how to draw QGraphicsPolygonItem

hobbyProgrammer

Hi,

I am trying to draw a polygon in my graphicsScene.
The polygon exists of QPoints that are stored inside qv_points (qv = QVector)

void graphicsView::addPoly()
{
    qDebug() << "Add polygon";

    QPolygonF polygon;
    polygon << qv_points;

    QBrush brush;
    brush.setColor(GREEN);

    QGraphicsPolygonItem *newPoly = new QGraphicsPolygonItem();
    newPoly->setPolygon(polygon);
    newPoly->setBrush(QBrush(GREEN, Qt::SolidPattern));
    scene->addItem(newPoly);
}

It comes into the function since I can see "Add polygon" in the debugger.
Does anyone know why my polygon is not being drawn?

mrjj

hi
The debugger for visual studio is a separate download called
Debugging tools from MS.

When you press Ctrl+N , you are not creating a new instance of the
graphicsView or something like that ?

JonB

@hobbyProgrammer
~~I know nothing about this area, but is it right that your QPolygonF polygon goes out of scope? Is setPolygon(polygon) going to copy it?~~ Yep, @mrjj says it will do that!

Or, does your polygon (qv_points) have the right number of sensible points?

mrjj

Hi
Check the actual points as the code works fine.

 QPolygonF polygon;
    polygon  << QPointF( 10, 10 ) << QPointF( 0, 90 ) <<
                QPointF( 40, 70 ) << QPointF( 80,110 ) << QPointF( 70, 20 );

    QGraphicsPolygonItem *newPoly = new QGraphicsPolygonItem();
    newPoly->setPolygon(polygon);
    newPoly->setBrush(QBrush(Qt::green, Qt::SolidPattern));
    scene->addItem(newPoly);

alt text

@JonB Good point but it will make a copy of the QPolygonF so its not out of scope issue in this case.
But spot on with the qv_points.
I think thye might be in scene coordinates or view and not local.

hobbyProgrammer

@mrjj Hi thanks.
I found that it does fill the qv_points, but when it comes in the addPolygon function the qv_points is empty.

I have absolutely no clue why. This is my code:
addPolygon is called by using the shortcut Ctrl+N (new polygon).

void graphicsView::mousePressEvent(QMouseEvent *event)
{


    if(event->button() == Qt::LeftButton)
    {
        bool point_exists = false;

        for(int i = 0; i < qv_points.size(); i++)
        {
            if(qv_points.at(i).x() <= mapToScene(event->pos()).x()+2.5 &&
               qv_points.at(i).x() >= mapToScene(event->pos()).x()-2.5 &&
               qv_points.at(i).y() <= mapToScene(event->pos()).y()+2.5 &&
               qv_points.at(i).y() >= mapToScene(event->pos()).y()-2.5)
            {
                point_exists = true;
            }
        }
        if(point_exists)
        {
            qDebug() << "point exists";
        }
        else if(!point_exists || qv_points.isEmpty())
        {
            addPoint(mapToScene(event->pos()));
        }
    }
}

void graphicsView::addPoly()
{
    qDebug() << "Add polygon";

    qDebug() << qv_points;
    QPolygonF polygon;
    polygon << qv_points;
    qDebug() << polygon;

    QBrush brush;
    brush.setColor(GREEN);

    QGraphicsPolygonItem *newPoly = new QGraphicsPolygonItem();
    newPoly->setPolygon(polygon);
    newPoly->setBrush(QBrush(GREEN, Qt::SolidPattern));
    scene->addItem(newPoly);
}

void graphicsView::addPoint(QPointF pos)
{
    qv_points << pos;
    qDebug() << qv_points;
    QGraphicsEllipseItem *newEllipse = new QGraphicsEllipseItem();

    double x,y;
    QPointF point = pos;
    x = point.x()-2.5;
    y = point.y()-2.5;

    newEllipse->setRect(QRectF(x,y,5,5));
    newEllipse->setBrush(QBrush(Qt::red, Qt::SolidPattern));
    newEllipse->setFlags(QGraphicsEllipseItem::ItemIsMovable | QGraphicsEllipseItem::ItemIsSelectable);

