Use QPainter with QPixmap contain QImage

[kevin32]

Hi everyone, again me can you help me please ?

I convert Mat image to qimage and display in qlabel

QImage img= QImage((uchar*) frame.data, frame.cols, frame.rows, frame.step, QImage::Format_RGB888);
    // create Qpixmap from Qimage
    QPixmap pix = QPixmap::fromImage(img);
 // display the pixmap on the label
    ui->label->setPixmap(pix);

I want to use painter for display other image, my code dont work

   QPixmap pix = QPixmap::fromImage(img);
// problem here
        QPainter *painter=new QPainter(pix);
        painter->drawPixmap(0, 0, 100, 100, QPixmap("C://Users//user//Pictures//a.png"));
        painter->end();
       ui->label->setPixmap(*pix);

error: no matching function for call to 'QPainter::QPainter(QPixmap&)'
         QPainter *painter=new QPainter(pix);
                                           ^
..\Double_video\mainwindow.cpp:44:43: note: candidates are:
In file included from ..\..\..\..\Qt\5.5\mingw492_32\include\QtGui/QPainter:1:0,
                 from ..\Double_video\mainwindow.h:4,
                 from ..\Double_video\mainwindow.cpp:1:
..\..\..\..\Qt\5.5\mingw492_32\include\QtGui/qpainter.h:117:14: note: QPainter::QPainter(QPaintDevice*)
     explicit QPainter(QPaintDevice *);
              ^
..\..\..\..\Qt\5.5\mingw492_32\include\QtGui/qpainter.h:117:14: note:   no known conversion for argument 1 from 'QPixmap' to 'QPaintDevice*'
..\..\..\..\Qt\5.5\mingw492_32\include\QtGui/qpainter.h:116:5: note: QPainter::QPainter()
     QPainter();
     ^

how use qpainter with existing qpixmap and no create new ?

thanks :)

[A Former User]

It's QPainter::QPainter(QPaintDevice *device) so instead of...

QPainter *painter=new QPainter(pix);

it has to be...

QPainter *painter=new QPainter(&pix);

BTW: Do you really need to create the painter in this way? Maybe it's better to just:

QPainter painter(&pix);

[kevin32]

@Wieland thanks, but i have ```

 error: cannot convert 'QPixmap*' to 'QPainter*' in initialization
         QPainter *painter(&pix);

 // create Qpixmap from Qimage
    QPixmap pix = QPixmap::fromImage(img);


        QPainter *painter(&pix);
        painter->drawPixmap(0, 0, 100, 100, QPixmap("C://Users//user//Pictures//a.png"));
        painter->end();
       ui->label->setPixmap(pix);

Have you a idea please?

[A Former User]

You have a typo in your code:

QPainter *painter(&pix);

Must be:

QPainter painter(&pix);

[kevin32]

@Wieland thank you very much again, work very well :)

[A Former User]

@kevin32 Great :)