[Qt-interest] FBO & OpenGL drawing in a QGraphicsItem

[John Clayton]

Hi

I'm trying to create a qgraphicsitem that can draw the reflection of any other qgraphicsitem.  All my graphics items draw using OpenGL - so I'm assuming that there is always a GL context active.

My approach is this:
	a) have my reflection graphics item contain a pointer to another graphics item - the reflection uses this other graphics item as its 'source' content
	b) in the paint() method for the reflection, I will
		0. create an FBO the size of the source bounding rect
		1. request that the 'source' graphics item paint into the FBO
		2. use Composition_DestinationIn to write a linear gradient over the FBO content 
		3. take the FBO texture and draw this into the reflection graphics item

My problem is that at stage 2, I don't see the linear gradient being drawn into the FBO.  And at stage 3, I see the gradient, but the FBO texture *appears* to be corrupted. 

Here's the relevant code:	
	http://pastebin.com/bTRTW29W

and here's a picture showing the 4 stages of rendering.  Each picture relates to one saveFBO() call in the code.  Its obvious to see that the 2/3 pictures are wrong.  picture 2 should have a linear gradient imposed over it.  Picture 3 *should* in my opinion be the same as 2.
	http://idisk.me.com/john_clayton/Public/Pictures/Skitch/rubbish-20101227-094335.jpg

I believe that its something to do with the OpenGL state so far, as commenting out the FBO painters save/restore calls changes the appearance of the pictures.  

Any help is hugely appreciated.  Thank you.

--

John