Initialising OpenCV-VideoCapture-class in another multithreading class

Question

Im trying to initialise VideoCapture capturedevice once in my multithreading program. Once initialised it should serve in the acquireImage-thread as the image-buffer, which is filled by a webcam. The following code shows me a "OpenCV Error: Assertion failed (func != 0) in cv::imshow"-error during runtime, which means VideoCapture capturedevice is never really initialised and therefore imshow doesnt have necessary data, but why?

The code was constructed based on this question: Correctly using mutex in OpenCL-OpenCV-Realtime-Threads? Basically it uses two threads, one processes images originating from a usb-webcam and the second will find faces on those images...but does nothing at the moment for code-simplicity.

#include <memory>
#include <thread>
#include <mutex>
#include <condition_variable>
#include <vector>
#include <cmath>
#include <opencv2/objdetect/objdetect.hpp>
#include <opencv2/highgui/highgui.hpp>
#include <opencv2/imgproc/imgproc.hpp>
#include "opencv2/ocl/ocl.hpp"
#include "opencv2/opencv.hpp"
#include <functional>

using namespace std;
using namespace cv;

typedef unsigned char uchar;
typedef unsigned int uint;

class FaceDetector
{
mutex imageLock, facesLock;
condition_variable imageAqcuired;
bool newImageAvailable;

Mat _img;
Mat _imgToWorkOn;
Mat _faceImages;
bool quit;

VideoCapture captureDevice;
int device_id;
FaceDetector (int _device_id);

void acquireImage()
{
while (!quit)
{

    unique_lock<mutex> ulock(imageLock);
    imageAqcuired.wait(ulock,[&](){return !newImageAvailable;}); //only take new image after current one was consumed

    Mat captureFrame;
    captureDevice>>captureFrame;
    transpose(captureFrame,captureFrame);
    flip(captureFrame,captureFrame,1);
    _img = captureFrame.clone();

    ulock.unlock();
    newImageAvailable = true;
    imageAqcuired.notify_one(); //notify that a new image is available
}
}
void processImage()
{
while (!quit)
{
    unique_lock<mutex> ulock(imageLock);
    imageAqcuired.wait(ulock,[&](){return newImageAvailable;}); //wait untill a new image is available
    _imgToWorkOn = _img.clone();
    ulock.unlock();
    newImageAvailable = false;
    imageAqcuired.notify_one(); //notify the current image can be replaced by a newer one
    unique_lock<mutex> lockFace(facesLock);
    //arbeit
    lockFace.unlock();
}
}

public:
FaceDetector() : newImageAvailable(false) {}
void start() {
quit = false;
thread t1(&FaceDetector::acquireImage,this);
t1.detach();
thread t2(&FaceDetector::processImage,this);
t2.detach();
}
void stop() {
quit = true;
}
Mat getImage() {
if (quit)
    return Mat();
lock_guard<mutex> lock(imageLock);
return _img;
}

Mat getProcessedImage() {
if (quit)
    return Mat();
lock_guard<mutex> lock(facesLock);
return _faceImages;
}

};

FaceDetector::FaceDetector(int _device_id)
{

device_id = _device_id;
captureDevice.open(device_id);
captureDevice.set(CV_CAP_PROP_FRAME_WIDTH,620);             //erst jetzt cam.set weil sonst system-pause nicht funzt
captureDevice.set(CV_CAP_PROP_FRAME_HEIGHT,480);

}

int main()
{
bool quit(false);
FaceDetector faceDet;
faceDet.start();
thread input([](bool &quitFlag) { getchar(); quitFlag = true; },ref(quit)); //stop on user press Enter
input.detach();
while (!quit) {


Mat img = faceDet.getImage();
Mat imgc = img.clone();

imshow("links", imgc);

/*
imgs = faceDet.getProcessedImage();
Mat imgsc = imgs.clone();

imshow("gsichter", imgsc);
*/
waitKey(30);
this_thread::sleep_for(chrono::milliseconds(33)); //no need to show more than 30 fps...
}
faceDet.stop();
return 0;
}

Edit: I tried to include your answer, still getting "Opencv Assertion Error in imshow".

