Draw line on image using QPainter

[Qt Coder]

I m using Qt 4.6.2 on windows xp

I need to draw horizontal ,vertical lines and an arc over the image .


My code is

#include <QtGui>
 
     QPixmap pixmap;
     QPainter painter;    
     QPen DarkGray((QColor::QColor(69,86,96)),1);
 
     pixmap.load(QString::fromUtf8(":/BiFold/Images/Plane.PNG"));
     painter.begin(&pixmap);
     painter.setPen(DarkGray);
     painter.drawLine(250,300,500,300);
     painter.end();
    ui->lbl_plane->setGeometry(QRect(100,150,611,301));
     ui->lbl_plane->setPixmap(pixmap);

#include <QtGui>

     QPixmap pixmap;
     QPainter painter;    
     QPen DarkGray((QColor::QColor(69,86,96)),1);
   
     pixmap.load(QString::fromUtf8(":/BiFold/Images/Plane.PNG"));
     painter.begin(&pixmap);
     painter.setPen(DarkGray);
     painter.drawLine(250,300,500,300);
     painter.end();
    ui->lbl_plane->setGeometry(QRect(100,150,611,301));
     ui->lbl_plane->setPixmap(pixmap);

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But only image is displayed ,no line is drawn....

[high_flyer]

you can only draw in a paintEvent().

[JohannesMunk]

You just need to setup your pixmap before you assign the painter:

#include <QtCore>
#include <QtGui>
 
int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
 
    QLabel l;
 
    QPixmap pixmap;
    pixmap.load(QString::fromUtf8("E:/2010-02-25_112406.png"));
 
    QPainter painter(&pixmap);
    QPen Red((QColor::QColor(255,0,0)),1);
    painter.setPen(Red);
    painter.drawLine(50,50,250,250);
    l.setPixmap(pixmap);
    l.show();
 
    return a.exec();
}

#include <QtCore>
#include <QtGui>

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);

    QLabel l;

    QPixmap pixmap;
    pixmap.load(QString::fromUtf8("E:/2010-02-25_112406.png"));

    QPainter painter(&pixmap);
    QPen Red((QColor::QColor(255,0,0)),1);
    painter.setPen(Red);
    painter.drawLine(50,50,250,250);
    l.setPixmap(pixmap);
    l.show();

    return a.exec();
}

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Joh

[high_flyer]

@Joh,
did you try the code you posted?
This code should not work.
At the very least in runtime a Qt warning should be printed, saying that QPainter can not be used outside the paintEvent().

[JohannesMunk]

Sure I tried it! That's why I posted it as a complete application.

It doesn't show any warnings on the console, either. Why should it not be possible to use a QPainter on an 'offline' QPixMap?

Joh

[high_flyer]

you are right, QPixmap is not a widget.
Sorry.

[JohannesMunk]

Aah! Allright! That was the source of the confusion!

Yes, and not beeing able to draw onto a widget outside the paintEvent makes perfect sense.

Hope we didn't confuse the OP :->

Joh

[Qt Coder]

I m still not able to draw line on image

My code is

QPixmap pixmap; 
 
     QPen GreenPen((QColor::QColor (0,255,0,255)),1); 
 
     ui->lbl_plane->setGeometry(QRect(100,150,611,301));
     pixmap.load(QString::fromUtf8(":/BiFold/Images/Plane.PNG"));
     ui->lbl_plane->resize(pixmap.size());
 
        QPainter painter(&pixmap);      
        painter.setPen(GreenPen);
        painter.drawLine(250,300,500,300);       
        ui->lbl_plane->setPixmap(pixmap);
        ui->lbl_plane->show();

QPixmap pixmap; 
    
     QPen GreenPen((QColor::QColor (0,255,0,255)),1); 
  
     ui->lbl_plane->setGeometry(QRect(100,150,611,301));
     pixmap.load(QString::fromUtf8(":/BiFold/Images/Plane.PNG"));
     ui->lbl_plane->resize(pixmap.size());

        QPainter painter(&pixmap);      
        painter.setPen(GreenPen);
        painter.drawLine(250,300,500,300);       
        ui->lbl_plane->setPixmap(pixmap);
        ui->lbl_plane->show();

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The image format is "Format_RGB32"

what I m missing?

[high_flyer]

give your pixmap a size.

[Qt Coder]

I have added line

QPixmap pixmap(533,292);

QPixmap pixmap(533,292);

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which is my image size to above code but still no line is drawn on image....

[high_flyer]

try adding painter.end() after the drawLine().

[high_flyer]

Try the following code.
Its works I tested it.
Once it works for you see what it is you are doing wrong:

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
 
    QLabel l;
    QPixmap pixmap(500,500);
    QPainter painter(&pixmap);
    QPen Red((QColor::QColor(255,0,0)),1);
    painter.setPen(Red);
    painter.drawLine(50,50,250,250);
    l.setPixmap(pixmap);
    l.show();
 
    return a.exec();
}

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);

    QLabel l;
    QPixmap pixmap(500,500);
    QPainter painter(&pixmap);
    QPen Red((QColor::QColor(255,0,0)),1);
    painter.setPen(Red);
    painter.drawLine(50,50,250,250);
    l.setPixmap(pixmap);
    l.show();

    return a.exec();
}

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