Re: [osg-users] Multiple ClearNodes in a scene, or 2D layering of 3D geometry
Hi Chris, On Fri, May 15, 2009 at 2:10 AM, Chris 'Xenon' Hanson xe...@alphapixel.com wrote: I will expand on the abstract explanation, and then reveal my optimal solution -- which is a bit of a hack, but is still the most elegant. Basically we have VPB terrain for a very large area, but low detail. Within it, we have a smaller world, which the user cannot leave. This terrain is also built with VPB, at a much higher detail. This small world needs to be rebuilt frequently with different data, and we don't wish to rebuild the entire surrounding world each time. this is because the surrounding world doesn't change, and we don't want to spend all the time rebuilding the surrounding world to the same degree of detail as the inset local world. It's kind of like a skydome/box/environment technique, but it's a world-dome/box/environment. It has to be rendered with Z-test and Z-write enabled to sort out self-occlusion, whereas a skydome can often skip Z-test/z-write as long as it's a sphere and you use backface culling. Depth partioning then :-) The solution is to render the surrounding world, then wipe the Z-buffer, then render the local world. this means that even if the local world terrain surface drops below the surrounding world in the middle, it still obscures the exterior world. Things could get weird close to the edge of the local world, but we prevent that from being an issue by restrictions on the local world and near-edge movement. What I did was this: Render surrounding world in RenderBin -3. Create an empty piece of dummy Geometry (with a Geode) and render it in RenderBin -2. Render the local world in RenderBin -1. The dummy geometry has a draw callback on it that issues a raw OpenGL glClear() only on the Z-buffer channel. It doesn't disrupt the OpenGL state in any way, just wipes the Z-buffer. It works great. no second cameras to track, no RTT, no goofy stuff. The only cost is a full-screen Z-clear operation. Thanks to everyone for their suggestions. So you're simulating using two slave Camera's for each of the depth ranges. osgViewer is capable of doing depth partioning although the osgdepthpartition example hasn't yet been ported across to take advantage of the flexibility of osgViewer, it is something on my TODO list. By putting the management of the dept partitioning into the viewer rather than the scene you remove viewing specific code form the scene graph, and make it possible to load any scene into the viewer and have it manage the depth partitioning automatically. Robert. ___ osg-users mailing list osg-users@lists.openscenegraph.org http://lists.openscenegraph.org/listinfo.cgi/osg-users-openscenegraph.org
Re: [osg-users] Multiple ClearNodes in a scene, or 2D layering of 3D geometry
Hi Chris, On Thu, May 14, 2009 at 1:01 AM, Chris 'Xenon' Hanson xe...@alphapixel.com wrote: The Goal: Render both of them, but with object A _always_ obscured by object B, regardless of what their Z values (which are mostly coincident) would normally dictate. This type of task is exactly what the stencil buffer is good for. The OSG supports stencil buffer natively. See the osgreflect example. Robert. ___ osg-users mailing list osg-users@lists.openscenegraph.org http://lists.openscenegraph.org/listinfo.cgi/osg-users-openscenegraph.org
Re: [osg-users] Multiple ClearNodes in a scene, or 2D layering of 3D geometry
Robert Osfield wrote: This type of task is exactly what the stencil buffer is good for. The OSG supports stencil buffer natively. See the osgreflect example. I thought of that too, but it seemed like overkill since all I needed was a bulk Z clear at the right time. Plus, I couldn't immediately describe to myself how the stencil buffer would accomplish this. Don't you still need to wipe the Z values in order for the Object B fragments to overwrite the Object A fragments (both of which have to be generated with Z write and Z test enabled to self-occlude properly)? Robert. -- Chris 'Xenon' Hanson, omo sanza lettere Xenon AlphaPixel.com PixelSense Landsat processing now available! http://www.alphapixel.com/demos/ There is no Truth. There is only Perception. To Perceive is to Exist. - Xen ___ osg-users mailing list osg-users@lists.openscenegraph.org http://lists.openscenegraph.org/listinfo.cgi/osg-users-openscenegraph.org
Re: [osg-users] Multiple ClearNodes in a scene, or 2D layering of 3D geometry
Hi Chris, Paul suggested using multiple Camera objects in the Scene and playing with the heirarchy, reference frame and identity matrix to make a sub-camera track its parent. That's probably what I would have tried too. It seems weird to me that the ClearNode clears at the start of rendering and not just when it's reached in the draw traversal, but perhaps that's the intended behavior. Let us know what solution you come up with, it's an interesting problem. Just out of curiosity, what is the context for which you need this? J-S -- __ Jean-Sebastien Guayjean-sebastien.g...@cm-labs.com http://www.cm-labs.com/ http://whitestar02.webhop.org/ ___ osg-users mailing list osg-users@lists.openscenegraph.org http://lists.openscenegraph.org/listinfo.cgi/osg-users-openscenegraph.org
Re: [osg-users] Multiple ClearNodes in a scene, or 2D layering of 3D geometry
On Thu, May 14, 2009 at 1:32 PM, Chris 'Xenon' Hanson xe...@alphapixel.com wrote: Robert Osfield wrote: This type of task is exactly what the stencil buffer is good for. The OSG supports stencil buffer natively. See the osgreflect example. I thought of that too, but it seemed like overkill since all I needed was a bulk Z clear at the right time. Plus, I couldn't immediately describe to myself how the stencil buffer would accomplish this. Don't you still need to wipe the Z values in order for the Object B fragments to overwrite the Object A fragments (both of which have to be generated with Z write and Z test enabled to self-occlude properly)? Well if just want to do a clear of the z buffer then the tool to use an osg::Camera for the second pass, and have this camera use a relative reference frame and identity view and projection matrices so it doesn't change the view, but enable the clear of the depth buffer but not the colour buffer. This is very similar to how one can do HUD's using Camera's, the only different being the how one sets the ReferenceFrame. FYI, ClearNode controls the clear of the RenderStage that it's nested within. A pre/post draw Camera is implemented in the rendering back-end as a RenderStage (a nested Camera just ruses it RenderStage it's within). But... you don't really need the ClearNode has the Camera has it's own settings you can use. Robert. ___ osg-users mailing list osg-users@lists.openscenegraph.org http://lists.openscenegraph.org/listinfo.cgi/osg-users-openscenegraph.org
