Very slow repainting ofPixmaps in QGraphicsView images

[drexiya]

Using the QGraphicsView and QGraphicsScene to render pixmaps generated from a QImage. This is on QT 4.4.3.

The pixmaps are generated and placed onto the scene from a QGraphicsItemGroup derived class, and manipulated through the returned QGraphicsPixmapItem*.

The image is a long thin image, that is viewed by scrolling through and I draw graphics path items on.

This works fine on Windows, but experiences major problems in Linux. Scrolling is very slow. Also drawing onto the view is also very slow. In fact, the whole application slows down hugely. This has made the application unusable.

The application appears to be spending very little time generating the pixmaps and painting the QGraphicsItemGroup. The time spent drawing the path items is also insignificant.

It seems that it is redrawing the image itself that is very slow. If I override the GraphicsView:aintEvent function and put a timer in there, this is very slow. (400-800ms)

It this a known problem in Linux? It works fine in Windows. (The same paintEvent takes around 5-15ms max)

Has anyone else experienced this? Is there a workaround for Linux?


Code in : QGraphicsItemGroup - looks like this:

(note: this takes max 2-3 ms)

void BoreholeImage::paint(QPainter* painter,
                          const QStyleOptionGraphicsItem* option,
                          QWidget* widget)
{
 
    if (m_graphicsItem->type() == QGraphicsPixmapItem::Type) {
        // We've already loaded the image to display.
        QGraphicsItemGroup::paint(painter, option, widget);
 
        // Is there anything to clean up?  See below.
        if (m_graphicsItemToDelete) {
            delete m_graphicsItemToDelete;
            m_graphicsItemToDelete = 0;
        }
 
        return;
    }
 
 
    m_stream->setSeekPosition(boundingRect().y());
    QImage image = m_stream->getImage();
    QPixmap pixmap = QPixmap::fromImage(image);
    QGraphicsPixmapItem* pixmapItem = scene()->addPixmap(pixmap);
    qDebug() << "BoreholeImage" << "pixmap at" << boundingRect().y();
    pixmapItem->setPos(0.0, boundingRect().y());
    pixmapItem->scale(1.0, boundingRect().height() / pixmap.height());
    pixmapItem->setZValue(0.0);
 
    removeFromGroup(m_graphicsItem);
    scene()->removeItem(m_graphicsItem);
 
    // Can't delete the unused rectItem here because this routine's caller
    // uses it again after this routine returns.  Delete it on the next call.
    m_graphicsItemToDelete = m_graphicsItem;
 
    m_graphicsItem = pixmapItem;
    addToGroup(m_graphicsItem);
    QGraphicsItemGroup::paint(painter, option, widget);
}

void BoreholeImage::paint(QPainter* painter,
                          const QStyleOptionGraphicsItem* option,
                          QWidget* widget)
{
  
    if (m_graphicsItem->type() == QGraphicsPixmapItem::Type) {
        // We've already loaded the image to display.
        QGraphicsItemGroup::paint(painter, option, widget);

        // Is there anything to clean up?  See below.
        if (m_graphicsItemToDelete) {
            delete m_graphicsItemToDelete;
            m_graphicsItemToDelete = 0;
        }
	
        return;
    }


    m_stream->setSeekPosition(boundingRect().y());
    QImage image = m_stream->getImage();
    QPixmap pixmap = QPixmap::fromImage(image);
    QGraphicsPixmapItem* pixmapItem = scene()->addPixmap(pixmap);
    qDebug() << "BoreholeImage" << "pixmap at" << boundingRect().y();
    pixmapItem->setPos(0.0, boundingRect().y());
    pixmapItem->scale(1.0, boundingRect().height() / pixmap.height());
    pixmapItem->setZValue(0.0);

    removeFromGroup(m_graphicsItem);
    scene()->removeItem(m_graphicsItem);
    
    // Can't delete the unused rectItem here because this routine's caller
    // uses it again after this routine returns.  Delete it on the next call.
    m_graphicsItemToDelete = m_graphicsItem;

    m_graphicsItem = pixmapItem;
    addToGroup(m_graphicsItem);
    QGraphicsItemGroup::paint(painter, option, widget);
}

To copy to clipboard, switch view to plain text mode