Hide area of QGraphicsItem that is out of boundary

Question

I have a QGraphicsPixmap item in a QGraphicsScene. The item has flags set to ItemIsMovable, and ItemIsSelectable. How do I ensure that when the item is moved out of a certain boundary - it can be a QGraphicsScene or just a fixed frame size at fixed coordinates - the part becomes hidden?

Eg.

The left part of the basketball becomes hidden.

You want it to be shown only if it is inside an area, for example the rectangle, and if it is outside that part is cut. I am right? — eyllanesc, May 23 '18 at 22:18

eyllanesc · Accepted Answer · 2018-05-24T04:11:53.247

2

You have to use setClipPath().

In the following code I have created a class that inherits from QGraphicsPixmapItem (the same could do with other classes that inherit from QGraphicsItem) and I created the method setBoundaryPath() that receives a QPainterPath that indicates the visible area, for example in the code use:

QPainterPath path;
path.addRect(QRectF(100, 100, 400, 200));

That QPainterPath is a rectangle whose topleft is the point (100, 100) of the QGraphicsScene with size of 400 in width and 200 in height.

#include <QApplication>
#include <QGraphicsRectItem>
#include <QGraphicsView>

class GraphicsPixmapItem: public QGraphicsPixmapItem{

public:
    GraphicsPixmapItem(const QPixmap & pixmap,  QGraphicsItem *parent = 0):
        QGraphicsPixmapItem(pixmap, parent)
    {
        setFlag(QGraphicsItem::ItemIsMovable, true);
        setFlag(QGraphicsItem::ItemIsSelectable, true);
    }
    void paint(QPainter *painter, const QStyleOptionGraphicsItem *option, QWidget *widget){
        if(!m_boundaryPath.isEmpty()){
            QPainterPath path = mapFromScene(m_boundaryPath);
            if(!path.isEmpty())
                painter->setClipPath(path);
        }
        QGraphicsPixmapItem::paint(painter, option, widget);
    }

    QPainterPath boundaryPath() const{
        return m_boundaryPath;
    }
    void setBoundaryPath(const QPainterPath &boundaryPath){
        m_boundaryPath = boundaryPath;
        update();
    }

private:
    QPainterPath m_boundaryPath;
};

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    QGraphicsView view;
    QGraphicsScene scene(0, 0, 600, 400);
    view.setScene(&scene);
    view.setBackgroundBrush(QBrush(Qt::gray));

    GraphicsPixmapItem *p_item = new GraphicsPixmapItem(QPixmap(":/ball.png"));
    p_item->setPos(100, 100);

    // Define the area that will be visible
    QPainterPath path;
    path.addRect(QRectF(100, 100, 400, 200));

    p_item->setBoundaryPath(path);
    scene.addItem(p_item);

    // the item is added to visualize the intersection
    QGraphicsPathItem *path_item = scene.addPath(path, QPen(Qt::black), QBrush(Qt::white));
    path_item->setZValue(-1);
    view.show();
    return a.exec();
}

You can find the example code in this link.

edited May 24 '18 at 04:11

answered May 24 '18 at 00:16

eyllanesc

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I wonder what's the `certain boundary` which the OP means. Why not just use a layer contains transparent stack on the basketball... – JustWe May 24 '18 at 01:40
I've edited the question to clarify `certain boundary`. There is no fixed approach at the moment. – ALH May 24 '18 at 03:36
@ALH a QGraphicSscene is everyone in QGraphicsView, that has no limits, explain how you determine that area, do not confuse sceneRect () with QGraphicsScene. in my example I add an item so that the area is visible but you can delete the path_item, in my example it is determined by the QPainterPath. – eyllanesc May 24 '18 at 03:41
@ALH I have added a clearer explanation to my answer, please review it. – eyllanesc May 24 '18 at 04:14
Hmm, with your code, it still seems like the image is visible on the grey background. – ALH May 24 '18 at 04:56
@ALH have you tried my code ?, the images that I have published prove it, you could show me what you get. – eyllanesc May 24 '18 at 08:06
@Jiu If my code works, you could give it an upvote. :D – eyllanesc May 24 '18 at 09:54
The answer gives correct result as I understand the question...But _it still seems like the image is visible on the grey background_ How could this possible? – JustWe May 24 '18 at 09:58
@Jiu What I think is that the askers do not know how to correctly implement the solution in their own project, that happens a lot with the beginners. ;-D – eyllanesc May 24 '18 at 10:00
Thanks all. The given answer works. I had a line: `painter->drawPixmap(offset(), pixmap());` in the `paint` function before `setClipPath`, which seemed to override the clipPath. I got it to work by switching the order. – ALH May 24 '18 at 16:52

Hide area of QGraphicsItem that is out of boundary

1 Answers1