Draw line on image using QPainter

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

Code:

---

#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);

---

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

you can only draw in a paintEvent().

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

Code:

---

#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();
}

---

Joh

@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().

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

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

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

I m still not able to draw line on image

My code is

Code:

---

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();

---

The image format is "Format_RGB32"

what I m missing?

give your pixmap a size.

I have added line

Code:

---

QPixmap pixmap(533,292);

---

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

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

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

Code:

---

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();
}

---

It works for me also ...

But only if I dont add image to pixmap.

as soon as I add image

Code:

---

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

---

it shows only image no red line..

show your full code.

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


main.cpp

Code:

---

#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();
}

---

bifold.h

Code:

---

#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

---

bifold.cpp

Code:

---

#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;
    }
}

---

doorcontrol1.h

Code:

---

#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

---

doorcontrol1.cpp

Code:

---

#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(); 
 
 
 
 }

---

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 :->

what is mean by "Cross posting :-> " ?????

you reimplemented the paintEvent() in your dialog, and left it blank.
So your dialog does not do any painting,I am not sure if this will cause its children not to draw as well.
Delete the paintEvent() re implementation and see if this helps.

I have removed reimplementation of paintEvent () from header as well as source file

but still no line is drawn on image .....

in BiFold construcotr add:
ODoorControl1->show();

It's already showing DoorControl1 Dialog with Plane's image...

still as you said I added ODoorControl1->show(); in Bifold constructor ,but no line still drawn..

you have to simplify and go step at a time.
Put your dialog in main, and show it.
See if you get the line drawn then.

I have added draw line code in main.cpp itself but still it only shows image and no line is drawn

main.cpp

Code:

---

#include <QtGui/QApplication>
 
#include <QtCore>
#include <QtGui>
#include <QLabel>
//#include "bifold.h"
 
 
int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
 
   // BiFold w;
  //  w.show();
    QLabel l;
    QPixmap pixmap(500,500);
    pixmap.load(QString::fromUtf8(":/BiFold/Images/Plane.PNG"));
 
    QPainter painter(&pixmap);
    l.resize(pixmap.size());
 
 
    QPen Red((QColor::QColor(255,0,0)),1);
    painter.setPen(Red);
    painter.drawLine(250,300,500,300);
    painter.end();
    l.setPixmap(pixmap);
    l.show();
    return a.exec();
}

---

Is this a fault of my image,but I tried changing the image still no line is drawn..

I read in documentation of QPainter "Painting on a QImage with the format QImage::Format_Indexed8 is not supported."


But my image format is QImage::Format_RGB32.


I m not able to upload image here,it says "upload failed"


pls show me right direction ,I cannot proceed my work if this issues is not solved.

Are you sure that your resource file path is good?

Your code with some minor tweaks and a local file:

Code:

---

#include <QtCore>
#include <QtGui>
#include <QDebug>
 
int main(int argc, char *argv[])
{ 
    QApplication app(argc, argv);
 
    QLabel l;
    QPixmap pixmap;
 
    bool goodLoad = pixmap.load(QString::fromUtf8("./test.png"));
    qDebug() << goodLoad << pixmap.isNull() << pixmap.size() << pixmap.depth();
 
    QPainter painter(&pixmap);
    QPen Red((QColor::QColor(255,0,0)),1);
    painter.setPen(Red);
    painter.drawLine(40, 40, 200, 280);
    painter.end();
 
    pixmap.save("./test2.png");
 
    l.setPixmap(pixmap);
    l.show();
 
    return app.exec();
}

---

Input is:
Attachment 4840
Output is:

Code:

---

true false QSize(240, 320) 24

---

and
Attachment 4839

Edit: the forum converted the PNG files to JPG

Finally the line is visible over the image

The co-ordinates I used for drawing line were main culprit here
painter.drawLine(250,300,500,300);

Code:

---

also
 
 qDebug() << goodLoad << pixmap.isNull() << pixmap.size() << pixmap.depth();
 
shows
 
true false QSize(533, 292) 32

---

if I use ur co-ordinate,

painter.drawLine(40, 40, 200, 280);

line is drawn.....

My next query is

Can we directly paint QDialog ?

I need to draw some arcs, squares and lines on QDialog

You can paint on any widget, but only in a paintEvent().

hai to every one am new to qt actually am trying to draw line on image (qpixmap),i had tried all coeds u have given above but still i didnt get the result plz can any one give full code to me thanking you and sorry for my bad english

How to drag that line over the image?
I tried to drag. But it does not update.