[Qt-interest] Layers in Graphics Framework

[Ross Bencina]

AK Wrote:
> I am trying to implement layers support for my app.
>
>
> - A item can be in one or more layer.

This sounds like an unusual situation, usually layers contain items (ie 
items have only one layer)

If not for that, you could use a parent GraphicsItem for each layer (not 
that it will solve your problems).

You may need to keep track of layer membership completely outside the 
standard Qt framework. e.g. define your own QGraphicsItem subclass that 
keeps track of which layers it is in, and only paints itself when the layer 
is enabled in the current view.

You'd need to implement similar hacks for hit-testing.

> - A view displays one / more layer
> - If current display view doesn't display a layer assigned to an items, 
> that item is not shown. Also its not available to hit-testing methods ( 
> items( QPoint  etc.).

As I understand it, the Qt Graphics View framework doesn't really support 
this kind of "same scene graph, render different item subsets" approach 
since the state of the scene graph is exactly what is rendered. Having 
different items visible in different views, but as part of the same scene 
isn't directly supported (aside from supporting it in your own paint 
routines, which might not be so hard, something like:

void paint(){ if( !myLayerIsVisibleInThisView() ) return; /* draw the item 
*/ }

> Off course, I would not like to replicate the scene, keeping them in sync 
> could be costly.

There may be inefficiencies if you _don't_ do this. For example, when you 
alter items in a scene, it will generate a damage list of what regions need 
to be re-rendered. This would be propogated to all views, even though the 
damage would only be relevent to some views.


> What will be the best approach to achieve it???

It's a good question.. I wonder what other people think...

Ross