Open Scene Graph Visualization II MSIM 842, CS 795/895

1. Open Scene GraphVisualization IIMSIM 842, CS 795/895 Instructor: Jessica Crouch

2. Implementation work • Time constraints and algorithmic complexity make it impractical to implement very many of the algorithms we’ll discuss • We’ll introduce OpenSceneGraph and do a little programming • It’s a useful skill • Good to learn after OpenGL • We’ll do enough to prepare you to pursue it further on your own • Our primary focus in class will remain on the readings

3. VTK Alternative • VTK was another strong possibility • OSG implements more graphics and simulation oriented features • VTK Implements more visualization, 2D & 3D data processing algorithms

4. Scene Graphs • Datastructure: Directed Acyclic Graph (DAG) • Usually a tree (only one parent per node) • Represents object-based hierarchy of geometry • Leaves contains geometry (triangles, etc.) • Each node holds pointers to children • Children can be • Group • Geometry • Matrix transform • Others…

5. Scene Graphs • Spatial transforms represented as graph nodes (rotation, translation, scaling, etc.) tricycle T T Seat Front Group Back wheels Handle bars Left wheel Right wheel T Front Wheel

6. Scene Graphs & Bounding Volumes • Basic idea: • Augment scene graphs with bounding volume data (spheres or blocks) at each node • Sometimes called “Bounding Volume Hierarchy” (BVH) • By applying clipping/culling tests to the bounding volumes, prune entire branches of the tree and possibly avoid processing many triangles

7. Scene graph example scene graph circles=BVs root

8. Culling: Overview • Hierarchical view-frustum culling • Use bounding volumes • Detail culling • Choose resolution of rendering based on object’s distace to camera

9. View-Frustum Culling • If a bounding volume (BV) is outside the view frustum, then the entire contents of that BV is also outside (not visible) • Avoid further processing of such BV’s and their containing geometry

10. root Example of Hierarchical View Frustum Culling camera

11. Scene Graphs & Detail Control • In complex scenes, a large percentage of time is wasted drawing tiny triangles • What happens if a triangle projects to one pixel or less? • Goal: Use scene graph to reduce workload of insignificant triangles

12. Detail Culling • Idea: Objects whose projected BV occupy less than N pixels are culled • This is an approximating algorithm since the triangles you cull may actually contribute to the final image • Advantage: trade-off quality/speed

13. Example of Detail Culling Images courtesy of ABB Robotics Products, created by Ulf Assarsson • Not much difference, but 80-400% faster • Good when moving detail culling OFF detail culling ON

14. Level-of-Detail Rendering • Use different levels of detail at different distances from the viewer • More triangles closer to the viewer

15. LOD rendering • Not much visual difference, but a lot faster • Use area of projection of BV to select appropriate LOD

16. Car chair Scene graph with LODs

17. Scene Graphs:State Changes • State changes affect the way the graphics pipeline execute • Recall pipelining from your architecture class • A change in state may require • flushing the pipeline • flushing the cache (on the graphics card) • Binding to a texture is one of the most expensive state changes

18. Scene Graphs:State Changes • Ideally, a scene would be drawn with the fewest possible state changes • Most expensive state changes would be minimized at the expense of faster state changes • How to accomplish this? • Must sort triangles according to their state • Manually? • Automatically?

19. Scene Graphs:State Changes • Scene graphs are used for state sorting • State information is stored with the nodes • Optimized rendering algorithms take advantage of groupings of same-state primitives • Scene graph hierarchy can be (algorithmically) rearranged for rendering into a state graph • Branchings represent state changes • State tends to be static

20. Open Scene Graph • Library for scene graphs • Built on top of OpenGL • C++ API • Object oriented • Open source • Growing popularity in graphics community • Commercially used (Boeing, NASA, …) • Web site: • http://www.openscenegraph.org

21. Open Scene Graph • Supports: • View frustum culling • Occlusion culling • Small feature culling • Level Of Detail nodes • State sorting • Many other features

22. OSG Distribution Contains • Core OSG – Focus on understanding this first • NodeKits – implements more node types than available in the core • Plugins – I/O for different file types • Interoperability libs – for using OSG with various packages, languages • Examples

23. Core OSG osg::Node - Base class for all nodes in the scene graph.

24. osg::Group - General group node which maintains a list of children.

25. Transform Nodes osg::Transform - A Transform is a group node for which all children are transformed by a 4x4 matrix. It is often used for positioning objects within a scene, producing trackball functionality or for animation.

26. Geometry Nodes osg::Geode - A Geode is a "geometry node", that is, a leaf node on the scene graph that can have "renderable things" attached to it. Renderable things are represented by objects from the Drawable class, so a Geode is a Node whose purpose is grouping Drawables.

27. Drawables Pure virtual base class for drawable geometry. Everything that can be rendered is implemented as a class derived from Drawable. A Drawable is not a Node, and therefore it cannot be directly added to a scene graph. Instead, Drawables are attached to Geodes, which are scene graph nodes. The OpenGL state that must be used when rendering a Drawable is represented by a StateSet. Drawables can also be shared between different Geodes, so that the same geometry (loaded to memory just once) can be used in different parts of the scene graph.

28. Plugins for file I/O • Has plugins to support reading/writing lots of graphics file formats and 3D models: • 3D database loaders include • OpenFlight (.flt) • TerraPage (.txp) • LightWave (.lwo) • Alias Wavefront (.obj) • Carbon Graphics GEO (.geo) • 3D Studio MAX (.3ds) • Peformer (.pfb) • Quake Character Models (.md2) • Direct X (.x) • Inventor Ascii 2.0 (.iv)/ VRML 1.0 (.wrl) • Designer Workshop (.dw) • AC3D (.ac) • native .osg ASCII format. • Image loaders include • .rgb • .gif • .jpg • .png • .tiff • .pic • .bmp • .dds • .tga

29. OSGEdit • Helps you compose scenes for OSG • Open source • Import models created with other programs, arrange your tree structure and transforms http://osgedit.sourceforge.net/

30. Open Scene Graph • OSG documentation is incomplete • No textbook • Doxygen code documentation • Some online tutorials and example programs • Takes time to learn your way around • Useful As-Is, development work continues

31. References • www.openscenegraph.org • http://www.nps.navy.mil/cs/sullivan/osgTutorials/SceneGraphOverview.htm • http://www.realityprime.com/scenegraph.php

32. Your To-Do List • Email me (jrcrouch@cs.odu.edu) your paper preferences by Sunday night. • Download and build OSG. See instructions on course web site. • Read through and run the Example osgAnimate. Draw the scene graph tree constructed in osgAnimate.cpp and bring to the next class. For each node, list object name and type. • Read the two assigned papers for the next class.