QGraphicsScene/QGraphicsView performance

Question

I am using QGraphicsScene/QGraphicsView pair in my project I have performance issue with this pair. I added my custom graphics items to scene and displayed the contents with view. After that my custom graphics items paint method continuously called by scene(just like infinite loop). This makes %25 of CPU usage(approximately 400 items on scene). What may cause this behaviour?

Here is one my item implementation:

class LevelCrossingItem : public QGraphicsWidget
{
public:
    LevelCrossingItem(QString _id,qreal _x,qreal _y);
    ~LevelCrossingItem();
    QRectF boundingRect() const;
    QSizeF sizeHint(Qt::SizeHint which, const QSizeF &constraint  = QSizeF()) const;
    void paint(QPainter *painter, const QStyleOptionGraphicsItem *option, QWidget *widget /* = 0 */);
    void readStateBits();
    bool isClosed();
    bool isGateArmBroken();
    bool isOpenedDuringRouteTanzimCompleted();
    bool hasDataConsistencyWarning();
    int type() const {return Type;}
private slots:
    void setVisible(bool);
private:
    enum {Type = FIELDLEVELCROSSING};
    QString m_id;
    QString m_source;
    short m_closedState;
    short m_brokenGateArmState;
    short m_openedDuringRouteTanzimCompletedState;
    short m_dataConsistencyWarningState;
    QBitArray stateBitArray;
    qreal x,y;
    QSvgRenderer *renderer;
};  

#include "levelcrossing.h"

LevelCrossingItem::LevelCrossingItem(QString _id,qreal _x,qreal _y):m_id(_id),x(_x),y(_y),stateBitArray(4)
{
    m_source = LEVELCROSSING_RESOURCE_PATH.arg("open");
    renderer = new QSvgRenderer;
    setStateArray(stateBitArray);
    setZValue(-0.5);
}

LevelCrossingItem::~LevelCrossingItem()
{
    delete renderer;
}

void LevelCrossingItem::setVisible(bool visible)
{
    QGraphicsItem::setVisible(visible);
}

QRectF LevelCrossingItem::boundingRect() const
{
    return QRectF(QPointF(x,y),sizeHint(Qt::PreferredSize));
}

QSizeF LevelCrossingItem::sizeHint(Qt::SizeHint which, const QSizeF &constraint) const
{
    return QSizeF(50,270);
}

void LevelCrossingItem::readStateBits()
{
    m_closedState = property("Closed").toInt();
    m_brokenGateArmState = property("Broken").toInt();
    m_openedDuringRouteTanzimCompletedState = property("OpenedOnRouteWarning").toInt();
    m_dataConsistencyWarningState = property("DataConsistencyWarning").toInt();

    stateBitArray.setBit(0,qvariant_cast<bool>(m_closedState));
    stateBitArray.setBit(1,qvariant_cast<bool>(m_brokenGateArmState));
    stateBitArray.setBit(2,qvariant_cast<bool>(m_openedDuringRouteTanzimCompletedState));
    stateBitArray.setBit(3,qvariant_cast<bool>(m_dataConsistencyWarningState));

    setStateArray(stateBitArray);
}

void LevelCrossingItem::paint(QPainter *painter, const QStyleOptionGraphicsItem *option, QWidget *widget )
{
    Q_UNUSED(option);
    Q_UNUSED(widget);

    readStateBits();

    m_closedState == Positive ? m_source = LEVELCROSSING_RESOURCE_PATH.arg("closed")
        : m_source = LEVELCROSSING_RESOURCE_PATH.arg("open");
    m_brokenGateArmState == Positive ? m_source = LEVELCROSSING_RESOURCE_PATH.arg("broken")
        : m_source = m_source;

    if(m_openedDuringRouteTanzimCompletedState == Positive)
    {
        setWarningVisible(OOR_WRN.arg(name()).arg(interlockingRegionId()),true);
        if(stateChanged())
            emit itemAlarmOccured(m_id,LevelCrossingIsOpenDuringTanzimCompleted);
    }
    else
         setWarningVisible(OOR_WRN.arg(name()).arg(interlockingRegionId()),false);

    if(m_dataConsistencyWarningState == Positive)
    {   
        setWarningVisible(DC_WRN.arg(name()).arg(interlockingRegionId()),true);
        if(stateChanged())
            emit itemAlarmOccured(m_id,LevelCrossingDataConsistency);
    }
    else
        setWarningVisible(DC_WRN.arg(name()).arg(interlockingRegionId()),false);

    renderer->load(m_source);
    renderer->render(painter,boundingRect());
}

bool LevelCrossingItem::isClosed()
{
    return m_closedState == Positive;
}

bool LevelCrossingItem::isGateArmBroken()
{
    return m_brokenGateArmState == Positive;
}

bool LevelCrossingItem::isOpenedDuringRouteTanzimCompleted()
{
    return m_openedDuringRouteTanzimCompletedState == Positive;
}

bool LevelCrossingItem::hasDataConsistencyWarning()
{
    return m_dataConsistencyWarningState == Positive;
}

I read x and y coordinates from xml file. For this item x and y coordinates are 239,344 respectively

ak. ak. · Accepted Answer · 2011-02-14T11:14:10

Most likely your graphics item has mistakes in the implementation. I had similar behavior, but I was unable to figure out what exactly causes it. I suspect that this happens when you draw outside of the bounding rect. This triggers some clean up routine, which in turn causes redraw of the item, and here goes the loop. Eventually I resolved the issue by carefully inspecting the implementation of my custom graphics item and making sure that:

These is no painting outside of the MyGraphicsItem::boundingRect. Use painter->setClipRect(boundingRect())
MyGraphicsItem::shape does not cross with the MyGraphicsItem::boundingRect.
If you override any other QGraphicsItem functions, make sure that your implementation is correct.

Hope this helps. Feel free to post the source code of your graphics item, it will be easier to find the problem.

QGraphicsScene/QGraphicsView performance

1 Answers