#include <memory>
#include <thread>
#include <mutex>
#include <condition_variable>
#include <vector>
#include <cmath>
#include <opencv2/objdetect/objdetect.hpp>
#include <opencv2/highgui/highgui.hpp>
#include <opencv2/imgproc/imgproc.hpp>
#include "opencv2/ocl/ocl.hpp"
#include "opencv2/opencv.hpp"
#include <functional>

using namespace std;
using namespace cv;

typedef unsigned char uchar;
typedef unsigned int uint;

class FaceDetector
{
mutex imageLock, facesLock;
condition_variable imageAqcuired;
bool newImageAvailable;

Mat _img;
Mat _imgToWorkOn;
Mat _faceImages;
bool quit;

VideoCapture captureDevice;
int device_id;

void acquireImage()
{
while (!quit)
{

    unique_lock<mutex> ulock(imageLock);
    imageAqcuired.wait(ulock,[&](){return !newImageAvailable;}); //only take new image after current one was consumed

    Mat captureFrame;
    captureDevice>>captureFrame;
    //transpose(captureFrame,captureFrame);
    //flip(captureFrame,captureFrame,1);
    _img = captureFrame.clone();

    ulock.unlock();
    newImageAvailable = true;
    imageAqcuired.notify_one(); //notify that a new image is available
}
}
void processImage()
{
while (!quit)
{
    unique_lock<mutex> ulock(imageLock);
    imageAqcuired.wait(ulock,[&](){return newImageAvailable;}); //wait untill a new image is available
    _imgToWorkOn = _img.clone();
    ulock.unlock();
    newImageAvailable = false;
    imageAqcuired.notify_one(); //notify the current image can be replaced by a newer one
    unique_lock<mutex> lockFace(facesLock);
    //arbeit
    lockFace.unlock();
}
}

public:

FaceDetector() : newImageAvailable(false) {}
void start() {
quit = false;
thread t1(&FaceDetector::acquireImage,this);
t1.detach();
thread t2(&FaceDetector::processImage,this);
t2.detach();
}
void stop() {
quit = true;
}
Mat getImage() {
if (quit)
    return Mat();
lock_guard<mutex> lock(imageLock);
return _img;
}

Mat getProcessedImage() {
if (quit)
    return Mat();
lock_guard<mutex> lock(facesLock);
return _faceImages;
}

FaceDetector::FaceDetector(int _device_id)
{
VideoCapture captureDevice;
device_id = _device_id;
captureDevice.open(device_id);
captureDevice.set(CV_CAP_PROP_FRAME_WIDTH,620);             //erst jetzt cam.set weil sonst system-pause nicht funzt
captureDevice.set(CV_CAP_PROP_FRAME_HEIGHT,480);
}

};

int main()
{
bool quit(false);
FaceDetector faceDet(0);
faceDet.start();
thread input([](bool &quitFlag) { getchar(); quitFlag = true; },ref(quit)); //stop on     user press Enter
input.detach();
while (!quit) {


Mat img = faceDet.getImage();
Mat imgc = img.clone();

imshow("links", imgc);

/*
imgs = faceDet.getProcessedImage();
Mat imgsc = imgs.clone();

imshow("gsichter", imgsc);
*/
waitKey(30);
this_thread::sleep_for(chrono::milliseconds(33)); //no need to show more than 30 fps...
}
faceDet.stop();
return 0;
}

I also tried following code, but still getting said Assertion Error.

public:
FaceDetector(int device_id) : newImageAvailable(false) {}
....
void init() {
VideoCapture captureDevice;
captureDevice.open(device_id);
captureDevice.set(CV_CAP_PROP_FRAME_WIDTH,620);             //erst jetzt cam.set weil sonst system-pause nicht funzt
captureDevice.set(CV_CAP_PROP_FRAME_HEIGHT,480);
}

int main()
{
FaceDetector faceDet(0);
faceDet.init();
faceDet.start();
}

the VideoCapture is already running in a thread of its own. you won't gain anything, wrapping another thread around it. if you're doing cascade-detection, that's a candidate. — berak

Pervez Alam Pervez Alam · Accepted Answer · 2014-08-21T06:55:57

In main() function You are not creating object faceDet using constructor FaceDetector::FaceDetector(int _device_id) but you are using default constructor. This means you are not opening captureDevice at all.

Edit for correction

In the declaration, make FaceDetector::FaceDetector(int _device_id) public. Now in main(), create object faceDet using this constructor, you need to call like this:

FaceDetector faceDet(0); // I have taken 0 as as default camera ID, you can add other value like 1 or 2 etc depending the choice of camera too.

This should be be working now, please let me know of any problem should you face.

Initialising OpenCV-VideoCapture-class in another multithreading class

1 Answers