Hi Robert Thanks for this completness of your answer, it could be usefull for others
I misinterpreted your review of my Readback callback, i though you suggested removing a drawcb from updatecb wasn't thread safe: it induces the idea there were a case (unknow to me) where draw and update were concurrents..:/ Thanks to lever this unrational doubt :) Cheers robertosfield wrote: > Hi Julien, > > On 7 February 2018 at 18:16, Julien Valentin <> wrote: > > > Perhaps my brain bugs, > > but AFAIK there's no way to mod a scenegraph at runtime because it's shared > > among threads. > > > > What do you mean by this, as it's really just a vague statement that > could have so many different interpretations. > > Most OSG applications modify the scene graph during the update or > event phases of the frameloop. Some even modify it during cull and > draw, but these just need to be more careful to handle the possibility > of multi-threaded draw. > > > > > Do you think a mutual exclusion "zone" should be defined in the main loop > > could be a good way to proceed? > > We'd then could define a standard way to mod sg executed in the "zone" > > > > The "zone" for the OSG is between the viewer.advance() and > viewer.renderingTraversals(), the viewer by design runs the main frame > loop single threaded and these traversals are all single threaded, > except when the DRAW_THREAD_PER_CONTEXT or > CULL_THREAD_PER_CAMERA_DRAW_THREAD_PER_CONTEXT are used, in which case > you need to set the DataVariance of the StateSet and Drawable that > contain dynamic data so that draw dispatch can hold back the main > thread till all the dynamic objects have been dispatched into the > OpenGL FIFO. > > The OSG is design to multi-thread cull and draw traversals. > > The OSG also is designed to have a DatabasePager for dynamically > loaded/computed new scene graph elements that are prepared in separate > background threads. > > You can also create your own threads and have them do work in the > background, you just need to make sure you are modifying bits of the > main scene graph that is being rendered. > > If you want some generic mechanism to allow you to modify any part of > the scene graph at any time you need to take a step back and wonder > what you are really asking for. Any scene graph, not just the OSG, > has to create a world and capture that world that is static for the > instance in time you take the picture of it, this means that the cull > and draw traversals really need to work on a static representation > during their traversals, for this instance in time all the operations > done need to be done with a single time stamp, otherwise you'd get > different objects all moving at different points and ended up with an > unholly mess on screen. > > This means that there is natural rhythm to what is done when, which is > why the OSG has a single FrameStamp that is updated by > viewer.advance() and all the update, event, cull and draw traversals > all share this same FrameStamp. You might have multiple threads > reading or writing, by they need to be synchronized. The osgViewer > itself will provide much of this threading and high level > synchronization for you. > > It's not a general purpose do whatever you want when ever you want > free for all, but it's a scene graph so it has to have this syncronity > to it. There are exceptions that some applications might need but for > these they need to decide exactly what they need to do and when and > use their own multi-buffering or mutexing to manage it. Since what > these exception are is completely open ended there is no way the OSG > can provide an API for it. > > Robert. > _______________________________________________ > osg-users mailing list > > http://lists.openscenegraph.org/listinfo.cgi/osg-users-openscenegraph.org > > ------------------ > Post generated by Mail2Forum ------------------ Read this topic online here: http://forum.openscenegraph.org/viewtopic.php?p=72955#72955 _______________________________________________ osg-users mailing list [email protected] http://lists.openscenegraph.org/listinfo.cgi/osg-users-openscenegraph.org
