QImage in a QGraphics scene

Question

I am quite new to Qt. I am having troubles in inserting a QImage to a scene. Could somebody please tell me how to add a QImage to a QGraphicsScene?

Jeopardize

Answer 1

For this you would use a QGraphicsPixmapItem that you add to the scene like any other QGraphicsItem.

The QGraphicsPixmapItem can be initialized with a QPixmap which is a device-dependent representation of a bitmap and you can get it from a QImage for example with the static function QPixmap::fromImage().

Update (example code)

#include <QtGlobal>

#if QT_VERSION >= 0x050000
    #include <QtWidgets>
#else
    #include <QtGui>
#endif

int main(int argc, char *argv[]) {
    QApplication a(argc, argv);
    QImage image("test.png");

    QGraphicsPixmapItem item( QPixmap::fromImage(image));
    QGraphicsScene* scene = new QGraphicsScene;
    scene->addItem(&item);

    QGraphicsView view(scene);
    view.show();
    return a.exec();
}

Proctoscope

Hi, I tried this solution but it was not working for me. I have pasted my code below. – Jeopardize

Hi, I tried this solution but it was not working for me. I have pasted my code below. QGraphicsScene scene; QImage* qimage = new QImage("./IMG_2938.JPG"); QGraphicsPixmapItem pixmapItem(QPixmap::fromImage(*qimage)); scene.addItem(&pixmapItem); QGraphicsView view(&scene); view.show(); It doesn't show the image, just a window, but it just shows an emptybox.Hope someone can answer. – Jeopardize

I added some example code to my answer. This works for me (it is very close to what you posted). So two things come to mind: 1. Are you sure the image is found and loaded correctly? (check QImage::isNull() or use QImage::load() explictly and check the return value) 2. If you put objects on the stack instead of the heap, make sure they are still alive when they are used. – Proctoscope

The code in the example will not work after the function containing item returns, because item is destructed upon returning. – Wozniak

error: variable 'QGraphicsPixmapItem item' has initializer but incomplete type QGraphicsPixmapItem item( QPixmap::fromImage(image)); – Boeschen

The example was written for Qt4; I assume you are using Qt5 where the widgets got split of into their own module. I changed the example to compile for both cases. – Proctoscope

wouldn't the addPixmap() function of QGraphicsScene work? – Neddy

Answer 2

As @Neal suggested, addPixmap works much simpler and where the QGraphicsPixmapItem would not work for me for some unknown reason. Assuming you have a QGraphicsScene, QGraphicView and rest of program set up the below code will just display the image:

QString filename = "C:/image.png";
QImage image(fileName);
scene->addPixmap( QPixmap::fromImage(image)); 

Field