Hi, On Sunday, February 05, 2012 22:29:18 Heiko Schulz wrote: > Because no one, not Tim Moore, not me, not anyone, did say this! > The wiki article is btw by Tim Moore. > > But they said, that the bottleneck of GPU's is not the vertice count, but > other things. That never meant that you have to use many vertices as you > want. > > "... As many vertices as needed- not more or less!" ACK.
Currently OpenGL wise we are basically geometry setup bound - at least for the models. This really means that vertices are not an issue. That still means that for setting up that one draw with 3 triangles is about as heavy as setting up say 500 triangles, but the conclusion of this is *not* that you should schedule as many triangles ass possible. The right conclusion is to collapse as many triangles as *sensible* for culling into a single draw. A simple help is to switch on the onscreen stats in the viewer or flightgear and see if the yellow bar is longer than the orange one. A yellow bar longer than the orange one means that the cpu is not able to saturate the gpu. Again, beware this does in no way mean that you should write complicated shaders to saturate the gpu! This rather means that the geometry setup/state change time - the yellow one - must decrease! How to achieve that is a very long chain of right decisions. Starting with the modeler: Avoid having leaf geometry with very small parts. Collapse as much triangles as sensible for culling into the same leaf geometry. Sure if geometry needs a different texture or anything that requires a different OpenGL state you cannot collapse this into the same leaf. May be it makes sense then to collapse the textures into a common one. Then you might be able to collapse more geometry. Avoid animations that are not really required. If you need a transform node in between some geometry, the geometry below the transform needs to be drawn seperate which takes time on the cpu. Everything that needs geometry to be drawn seperately should be avoided to that level where it stops to make sense because of culling. May be introduce some level of detail. Not just avoid the whole model in some distance, but also provide a static model without animations, simple geometry for the mid range. May be provide some more culling friendly and detaild with the animations you need for the short range. Keep in mind that culling a model that you are close to should make you split the model into more parts that could possibly be culled away. But for culling a far away model is probably very sufficient to cull it either as a whole or not. Avoid state changes. Use as much common state as possible. Osg does a good job in sorting together draws using the same state, but if the overall scene contains plenty of different state osg cannot change that. A new texture is a new state for example. A new shader is a new state. .... Once your orange bar is longer than the yellow one, you can start to care for the shaders and their execution. When thinking about shaders, keep in mind that different GPU's perform very different on the same shader. Appart from OpenGL we spend a lot of time in scenegraph traversal. This is mostly due to plenty of structural and often needless nodes in the scenegraph. The reason or this is that historically the xml files did some implicit grouping for *every* animation in the xml file. To make that reilably work I had to introduce a huge amount of group nodes in the xml file loader. These really hurt today as they introduce a whole lot of group nodes that just have a single child which need to be traversed for update and for cull. Profiling shows that Group::traverse is the most used function in flightgear. The lowest hanging fruit could be to optimize away the redundant nodes from the top level model that gets loaded by a database pager request. We cannot do that recursively once a model part is loaded since the mentioned grouping nodes might be referenced in any level in the model hierarchy above the currently loaded model. So only the top level model could do this without braking tons of models. And regarding all that, even if your box already is gpu bound this does not mean that other driver/hardware combinations are gpu bound too. Always: As much as needed and as few as possible. Ok, there are plenty of other aspects of performance tuning a scene graph, but these are the ones I think are most important for flightgear as of today. Mathias ------------------------------------------------------------------------------ Try before you buy = See our experts in action! The most comprehensive online learning library for Microsoft developers is just $99.99! Visual Studio, SharePoint, SQL - plus HTML5, CSS3, MVC3, Metro Style Apps, more. Free future releases when you subscribe now! http://p.sf.net/sfu/learndevnow-dev2 _______________________________________________ Flightgear-devel mailing list Flightgear-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/flightgear-devel
