Drawing on QGraphicsView

[wysota]

I would make the line a regular item. I would create it when mouse button is pressed and destroy when it is released. As you'll have the line coordinates in scene coordinate system, calculating the distance regardless of the scale, etc. should be easy.

[wysota]

Here is an example of what I mean.

[Gopala Krishna]

Here is an example of what I mean.

Do you think an update() call is needed in each of the mouse events implemented in the reimplemented scene class? I tried commenting them and it worked fine.

[wysota]

It doesn't slow anything down, so why not? But you're right, it's not required.

[vermarajeev]

Thanks to wysota for his quick solution.
I made a mistake not to override QGraphicsScene. Anyway I'll try getting the solution overriding QGraphicsView and without overriding QGraphicsScene and see how it works.

JonathanForQT4-->
It was nice to see your quick solution. Thanks!!! wysota made my work easy.

Njoy and have fun as I'm with qt4.

//And it works... Anything is possible with qt4

[JonathanForQT4]

wysota: thanks for your code, it is pretty much the same thing I had already implemented after vermarajeev suggested this
The reason I keep posting is because I wanted to find out how to get it to work within drawForeground so that I could get the axis scale painting within there too....Or would you also suggest making the axis scale some items -- I think things would get too sluggish.

I was also thinking that you could add a widget to the scene....and the widget could be the axis scale....any ideas if this is doable?

I was reading this:

http://labs.trolltech.com/blogs/2007...on-the-canvas/

and this:

http://labs.trolltech.com/blogs/2007...s-and-widgets/

and that's why I ask if I should put my axis scale in a widget's paintEvent (like it was before in the picture I attached) and then add the widget overtop of the viewPort from QGraphicsView.

The whole thing seems stupid to me though because QGraphicsView is a inherits QWidget and I should be able to paint over the viewPort, arggggg!

vermarajeev: I have now added thanks for this first suggestion of making the measureDist line an QGraphicsItem

[hb]

I am also having the task of drawing an axis scale on a QGraphicsView, that is supposed to be at a fixed widget position, with fixed linewidth etc. I think I finally found a solution. Basically, you can just override drawForeground, turn off the world coordinate system mapping and draw directly in view coordinates. The following code would for example draw the axes as two plain lines:

void myGraphicsView::drawForeground(QPainter *painter, const QRectF &rect)
{
	int axesOffset = 10;
	int bottom = height() - 2*axesOffset;
	int right  = width()  - 2*axesOffset;
 
	painter->setWorldMatrixEnabled(false);
	painter->setPen(Qt::white);
	painter->drawLine(axesOffset,axesOffset,axesOffset,bottom);
	painter->drawLine(axesOffset,bottom,right,bottom);
}

void myGraphicsView::drawForeground(QPainter *painter, const QRectF &rect)
{
	int axesOffset = 10;
	int bottom = height() - 2*axesOffset;
	int right  = width()  - 2*axesOffset;

	painter->setWorldMatrixEnabled(false);
	painter->setPen(Qt::white);
	painter->drawLine(axesOffset,axesOffset,axesOffset,bottom);
	painter->drawLine(axesOffset,bottom,right,bottom);
}

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[Bitto]

No drawing is "real-time" drawing in Qt. Not on X11, not Windows, not OpenGL. There's always an asynchronous mechanism; anything else is bound to cause flicker, or be very inefficient at the very least.

If you want to understand how QGraphicsView::drawBackground and ::drawForeground work, you should create yourself a very small simple example with a 100x100 scene in a plain view, and see what happens if you draw shapes of various colors at different places. Start by drawing the scene's scene rect, filling it with a solid color, and see what happens when you resize the view, and move the scrollbars.

void CustomView::drawBackground(QPainter *painter, const QRectF &exposed)
{
    painter->fillRect(sceneRect(), Qt::blue);
}
 
void CustomView::drawForeground(QPainter *painter, const QRectF &exposed)
{
    // note: semi-transparent color, otherwise the items and
    // background won't shine through the foreground
    painter->fillRect(sceneRect(), QColor(64, 128, 192, 128));
}

void CustomView::drawBackground(QPainter *painter, const QRectF &exposed)
{
    painter->fillRect(sceneRect(), Qt::blue);
}

void CustomView::drawForeground(QPainter *painter, const QRectF &exposed)
{
    // note: semi-transparent color, otherwise the items and
    // background won't shine through the foreground
    painter->fillRect(sceneRect(), QColor(64, 128, 192, 128));
}

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Don't expect drawing to be "instantaneous"; that's not how Qt works. :-) Still, you can easily get frame rates beyond 100fps if you do things right.

Btw, I also recommend http://doc.trolltech.com/4.2/graphic...w-architecture as bed time reading, and if you look at the examples (elastic-nodes uses drawBackground, for example), you'll probably get the picture quite soon :-). Notice especially the part that explains the coordinate systems; since drawForeground and drawBackground render in scene coordinates, but QWidget:aintEvent() renders in device coordinates.

[JonathanForQT4]

thanks hb I was able to use the painter->setWorldMatrixEnabled(false); trick and can now re use my code from my old implementation of this problem

Cheers,
Jonathan