Draw a line in Scene with click mouse.

[AlvaroS]

Hello everyone:

First of all thanks a lot for reading this post and being able to help.

I would like to draw a line in a Scene.
User click on the Start point and on End point of the line.

i do this:

void MainWindow::mousePressEvent(QMouseEvent * e)
{

    if (generate_traj){
        number_click_press++;
        QPointF pt = e->pos();

        /*Paint in color each head arrow line*/
        QPainter painter1(this);
        QPen blue(Qt::blue); //Draw lines in blue
        blue.setCosmetic(true); //Users can select easier the lines.
        blue.setWidthF(0.3); //Lines width
        blue.setBrush(Qt::blue);
        painter1.setPen(blue);

        /*Fill brush each head arrow*/

        /*Add head arrow to the scene*/

        if (number_click_press>2){
            QLineF line_traj;
            line_traj.setP1(last_press);
            line_traj.setP2(pt);
            scene->addLine(line_traj,blue);
        }
        last_press=pt;
    }
}

if does not give me any errors. But line is not painted...

And when I do click on the scene it says me this:
QWidget::paintEngine: Should no longer be called
QPainter::begin: Paint device returned engine == 0, type: 1
QPainter::setPen: Painter not active

how can i do that?
thanks!

[mrjj]

Hi
You are not allowed to use QPainter OUTSIDE of paintEvent. :)

please see here
http://www.walletfox.com/course/qgraphicsitemruntimedrawing.php

[AlvaroS]

@mrjj hello, first of all thanks a lot for answering!

My code looks like this:

.CPP

void MainWindow::mousePressEvent(QGraphicsSceneMouseEvent *event)
{
        qDebug()<<"in musubclass mouse press event: ";

    if (generate_traj){
        number_click_press++;

        QPointF pt = event->scenePos();

        qDebug()<<"in musubclass mouse press event: "<< pt;



        /*Paint in color each head arrow line*/
        QPen blue(Qt::blue); //Draw lines in blue
        blue.setCosmetic(true); //Users can select easier the lines.
        blue.setWidthF(0.3); //Lines width
        blue.setBrush(Qt::blue);

        if (number_click_press>=2){
            QLineF line_traj;
            line_traj.setP1(last_press);
            line_traj.setP2(pt);
            scene->addLine(line_traj,blue);
        }
        last_press=pt;
    }
}

Header:

protected:
    void mousePressEvent(QGraphicsSceneMouseEvent *event);

The problem is that code never enters in that function....

However if i write QMouseEvent instead of QGraphicsSceneMouseEvent it enters but I need just that it enters if user click on the scene, and with QMouseEvent it enters if user clicks wherever.

Wha t am i doing wrong?

Thanks a lot!

[mrjj]

@AlvaroS said:

MainWindow::mousePressEvent

You override it for main window?
Normally you would use QGraphicsScene or QGraphicsView since
its the object u click on.

Also for mainwindow, I doubt it would use
QGraphicsSceneMouseEvent as that is for QGraphicsView.

Please see link. it does exactly what you want.
http://www.walletfox.com/course/qgraphicsitemruntimedrawing.php

It creates a custom scene.
class Scene : public QGraphicsScene

You can just use that class and then you can draw lines.

[AlvaroS]

@mrjj Thanks a lot for answering me.

I have just done what example says.

But now I have a problem to modify the scene in "scene" class.
The scene is declarated in mainwindow class as a public.
So when I try to modify the scene in "scene" class like:

        MainWindow.scene->addLine(line_traj,blue);

it says me this error:

/scene_mouse_event.cpp:35: error: expected unqualified-id before '.' token
MainWindow.scene->addLine(line_traj,blue);
^

Any ideas?

thanks a lot

[mrjj]

@AlvaroS said:

MainWindow.scene

hi
why with mainwin in front ?
If its a member of main window , just
scene->addLine(line_traj,blue);

[AlvaroS]

@mrjj If I do that it says me:

scene_mouse_event.cpp:35: error: 'scene' was not declared in this scope
scene->addLine(line_traj,blue);
^

[mrjj]

@AlvaroS

Well its say it dont know scene
so if you added to main wind , its pretty odd.

IN what class / function do u call
scene->addLine(line_traj,blue);

If not in mainwindow, then that is why.

[AlvaroS]

@mrjj No, I call in "scene" class because is here where I have to put mousepressevent.

So it is here where I have to modify the scene

[AlvaroS]

@mrjj So I need to modify scene which is declarated in mainwindow class in "scene" class.
For that reason I have declarated scene as a public in mainwindow but it does not work.

[mrjj]

@AlvaroS

Well, why do u need to modify scene from outside ?
and from what class do you try this?

Why cant you just do it in main window? then ?

Something seems off. :)

You would need a pointer to the mainwindow in that other
class to do it but that
is not good style.
You should give the other class the scene as pointer then.