Draw a line with draggable

[cskumar]

Hi to all,
I am trying to drag a line in QPainter using mouse event. It is working in without any label and etc. But I am trying to Draw a line over the image with drag able. The line is not update. I attached my code. Please help me to drag a line over the image.

MainWindow::MainWindow(QWidget *parent) :
QMainWindow(parent),
ui(new Ui::MainWindow)
{
ui->setupUi(this);
label1 = new QLabel(this);
QPixmap pm1("download.jpg");
label1->setPixmap(pm1);
}
 
void MainWindow :: mouseMoveEvent(QMouseEvent *e)
{
 
mFirstX = e->x();
mFirstY = e->y();
this->update();
 
}
 
void MainWindow :: paintEvent(QPaintEvent * e)
{
QImage tmp(label1->pixmap()->toImage());
QPainter painter(&tmp);
QPen paintpen(Qt::red);
painter.setPen(paintpen);
painter.drawLine(0,mFirstY,500,mFirstY);
label1->setPixmap(QPixmap::fromImage(tmp));
}

MainWindow::MainWindow(QWidget *parent) :
QMainWindow(parent),
ui(new Ui::MainWindow)
{
ui->setupUi(this);
label1 = new QLabel(this);
QPixmap pm1("download.jpg");
label1->setPixmap(pm1);
}

void MainWindow :: mouseMoveEvent(QMouseEvent *e)
{

mFirstX = e->x();
mFirstY = e->y();
this->update();

}

void MainWindow :: paintEvent(QPaintEvent * e)
{
QImage tmp(label1->pixmap()->toImage());
QPainter painter(&tmp);
QPen paintpen(Qt::red);
painter.setPen(paintpen);
painter.drawLine(0,mFirstY,500,mFirstY);
label1->setPixmap(QPixmap::fromImage(tmp));
}

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[anda_skoa]

If you want to move a line, not repeatedly add a line, you'll have to start with the clean image every time you draw.

Keep the loaded image as a member of the widget in which you implement your mouse handling.
Then, when you need to, draw the line on a copy of that image and then draw that image (or let it be drawn by QLabel).

Instead of hacking things into a QMainWindow derived class, I would suggest to do a custom widget:

- derive from QWidget
- add a constructor argument that takes the file name
- in the constructor load the image and keep it as a member
- in sizeHint() you return the size of the image
- in paintEvent() you draw the image and then the line
- in mouseMoveEvent() you change the stored coordinates and call update()

Cheers,
_

[cskumar]

Hi,
I drawn two lines over the image (i.e.) Horizontal and vertical. I can able to drag a lines. But I need to drag a line separately with highlight.

My code is,

void Widget :: paintEvent(QPaintEvent * )
{
        QImage tmp(label1->pixmap()->toImage());
        QPainter painter(&tmp);
        QPen paintpen(Qt::black);
        paintpen.setWidth(1);
        painter.setPen(paintpen);
        painter.drawLine(0,mFirstY,500,mFirstY);
 
        QPen paintpen1(Qt::red);
        paintpen1.setWidth(1);
        painter.setPen(paintpen1);
        painter.drawLine(mFirstX,0,mFirstX,200);
 
        label1->setPixmap(QPixmap::fromImage(tmp));
}

void Widget :: paintEvent(QPaintEvent * )
{
        QImage tmp(label1->pixmap()->toImage());
        QPainter painter(&tmp);
        QPen paintpen(Qt::black);
        paintpen.setWidth(1);
        painter.setPen(paintpen);
        painter.drawLine(0,mFirstY,500,mFirstY);

        QPen paintpen1(Qt::red);
        paintpen1.setWidth(1);
        painter.setPen(paintpen1);
        painter.drawLine(mFirstX,0,mFirstX,200);

        label1->setPixmap(QPixmap::fromImage(tmp));
}

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[anda_skoa]

I don't understand what you mean.
You are still drawing into the same image over and over again.

Cheers,
_

[cskumar]

hi,
i had drawn lines over image as Vertical and horizontal lines (i.e ) x -axis and y-axis and to drag the line using mouse event as per your idea , in this case while i dragging the line of Vertical means it as moving the horizontal line also and this the problem had faced ,Actually i need to drag the lines separately

[anda_skoa]

I would suggest you make dragging one line work before trying to attempt to do two.

Once you have one, the other one is trivial, you just need to decide how you want to distinguish between the two, e.g. direction of mouse movement or which line is closer to the point where the mouse press occurs, etc.

Cheers,
_

[d_stranz]

Why aren't you using QRubberBand for this? That's what it is designed for.