Hi Gianluca, The OSG allows you to create graphics context, make the context current and then dispatch rendering and do swap buffers yourself but this requires you to individually set everything up yourself.
What osgViewer does is provide all the basic functionality that 99% of users need out of the box, including handling context creation, multiple contexts, handling of database threading, viewer threading, event handling etc. It makes what would be a complicated task trivial, but with encapsulating all this functionality it has to make some assumptions about the way it's used, and I'm afraid your specific case isn't handled out of the box. You could if you want roll your own mini view with a pbuffer and do what you want completely behind the scenes so that the user doesn't know about it. This has the advantage that the pbuffer size can be different than the on screen size, it also avoid issues with other windows/toolboxes overlapping the window. The other route would be to stop the viewers threading, then make the context current and then insticate your won cull and draw to do the rendering, then release the context, and then start the viewer threading once more. Another route would be to use a viewer slave camera that sole job is to the render the screenshot, it has a NodeMask of 0x0 for all frames except the one you want to capture the screen shot, then you just switch it on for this frame. The slave camera would be rendered prior to the main frame - so you'd set the RenderOrder on this slave Camera to pre render. This approach would still require you to rendering a normal frame and have the extra cost of the rendering the scene, but for most apps you should be hit 60Hz anyway so it'd wouldn't be anything a viewer would notice, at most you'd get a frame drop when you added in the extra slave camera's rendering work. Robert. On Thu, Jan 28, 2010 at 9:46 AM, Gianluca Natale <gianluca.nat...@adstorino.it> wrote: > Thank you Paul. > The reason why I need to render in background is this: > > I need to save an image of my model with a different camera position and > orientation > than that currently visible to the user on the screen (i.e. in the front > buffer). > So, I thought that I could draw my model from a different point of view > in the back buffer, without swapping. > Then get it by reading pixels from the back buffer, and saving the pixmap in > an image file. > Finally, restore the camera position and orientation, > and resume in swapping back and front buffers after drawing. > So, the complete operation would be completely hidden to the user. > I always did it using standard OpenGL calls (glDrawBuffers(...), > glReadPixels(...), etc...) > since I could control whether and when swapping the buffers or not. > I wondered why OSG does not allow such an operation, I thought it is a > limitation. > But now you're saying that OSG provides a better way to do that. > Could anyone give me some more tips and suggestions (I'm still a newbie to > OSG)? > > Thanks in advance > Gianluca > > > -----Original Message----- > From: osg-users-boun...@lists.openscenegraph.org > [mailto:osg-users-boun...@lists.openscenegraph.org] On Behalf Of Paul Martz > Sent: mercoledì 27 gennaio 2010 19.25 > To: OpenSceneGraph Users > Subject: Re: [osg-users] drawing in back buffer without swapping? > > From a little code digging, there doesn't appear to be a way to turn > this off. ViewerBase::startThreading always adds a SwapBuffersOperation > per render thread, and SwapBuffersOperation::operator() always calls > swapBUffersImplementation and clear. > > Maybe there's a way to delete that Operation and add your own. > > Or you could derive your own GraphicsContext that allows you to make > swapBuffersImplementation a no-op, I guess. > > Or you could set up the Viewer embedded, in which case creating the > context and swapping is up to you and no longer managed by osgViewer. > > But I guess I'd wonder why you need to do this in the first place. If > you need to partially render, then do some app code, then render some > more before swapping, OSG has many facilities for this already. So can > you explain why you think you need to render into the back buffer but > not swap? > -Paul > > > Gianluca Natale wrote: >> >> >> Hi all. >> I have an osg::viewer and its associated graphics context, that is a >> double buffered context. >> >> I need to draw in the back buffer without swapping the back and front >> after rendering traversal. >> >> How can I do that? >> >> I cannot find a way to disable the swapping after drawing in >> osgViewer::Viewer and in osg::GraphicsContext. >> >> >> >> Thanks in advance >> >> Gianluca >> >> >> >> >> ------------------------------------------------------------------------ >> >> _______________________________________________ >> osg-users mailing list >> email@example.com >> http://lists.openscenegraph.org/listinfo.cgi/osg-users-openscenegraph.org > _______________________________________________ > osg-users mailing list > firstname.lastname@example.org > http://lists.openscenegraph.org/listinfo.cgi/osg-users-openscenegraph.org > > _______________________________________________ > osg-users mailing list > email@example.com > http://lists.openscenegraph.org/listinfo.cgi/osg-users-openscenegraph.org > _______________________________________________ osg-users mailing list firstname.lastname@example.org http://lists.openscenegraph.org/listinfo.cgi/osg-users-openscenegraph.org