Program throws this error and SIGSEV when i try to set a pixmap for a Qlabel: glib-gobject-critical ** g_object_unref: assertion 'g_is_object (object)' failed qt

[Manu_NaFu]

Hello, I have the following problem: I have an opencv::mat. When i resize it and create a Qimagen with that in a thread, I send it to the mainWindow (via slot-signal print_img()) to use it as a pixmap in a label. However, I get a SIGSEV and the error I wrote on the title.

I will put the code here, thanks in advance:

void MainWindow::print_img(const QImage &img)
{

    ui->map->setPixmap(QPixmap::fromImage(img.rgbSwapped()));

}

//simulation.cpp - run()
while(something)
{
        cv::Mat mat = map.getMap();
        cv::resize(mat,mat,cv::Size(800,800));
        const uchar *qImage = (const uchar*)mat.data;
        QImage img(qImage, mat.cols, mat.rows, mat.step, QImage::Format_RGB888);
        emit(print_img(img));
        usleep(1000);
}

[SGaist]

Hi,

The QImage constructor you are using does not make a copy of the data from the buffer. That buffer is likely already destroyed when the slot executes.

[Manu_NaFu]

@SGaist Does it matter? Shouldn't it put the pixmap first?

If it does, what constructor should I use?

[SGaist]

From the looks of it you are using multiple threads so yes it does matter.

As the easiest way to do that, use the copy method of QImage.

[Manu_NaFu]

@SGaist I created a QImage in my class and did this:

        QImage img(qImage, mat.cols, mat.rows, mat.step, QImage::Format_RGB888);
        *image = img.copy();
        emit(print_img(*image));

But I still get the error.

I tried eliminating the oart where i resize my map and then the image is loaded fine, but with the dimensions i do not want.

The map i resize is the map where i get my image:

cv::Mat mat = map.getMap();

And I resize it with:

cv::resize(map_,map_,cv::Size(width, heigth));

Thanks for the help.

[Christian Ehrlicher]

@Manu_NaFu said in Program throws this error and SIGSEV when i try to set a pixmap for a Qlabel: glib-gobject-critical ** g_object_unref: assertion 'g_is_object (object)' failed qt:

*image = img.copy();

Why a pointer here?

[Manu_NaFu]

@Christian-Ehrlicher because it doesnt let me declare a QImage in the class so I created a Pointer.

[jsulm]

@Manu_NaFu said in Program throws this error and SIGSEV when i try to set a pixmap for a Qlabel: glib-gobject-critical ** g_object_unref: assertion 'g_is_object (object)' failed qt:

because it doesnt let me declare a QImage in the class

There shouldn't be a problem declaring QImage in your class. How did you declare it and what was the error?

[Manu_NaFu]

@jsulm O declared it as: QImage img;

And it said I needed a constructor.

[jsulm]

@Manu_NaFu Did you include QImage header file?

[Manu_NaFu]

@jsulm I did, i created the pointer as QImage *image;
Instead of a normal variable.

[Christian Ehrlicher]

@Manu_NaFu said in Program throws this error and SIGSEV when i try to set a pixmap for a Qlabel: glib-gobject-critical ** g_object_unref: assertion 'g_is_object (object)' failed qt:

Instead of a normal variable.

What is a 'non-normal' variable??

Again: there is no need to declare it as pointer and if you declare it as such you a) have to properly create it with new and b) should clean it up later on. But both is not needed when you declare it as simple object.

[Manu_NaFu]

@Christian-Ehrlicher Somehow now I could declare it. Anyway, it seems to me a problem related to cv::Mat memory allocation instead of Qt. It only fails when I resize the cv::Mat from where i get the image to use as pixmap, so now I don't really know if I should have posted this here.

[Manu_NaFu]

@Manu_NaFu Update: it fails when I both resize the cv::Mat and try to use it as pixmap. If I only resize the mat works fine, and if I only use it as pixmap, it also works fine, but for any reason I cannot do both

[JonB]

@Manu_NaFu
To be clear: are you resizing your cv data before or after you assign it to the QImage? If the QImage or whatever is using the original data buffer passed to it, you cannot execute anything which changes that buffer. Just checking..

[Manu_NaFu]

@JonB I have a class called map, which contains an image, I resize that image at the start of the program, for example to be 200x200. Then i get that image and resize it to be the size of the Qlabel, which is 800x800, then i transform it into a QImage to use it for the pixmap. For that purpose, I do the following:

mat = map.getMap();
cv::resize(mat,mat,cv::Size(800,800));
QImage img((const uchar*)mat.data, mat.cols, mat.rows, mat.step, QImage::Format_RGB888);
emit(print_img(img.copy()));
usleep((10000/speed_)*1000);

This code is in a while loop

[Manu_NaFu]

I solved the issue by moving the map from the class variables to the run function(), so I do not have to resize the map.