Question

I’m trying to make a function similar to impoly (from matlab) in Qt. Right now, I have a subclass of QGraphicsView and have set the virtual function “drawBackground” as:

void roiwindow::drawBackground(QPainter *painter, const QRectF &rect)
{
painter->save();
painter->drawImage(rect, *refimage);
painter->restore();
}

This works great and is basically exactly what I want as far as the background layer. Now, I’m trying to add circles that will eventually act as nodes for the polygon. I did this by using:

QGraphicsView *view = new QGraphicsView(this);
view->show();
QGraphicsEllipseItem *e1;
e1 = this->addEllipse(20, 20, 30, 30, QColor(0, 0, 0), Qt::white);
e1->setFlags(QGraphicsItem::ItemIsMovable | QGraphicsItem::ItemIsMovable);

This is sort of what I want. But, there is a problem that when I click and drag the ellipses, the background of the ellipse is a scaled down version of *refimage… This leaves sort of a streak across the screen but it disappears when I click on another window or minimize the window. Does QGraphicsItem also call drawBackground? If so it seems to only call it when the item is being dragged. Any suggestions for how I can code this better? Thanks.

Answer 1

The streaks are occurring because you probably have QGraphicsView set to the default viewport update mode, which is QGraphicsView::MinimalViewportUpdate. In this case, you need QGraphicsView::FullViewportUpdate. The reason is that when you move items around, the background behind them needs to be redrawn, but when you have MinimalViewportUpdate set, only the items will be redrawn (unless there is a full viewport update, e.g., from minimizing/maximizing the window).