The sync blocks on a swap buffers call, not on the first OpenGL call (unless by accident it's an actual OpenGL blocking call). Non-blocking OpenGL calls run asynchronous on the back buffer (traditional double buffering). A glGet call is an actual OpenGL blocking call as it forces a pipe flush. Making blocking calls when trying to run real time thoroughly defeats the purpose.
Giovanni, what you're seeing is typical behavior when syncing with the vertical retrace. To maintain real-time at 50Hz each frame must be rendered in less than 20ms (1/50). If a frame just happens to take 21ms, then the buffer swap will block for 19ms before actually swapping buffers. Hence, your frame rate is cut completely in half (21ms + 19ms = 40ms = 25Hz). And it also introduces nasty temporal aliasing. I'm not aware of another way to synchronize with such regularity as the monitor retrace. I'm guessing that deterministic hardware is required given the nature of something like OpenSceneGraph on a PC. Although, you can achieve near real-time by things like database tuning and pre-emptive rendering. But, nothing to guarantee actual real time. Extra hardware is not needed to run at multiples of the frame rate. Just set the retrace rate to the desired frame rate and run sync'd. Boom - multiples of the frame rate. BTW, there's something about this alleged '2 frame latency' charge that just doesn't pass the smell test. Mostly - ATI sure the hell wouldn't let 'em! Bob -----Original Message----- From: osg-users-boun...@lists.openscenegraph.org [mailto:osg-users-boun...@lists.openscenegraph.org] On Behalf Of Bunfield, Dennis AMRDEC/AEgis Sent: Wednesday, June 23, 2010 4:35 PM To: osg-users@lists.openscenegraph.org Subject: Re: [osg-users] OSG and vsync (UNCLASSIFIED) Classification: UNCLASSIFIED Caveats: FOUO The sync actually blocks on the first open gl call. So your osg update and cull stages will run then you will block on the draw stage until the vsync. Your problem is actually worse than 20ms w/o you knowing it. For Nvidia cards there is a built in 2 frames of latency. So even after your sync you won't see the image you updated come out the DVI until 2 render frames later. In order for you to do what you want you will need some expensive frame lock hardware with external syncs to run at a multiple of the frame rate. -----Original Message----- From: osg-users-boun...@lists.openscenegraph.org [mailto:osg-users-boun...@lists.openscenegraph.org] On Behalf Of Giovanni Ferrari Sent: Wednesday, June 23, 2010 3:28 AM To: osg-users@lists.openscenegraph.org Subject: [osg-users] OSG and vsync Hi, i'm developing an application that need vsync to avoid tearing but i'm having some problems. Here is the (really simplified) code structure of my application: Code: ... while ( !mViewer->done() ) { mGraph->Update(...); mViewer->frame(); } I've noticed frame() function is blocking when vsync is enabled. This means that i call the frame function, i stay blocked for something like 20ms (50hz PAL), and then i must terminate the Update call in the vsync period otherwise i won't be able to draw a new frame ( the graphic card draw the old content of frame buffer without changes performed in the update function. Changes will be performed in the next frame ). As you can immagine this is a big problem for real-time applications cause i'm introducing 20ms of retard. Is there a way to syncronize frame call without using vsync ? or can i scompose the frame function to be able to separate functions that operate on the graph from those that perform the rendering ? The second solution could help me cause i would be able to operate on the graph with mGraph->Update(...) even if the frame "part" that write the frameBuffer is blocked. I hope i've explained my problem clearly. Thank you! Cheers, Giovanni ------------------ Read this topic online here: http://forum.openscenegraph.org/viewtopic.php?p=29295#29295 _______________________________________________ osg-users mailing list osg-users@lists.openscenegraph.org http://lists.openscenegraph.org/listinfo.cgi/osg-users-openscenegraph.or g Classification: UNCLASSIFIED Caveats: FOUO _______________________________________________ osg-users mailing list osg-users@lists.openscenegraph.org http://lists.openscenegraph.org/listinfo.cgi/osg-users-openscenegraph.org _______________________________________________ osg-users mailing list osg-users@lists.openscenegraph.org http://lists.openscenegraph.org/listinfo.cgi/osg-users-openscenegraph.org
