Can I use OSG without Producer, and/or with already created MFC or WxWindow windows ?
You can use OSG with or without Producer, even if you are creating your own windows. See the osgsimple example for an example of using OSG with a generic windowing environment. OSG itself is windowing system agnostic, so setting up the window and OpenGL graphics context will be up to your application. osgsimple uses Producer, but is written in a way to provide you the skeleton to use your own windowing system. Alternatively, you can use Producer in your own windowing environment as well by setting each Camera’s RenderSurface to the window you’ve created.