Re: [osg-users] Multiple ClearNodes in a scene, or 2D layering of 3D geometry
I Robert -- No, I don't think stencil applies here. Chris might not have described the rendering task precisely. What he is trying to do is a sort of smart depth partition, in which he knows that part of his scene graph is always the background, and another part of always the foreground. So if he can just clear depth after rendering the background, he has all that he needs. Now that I see how similar this is to depth partitioning, I wonder if slave cameras are the way to go? They will both have the same view (but probably different near/far planes). Paul Martz Skew Matrix Software LLC http://www.skew-matrix.com +1 303 859 9466 -Original Message- From: osg-users-boun...@lists.openscenegraph.org [mailto:osg-users-boun...@lists.openscenegraph.org] On Behalf Of Robert Osfield Sent: Thursday, May 14, 2009 2:38 AM To: OpenSceneGraph Users Subject: Re: [osg-users] Multiple ClearNodes in a scene,or 2D layering of 3D geometry Hi Chris, On Thu, May 14, 2009 at 1:01 AM, Chris 'Xenon' Hanson xe...@alphapixel.com wrote: The Goal: Render both of them, but with object A _always_ obscured by object B, regardless of what their Z values (which are mostly coincident) would normally dictate. This type of task is exactly what the stencil buffer is good for. The OSG supports stencil buffer natively. See the osgreflect example. Robert. ___ 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
Re: [osg-users] Multiple ClearNodes in a scene, or 2D layering of 3D geometry
Paul Martz wrote: I Robert -- No, I don't think stencil applies here. Chris might not have described the rendering task precisely. What he is trying to do is a sort of smart depth partition, in which he knows that part of his scene graph is always the background, and another part of always the foreground. Paul is right. I tried to simplify it to make it easier to explain, but probably messed it up. I will expand on the abstract explanation, and then reveal my optimal solution -- which is a bit of a hack, but is still the most elegant. Basically we have VPB terrain for a very large area, but low detail. Within it, we have a smaller world, which the user cannot leave. This terrain is also built with VPB, at a much higher detail. This small world needs to be rebuilt frequently with different data, and we don't wish to rebuild the entire surrounding world each time. this is because the surrounding world doesn't change, and we don't want to spend all the time rebuilding the surrounding world to the same degree of detail as the inset local world. It's kind of like a skydome/box/environment technique, but it's a world-dome/box/environment. It has to be rendered with Z-test and Z-write enabled to sort out self-occlusion, whereas a skydome can often skip Z-test/z-write as long as it's a sphere and you use backface culling. The solution is to render the surrounding world, then wipe the Z-buffer, then render the local world. this means that even if the local world terrain surface drops below the surrounding world in the middle, it still obscures the exterior world. Things could get weird close to the edge of the local world, but we prevent that from being an issue by restrictions on the local world and near-edge movement. What I did was this: Render surrounding world in RenderBin -3. Create an empty piece of dummy Geometry (with a Geode) and render it in RenderBin -2. Render the local world in RenderBin -1. The dummy geometry has a draw callback on it that issues a raw OpenGL glClear() only on the Z-buffer channel. It doesn't disrupt the OpenGL state in any way, just wipes the Z-buffer. It works great. no second cameras to track, no RTT, no goofy stuff. The only cost is a full-screen Z-clear operation. Thanks to everyone for their suggestions. BTW, should we document that there really should only be one ClearNode in the scene graph at a time, and what effect exactly it actually has? -- Chris 'Xenon' Hanson, omo sanza lettere Xenon AlphaPixel.com PixelSense Landsat processing now available! http://www.alphapixel.com/demos/ There is no Truth. There is only Perception. To Perceive is to Exist. - Xen ___ osg-users mailing list osg-users@lists.openscenegraph.org http://lists.openscenegraph.org/listinfo.cgi/osg-users-openscenegraph.org
Re: [osg-users] Multiple ClearNodes in a scene, or 2D layering of 3D geometry
Hi Chris, I will expand on the abstract explanation, and then reveal my optimal solution -- which is a bit of a hack, but is still the most elegant. [...] Wow, yeah that's a nice setup and good solution to it. I'll probably be able to use a similar technique in our simulators. Thanks for sharing more detail about your situation and solution! BTW, should we document that there really should only be one ClearNode in the scene graph at a time, and what effect exactly it actually has? Well, if the doxygen description of the class didn't clearly state that, then yes, it should be added, of course. I just read the description, and it contains other details but this (and the fact that it actually clears at the start of the frame and not when the node is reached in the draw traversal) is not mentioned. J-S -- __ Jean-Sebastien Guayjean-sebastien.g...@cm-labs.com http://www.cm-labs.com/ http://whitestar02.webhop.org/ ___ osg-users mailing list osg-users@lists.openscenegraph.org http://lists.openscenegraph.org/listinfo.cgi/osg-users-openscenegraph.org
