Is there a way to access the image on the QWidget's backing store?

Question

I'm doing some compositing inside the paintEvent() in a custom widget. Some of the compositing is done when some areas are already painted, and I need access to the current contents painted so far.

So, I'm looking for a way to access the image contents of the current backing store during a paintEvent. I've looked at QBackingStore, but there's nothing there that directly gives me access to the backing store bitmap. Is there some API, perhaps private, that could be used to provide that?

If not, I'll have to resort to painting on an explicit pixmap and rendering that pixmap onto the widget.

Answer 1

It is possible, but it is not portable. The QBackingStore is just a wrapper class around a QImage buffer on most platforms, but I suppose this is not guaranteed. I've researched this issue when writing the QuickWidget. A cast is needed:

QImage * image = dynamic_cast<QImage*>(backingStore()->paintDevice());
if (image != 0) // it's an image, do something with it

Be careful though not to cause the QImage to detach. Things such as resizing are off limits.

Check the QuickWidget out at:

https://code.google.com/p/quickwidget/