Draw line on image using QPainter

[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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[Qt Coder]

It works for me also ...

But only if I dont add image to pixmap.

as soon as I add image

pixmap.load(QString::fromUtf8(":/BiFold/Images/Plane.PNG"));

pixmap.load(QString::fromUtf8(":/BiFold/Images/Plane.PNG"));

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it shows only image no red line..

[high_flyer]

show your full code.

[Qt Coder]

Doorcontrol1 is a QDialog in which I need to show that image and lines.


main.cpp

#include <QtGui/QApplication>
#include <QtCore>
#include "bifold.h"
 
int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
 
    BiFold w;
    w.show();
    return a.exec();
}

#include <QtGui/QApplication>
#include <QtCore>
#include "bifold.h"

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

    BiFold w;
    w.show();
    return a.exec();
}

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bifold.h

#ifndef BIFOLD_H
#define BIFOLD_H
#include <QMainWindow>
 
namespace Ui {
    class BiFold;
}
 
class DoorControl1;
 
class BiFold : public QMainWindow {
    Q_OBJECT
public:
    BiFold(QWidget *parent = 0);
    ~BiFold();
 
    DoorControl1 *ODoorControl1;
 
protected:
    void changeEvent(QEvent *e);
 
private:
    Ui::BiFold *ui;
};
 
#endif // BIFOLD_H

#ifndef BIFOLD_H
#define BIFOLD_H
#include <QMainWindow>

namespace Ui {
    class BiFold;
}

class DoorControl1;

class BiFold : public QMainWindow {
    Q_OBJECT
public:
    BiFold(QWidget *parent = 0);
    ~BiFold();

    DoorControl1 *ODoorControl1;

protected:
    void changeEvent(QEvent *e);

private:
    Ui::BiFold *ui;
};

#endif // BIFOLD_H

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bifold.cpp

#include "bifold.h"
#include "ui_bifold.h"
#include "doorcontrol1.h"
#include <QMessageBox>
 
BiFold::BiFold(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::BiFold)
{
    ui->setupUi(this);  
    removeToolBar(ui->mainToolBar);                 //Removes tool bar from main window
 
    ODoorControl1 = new DoorControl1(this);
    ODoorControl1->setWindowFlags( Qt::FramelessWindowHint );
 
 
}
 
BiFold::~BiFold()
{
    delete ui;
}
 
void BiFold::changeEvent(QEvent *e)
{
    QMainWindow::changeEvent(e);
    switch (e->type()) {
    case QEvent::LanguageChange:
        ui->retranslateUi(this);
        break;
    default:
        break;
    }
}

#include "bifold.h"
#include "ui_bifold.h"
#include "doorcontrol1.h"
#include <QMessageBox>

BiFold::BiFold(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::BiFold)
{
    ui->setupUi(this);  
    removeToolBar(ui->mainToolBar);                 //Removes tool bar from main window

    ODoorControl1 = new DoorControl1(this);
    ODoorControl1->setWindowFlags( Qt::FramelessWindowHint );


}

BiFold::~BiFold()
{
    delete ui;
}

void BiFold::changeEvent(QEvent *e)
{
    QMainWindow::changeEvent(e);
    switch (e->type()) {
    case QEvent::LanguageChange:
        ui->retranslateUi(this);
        break;
    default:
        break;
    }
}

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doorcontrol1.h

#ifndef DOORCONTROL1_H
#define DOORCONTROL1_H
 
#include <QDialog>
 
namespace Ui {
    class DoorControl1;
}
 
class DoorControl1 : public QDialog {
    Q_OBJECT
public:
    DoorControl1(QWidget *parent = 0);
    ~DoorControl1();
    void DrawStuff();
 
 
protected:
    void changeEvent(QEvent *e);
    void paintEvent(QPaintEvent *event);
 
private:
    Ui::DoorControl1 *ui;
};
 
#endif // DOORCONTROL1_H

#ifndef DOORCONTROL1_H
#define DOORCONTROL1_H

#include <QDialog>

namespace Ui {
    class DoorControl1;
}

class DoorControl1 : public QDialog {
    Q_OBJECT
public:
    DoorControl1(QWidget *parent = 0);
    ~DoorControl1();
    void DrawStuff();


protected:
    void changeEvent(QEvent *e);
    void paintEvent(QPaintEvent *event);

private:
    Ui::DoorControl1 *ui;
};

#endif // DOORCONTROL1_H

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doorcontrol1.cpp

#include <QtGui>
#include <QtCore>
#include <QMessageBox>
#include "doorcontrol1.h"
#include "ui_doorcontrol1.h"
 
DoorControl1::DoorControl1(QWidget *parent) :
    QDialog(parent),
    ui(new Ui::DoorControl1)
{
    ui->setupUi(this);
    DrawStuff();
}
 
DoorControl1::~DoorControl1()
{
    delete ui;
}
 
void DoorControl1::changeEvent(QEvent *e)
{
    QDialog::changeEvent(e);
    switch (e->type()) {
    case QEvent::LanguageChange:
        ui->retranslateUi(this);
        break;
    default:
        break;
    }
}
 
void DoorControl1::paintEvent(QPaintEvent *event )
 {
 
}
 
void DoorControl1::DrawStuff()
 {   
 
     ui->lbl_plane->setGeometry(QRect(100,150,611,301));
 
       QPixmap pixmap(500,500);
       pixmap.load(QString::fromUtf8(":/BiFold/Images/Plane.PNG"));
 
       QPainter painter(&pixmap);
       ui->lbl_plane->resize(pixmap.size());
 
       QPen Red((QColor::QColor(255,0,0)),1);
       painter.setPen(Red);
       painter.drawLine(250,300,500,300);
       painter.end();
       ui->lbl_plane->setPixmap(pixmap);
       ui->lbl_plane->show(); 
 
 
 
 }

#include <QtGui>
#include <QtCore>
#include <QMessageBox>
#include "doorcontrol1.h"
#include "ui_doorcontrol1.h"

DoorControl1::DoorControl1(QWidget *parent) :
    QDialog(parent),
    ui(new Ui::DoorControl1)
{
    ui->setupUi(this);
    DrawStuff();
}

DoorControl1::~DoorControl1()
{
    delete ui;
}

void DoorControl1::changeEvent(QEvent *e)
{
    QDialog::changeEvent(e);
    switch (e->type()) {
    case QEvent::LanguageChange:
        ui->retranslateUi(this);
        break;
    default:
        break;
    }
}

void DoorControl1::paintEvent(QPaintEvent *event )
 {

}

void DoorControl1::DrawStuff()
 {   
    
     ui->lbl_plane->setGeometry(QRect(100,150,611,301));

       QPixmap pixmap(500,500);
       pixmap.load(QString::fromUtf8(":/BiFold/Images/Plane.PNG"));

       QPainter painter(&pixmap);
       ui->lbl_plane->resize(pixmap.size());

       QPen Red((QColor::QColor(255,0,0)),1);
       painter.setPen(Red);
       painter.drawLine(250,300,500,300);
       painter.end();
       ui->lbl_plane->setPixmap(pixmap);
       ui->lbl_plane->show(); 



 }

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[JohannesMunk]

Can't you put your code into something simpler? Just one class, and without an UI-File, that you didn't provide!

Reducing complexity often helps to find problems in the first place, so you might even solve it yourself!

Johannes

PS: "Cross posting" meant, that you posted, while I was writing my message. After I posted mine.. I saw your new message and changed mine .. to "cross posting". After I read your message, I edited again :->