Drawing on QGraphicsView

[JonathanForQT4]

Hey guys, I'm trying to draw some stuff within the viewport of QGraphicsView....when I do this within paintEvent it doesn't work for some reason.

Is the QGraphicsScene for the view overwriting what I am drawing (..although my scene's color is transparent)? Any help ideas would be appreciated.

Thanks,
Jonathan

[wysota]

In general you shouldn't draw on the graphics view directly from within the paint event. Instead use drawBackground() and drawForeground() methods.

[JonathanForQT4]

I just wrote drawForeground, since I want to draw a scale (for x and y) of the scene I have, I called drawForeground from paintEvent since I need it updated whenever I call update(). The painter I pass it is QPainter mainPainter(this); (from paintEvent) and the Rect I pass it is the rect I defined with setSceneRect....since coordinates are relative to the scene.

The scale would look like your are looking through a rifle scope and would be over the viewport. I got it working when I had defined a widget (everything written in the paintEvent) within a qscrollarea....but now I'm using QGraphicsView and am having trouble with drawing whatsoever.

thanks again,

Jonathan

[wysota]

But why do you reimplement the paint event in the first place? You should reimplement drawForeground or drawBackground instead.

[JonathanForQT4]

I did reimplement drawForeground...but it's not painting anything within the viewport....

[wysota]

How did you reimplement it? Can we see the code? Did you take into consideration that all coordinates are in the scene coordinate system?

[JonathanForQT4]

void someclass::drawForeground(QPainter *painter, const QRectF &rect){
 
	if(m_measureDistDragging){
		QPen tmp;
		tmp.setColor(QColor(230, 100, 50, 200));
		tmp.setWidth(1);
		painter->setPen(tmp);
		painter->drawLine(m_measureDistStartPoint, m_currentMousePos);
	}
 
}

void someclass::drawForeground(QPainter *painter, const QRectF &rect){

	if(m_measureDistDragging){
		QPen tmp;
		tmp.setColor(QColor(230, 100, 50, 200));
		tmp.setWidth(1);
		painter->setPen(tmp);
		painter->drawLine(m_measureDistStartPoint, m_currentMousePos);
	}

}

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when the mouse is clicked m_measureDistStartPoint is set to mapToScene(event->pos()) since when I use it for drawing it needs to be in scene coordinates. The same applies to m_currentMousePos.

When I am debugging I can see that the line is there, but when I am just using the program, I never see the line drawn. Any ideas?

//edit: btw. m_measureDistDragging is true....

[JonathanForQT4]

anyone have any ideas? I can see the line drawn when I am debugging and see the line right after this code is executed and the window pops up....I'm guessing it's overwritten in paintEvent or something?

[wysota]

Did you override the paint event?

[JonathanForQT4]

I tried the reimplementing paintEvent with the same code (the class was just a widget before and it worked then...no it's a QGraphicsView class....but should still work since QGraphicsView inherits QWidget....), but to no avail...then you said, "But why do you reimplement the paint event in the first place? You should reimplement drawForeground or drawBackground instead."
So...I tried foreground....I'm now assuming that paintEvent is drawing over it?

[wysota]

No. paintEvent calls drawForeground() after calling drawBackground() and drawItems(). It doesn't do anything after drawForeground.