    //add item to scene
    scene->addItem(newEllipse);

    qDebug() << pos;
}

this is what the debugger says:

QVector(QPointF(142,73))
QPointF(142,73)
QVector(QPointF(142,73), QPointF(122,305))
QPointF(122,305)
QVector(QPointF(142,73), QPointF(122,305), QPointF(289,323))
QPointF(289,323)
QVector(QPointF(142,73), QPointF(122,305), QPointF(289,323), QPointF(284,69))
QPointF(284,69)
Add polygon
QVector()
QPolygonF()

mrjj

Hi
Its very good you use qDebug() but would be even nicer if you added some text so
its easier to follow what is what.

qDebug() <<"graphicsView::addPoint - qv_points: " << qv_points;

Anyway, it looks very odd as list suddenly is empty.
It looks all is inside same class so unless you call clear on it
i see no way it suddenly goes empty.

However, looking at the posted code, there is also nowhere - where you add to the qv_points list.

hobbyProgrammer

@mrjj is there anyway to debug like in visual studio where you can step over etc.?

I would just like to see what qv_points does with each action.

hobbyProgrammer

@mrjj said in how to draw QGraphicsPolygonItem:

However, looking at the posted code, there is also nowhere - where you add to the qv_points list.

the first line of addPoint() is where the point is added to the qv_points.

void graphicsView::addPoint(QPointF pos)
{
    qv_points << pos;

mrjj

@hobbyProgrammer
Hi
Ah i miss that as i was looking for append or similar :)

Yes, just start the app in debug mode (f5)
Then place a break point.
Then press F10 to step or F11

hobbyProgrammer

@mrjj oh thanks!

I got the error: Unable to create a debugging engine.
but I didn't have any debuggers for MSVC. I did have two for minGW so I added those kits to my project and now I can view them hopefully.

hobbyProgrammer

This post is deleted!

hobbyProgrammer

@mrjj
I managed to see that it shows up empty in the addPolygon function, but when I add a new point it still shows the old points in the debugger.

This is what I get in the debugger:

qv_points in addPoint:  QVector(QPointF(27,40))
qv_points in addPoint:  QVector(QPointF(27,40), QPointF(50,229))
qv_points in addPoint:  QVector(QPointF(27,40), QPointF(50,229), QPointF(151,236))
qv_points in addPoint:  QVector(QPointF(27,40), QPointF(50,229), QPointF(151,236), QPointF(152,147))
addPoly() qv_points:  QVector()
addPoly() qv_points:  QVector()
addPoly() qv_points:  QVector()
qv_points in addPoint:  QVector(QPointF(27,40), QPointF(50,229), QPointF(151,236), QPointF(152,147), QPointF(245,129))

mrjj

hi
The debugger for visual studio is a separate download called
Debugging tools from MS.

When you press Ctrl+N , you are not creating a new instance of the
graphicsView or something like that ?

hobbyProgrammer

@mrjj yes I did! Thank you so much.

I had this in the create actions

GraphicsView gv = new GraphicsView;


    addPolyAct = new QAction(tr("&Add Poly..."), this);
    addPolyAct->setShortcut(QKeySequence::New);
    connect(addPolyAct, SIGNAL(triggered()), gv , SLOT(addPoly()));

since I promoted the widget inside the designer I changed it to:

    addPolyAct = new QAction(tr("&Add Poly..."), this);
    addPolyAct->setShortcut(QKeySequence::New);
    connect(addPolyAct, SIGNAL(triggered()), ui->graphicsView, SLOT(addPoly()));

Now everything works perfectly!
Thank you so much.

mrjj

@hobbyProgrammer

Ok. Good. :)
It was my only guess
how on earth the points list could become
empty.
Sometimes Im a lucky guesser :)