Is it possible to use QPainter with QGraphicsItem?

[william.dias]

Hi!

I am trying to create a generic QGraphicsItem and then use the QPainter to draw rectangles, circles and freehand lines inside the item. Does any body know if it is possible? Could you give me a exemple? If it is not, does any body have an alternative way to do this?

I am using QT 4.3 and Windows XP.

Thanks!

[jpn]

Every QGraphicsItem has a special virtual method QGraphicsItem::paint(). QPainter is passed as an argument. Take a look at docs, there's an example.

[william.dias]

Could you give me an example?
If I create a QGraphicsPixmapItem, how can i draw inside the item freehand lines?
Thanks!

[jpn]

• subclass/inherit (which ever term you're familiar with) QGraphicsPixmapItem just like you would subclass any class in C++
• reimplement/override (again, which ever term you're familiar with) paint() just like you would reimplement any virtual method in C++, see docs for exact function signature
• in reimplemented paint(), call the base class implementation to paint the pixmap and draw your custom lines afterwards using the painter passed as a parameter

Things like inheritance and member function overriding are not covered by Qt docs. They are basic OOP concepts one should know before one starts working with Qt at all.

[Bitto]

William Dias: Have you tried running the Chip Demo, then holding the secret Shift-button and then clicking and dragging on a chip?

This shows one way to draw freehand on an item. It's one practical approach with source code (chip.cpp) - the item tracks all mouse positions and generates a path by connecting the points.