Painter drawLine doubt

[arjunasd]

Hi

I have a QGraphicsView and I am trying to draw the lines on the widget.

Also, I found the following piece of code while browsing. Its in Qt3.
They have a QScrollView and they paint the lines as follows:

painter->drawLine(x1,y1, x2, y2);

change the values of x's and y's

painter->drawLine(x1,y1, x2, y2);

They comment the 1st draw clears the previous line and second draws the new line. How does this work? Sorry if its a naive question?

Arjun

[jpn]

QGraphicsView caches its content. It is not suggested to draw by hand on GV. Perhaps you could use QGraphicsLineItems intead?

[arjunasd]

Thanks. It works fine with QGraphicsLineItem.

Just out of curiosity, regarding the sample code I got from the internet. Its in Qt3.They have a QScrollView and they paint the lines as follows:

painter->drawLine(x1,y1, x2, y2);
change the values of x's and y's
painter->drawLine(x1,y1, x2, y2);

They comment the 1st draw clears the previous line and second draws the new line. How does this work?

Thanks
Arjun

[jpn]

How does this work?

Hard to say without seeing the exact code. Maybe they switch colors in between?

Anyway, the painting system has pretty much changed in Qt 4. One can no more paint outside QWidget::paintEvent(). For more details, see Porting to Qt 4 - Painting and Redrawing Widgets and The Arthur Paint System.