Issue with threading and opencv

[Kira]

@jsulm
Queued signals do not block anything - they are just in a queue. The only delay you will have is the slot when it is actually called - but that you will have anyway as you have to execute the code anyway, right? : Yes

Thanks for the help cleared a lot of implementation doubts.
Just a small question. If i have a thread which acquires the lock and there is one more thread which requests for the locked resource. How will i tell the other thread wait until and unless the lock in the thread have not been unlocked.
We can currently use the wait time but is there any way to tell when create a notification when the lock is released and tell the other thread to wait for that specific time.

And one more thing how to highlight specific lines which we are answering. Which you have did for my earlier questions. :)

[jsulm]

@Kira said in Issue with threading and opecv:

How will i tell the other thread wait until

That's exactly what a mutex is doing: it blocks the thread which tries to lock a mutex which is already locked by other thread, as soon as the other thread releases the mutex the OS will wake up one of the blocked (waiting) threads. So, you do not have to tell the thread anything.

[Kira]

@jsulm : Hello i have implemented the save image function in a slot as suggested by you.
But the problem is that the image is not being saved. I would like to share the given code:

signals:
void signalSaveImage(cv::Mat,String row,String column);
slots:
public slots:
void saveImage(Mat finalImage,String row, String column);
Slot Definition:

void MainWindow::saveImage(Mat finalImage, String row, String column)
{
    cout<<"Inside image save function"<<endl;
    cout<<"Row"<<row<<endl;
    cout<<"Column"<<column<<endl;
    String imagePath = "D:/testN/"
    String imageFormat = ".bmp";
    try {
           cv::imwrite(imagePath+row+"_"+column+imageFormat,finalImage);
       }
       catch (cv::Exception& ex) {
           fprintf(stderr, "Exception in saving image: %s\n", ex.what());
       }
       fprintf(stdout, "Saved PNG file with alpha data.\n");

    cout<<"\n Entering inside save mutex"<<endl;

    cout<<"\n Completed image save function"<<endl;


}

Signal to slot connection:
connect(this,SIGNAL(signalSaveImage(cv::Mat,String,String)),this,SLOT(saveImage(Mat,String,String)),Qt::QueuedConnection);

MainWindow.cpp
Emitting signal to save image:
emit signalSaveImage(image,to_string(row),to_string(column));

But my problem is that my images are not being saved.

[J.Hilk]

@Kira said in Issue with threading and opencv:

QueuedConnection

forces a copy of the arguments, The MetaSystem doesn't know the type cv::Mat -> can't be transmitted via QueuedConnection.

Check your console output you should have warning of unknown metatyps (declare and/or register)

[Kira]

@J.Hilk : I have registered it.
#include "mainwindow.h"
#include <QApplication>

int main(int argc, char *argv[])
{
qRegisterMetaType< cv::Mat >("cv::Mat");
QApplication a(argc, argv);
MainWindow w;
w.show();

return a.exec();

}

[J.Hilk]

@Kira

the data goes most likely out of scope, as the copy constructor of cv::Mat does actually no coping

I had the same issues in my time with opencv.

ended up converting it to a Image before transferring it from the opencv thread to the normal one.

[Kira]

@J.Hilk : Just for clarification should i replace this part with cv::clone
void saveImage(Mat finalImage,String row, String column);

[Kira]

@J-Hilk : will try and also i tried to run the program in debug mode and getting the following error.

Debugger redirects to following line of file of ostream:
// MANIPULATORS
template<class _Elem,
class _Traits> inline
basic_ostream<_Elem, _Traits>&
__CLRCALL_OR_CDECL endl(basic_ostream<_Elem, _Traits>& _Ostr)
{ // insert newline and flush stream
_Ostr.put(_Ostr.widen('\n'));
_Ostr.flush();
return (_Ostr);
}

And the program quits.
Can u please explain what may be the relevant reason for error

[jsulm]

@Kira Please post the stack trace after crash

[Kira]

@jsulm : I thought this is the stack trace after crash

[jsulm]

@Kira said in Issue with threading and opencv:

this is the stack trace after crash

no, it isn't

[Kira]

@jsulm : This is the only thing which i see after crash with exception triggered.
Is there any other way to do so

[jsulm]

@Kira In QtCreator if you start with debugger you will have a stack trace

[Kira]

@jsulm : Are you referring to this:

Please let me know in case of any issues

[jsulm]

@Kira Yes. Now you need to go from top to bottom until you hit first source code file from your project and check it at the line mentioned in the stack trace.

[Kira]

@jsulm : I tried doing it but the error is expected to come after thread running >150 times which is making the debugging process very difficult.
I have also sample test case on github can you please go through it for any possible issues.
https://github.com/HackersSpirit/ImageProcessor

[J.Hilk]

@Kira said in Issue with threading and opencv:

150 times

you can tell a breakpoint to be ignored for a certain amount of time

[jsulm]

@Kira I actually suggested to go through the stack trace after crash from top to bottom. No break points needed for that.

[Kira]

@jsulm :
@Kira said in Issue with threading and opencv:

@jsulm : Are you referring to this:

Please let me know in case of any issues

This is the stack trace which i get posted earlier referring from line no 1. It does not point to line of code where program has a break.
But line 16 and 17 point to the line of the code.
Should i refer those line for particular cause of the program to break
Please do let me know if i getting something wrong

[jsulm]

@Kira Check your code in that source code at line 16 and 17.