Initialising OpenCV-VideoCapture-class in another multithreading class - c++

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();
}

Answer 1

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.