', small_frame_c)
                        height34, width34 = small_frame_c.shape[:2]
                        # print fsizeee
                        if(width34<20):
                            print("face 2 small")
                            print(width34)
                            break
                        face_encodings_cam = fik.face_encodings(small_frame_c,[(0, width34, height34, 0)])

                        match = fik.compare_faces([obama_face_encoding], face_encodings_cam[0])
                        name = "Unknown"

                        if match[0]:
                            name = "Barack"

                        print(name)
                        break

                frame_id += 1

                # Hit 'q' on the keyboard to quit!
                if cv2.waitKey(1) & 0xFF == ord('q'):
                    break

Answer 3

Do not have much experience with using ThreadPool, but I always just use Process like shown below. You should be able to easily edit this code to fit your needs. I wrote this with your implementation in mind.

This code will get the number of cores and start however many worker processes that will all be implementing the desired function in parallel. They all share a Queue of frames for input and all put to the same output Queue for the main to get and show. Each Queue has a maximum size, in this case 5. This ensures that despite the CPU time it takes to process, it will always be relatively live time.

import numpy as np
import cv2

from multiprocessing import Process, Queue
import time

#from common import clock, draw_str, StatValue
#import video

class Canny_Process(Process):
    
    def __init__(self,frame_queue,output_queue):
        Process.__init__(self)
        self.frame_queue = frame_queue
        self.output_queue = output_queue
        self.stop = False
        #Initialize your face detectors here
        

    def get_frame(self):
        if not self.frame_queue.empty():
            return True, self.frame_queue.get()
        else:
            return False, None

    def stopProcess(self):
        self.stop = True
            
    def canny_frame(self,frame):
        # some intensive computation...
        gray = cv2.cvtColor(frame, cv2.COLOR_BGR2GRAY)
        edges = cv2.Canny(gray, 50, 100)
        
        #To simulate CPU Time
        #############################
        for i in range(1000000):
            x = 546*546
            res = x/(i+1)
        #############################
        'REPLACE WITH FACE DETECT CODE HERE'

        if self.output_queue.full(): 
            self.output_queue.get_nowait()
        self.output_queue.put(edges)

    def run(self):
        while not self.stop: 
            ret, frame = self.get_frame()
            if ret: 
                self.canny_frame(frame)


if __name__ == '__main__':

    frame_sum = 0
    init_time = time.time()

    def put_frame(frame):
        if Input_Queue.full(): 
            Input_Queue.get_nowait()
        Input_Queue.put(frame)

    def cap_read(cv2_cap):
        ret, frame = cv2_cap.read()
        if ret: 
            put_frame(frame)
        
    cap = cv2.VideoCapture(0)

    threadn = cv2.getNumberOfCPUs()

    threaded_mode = True

    process_list = []
    Input_Queue = Queue(maxsize = 5)
    Output_Queue = Queue(maxsize = 5)

    for x in range((threadn -1)):    
        canny_process = Canny_Process(frame_queue = Input_Queue,output_queue = Output_Queue)
        canny_process.daemon = True
        canny_process.start()
        process_list.append(canny_process)

    ch = cv2.waitKey(1)
    cv2.namedWindow('Threaded Video', cv2.WINDOW_NORMAL)
    while True:        
        cap_read(cap)
        
        if not Output_Queue.empty():
            result = Output_Queue.get()
            cv2.imshow('Threaded Video', result)
            ch = cv2.waitKey(5)

        if ch == ord(' '):
            threaded_mode = not threaded_mode
        if ch == 27:
            break
    cv2.destroyAllWindows()

This should do the trick just change my canny function to do your face detection. I wrote this from your code and compared the two. This is significantly faster. I am using multiprocessing.Process here. In python processes are truly parallel and threads are not quite because of the GIL. I am using 2 queues to send data back and forth between the main and the processes. Queues are both Thread and Process safe.

Answer 4

you may use this, multithreaded:

from imutils.video import VideoStream

# Initialize multithreading the video stream.
videostream = "rtsp://192.168.x.y/user=admin=xxxxxxx_channel=vvvv=1.sdp?params"
vs = VideoStream(src=videostream, resolution=frameSize,
                 framerate=32).start()

frame = vs.read()