Open Scene Graph
Visualization IIMSIM 842, CS 795/895Instructor:Jessica Crouch
Implementation workTime constraints and algorithmic complexity make it impractical to implement very many of the algorithms well discuss
Well introduce OpenSceneGraph and do a little programmingIts a useful skillGood to learn after OpenGLWell do enough to prepare you to pursue it further on your own
Our primary focus in class will remain on the readings
VTK AlternativeVTK was another strong possibility
OSG implements more graphics and simulation oriented features VTK Implements more visualization, 2D & 3D data processing algorithms
Scene GraphsDatastructure: 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 childrenChildren can beGroupGeometryMatrix transformOthers
Scene GraphsSpatial transforms represented as graph nodes (rotation, translation, scaling, etc.)
tricycleBack wheelsFront GroupLeft wheelRight wheelTTSeatHandle barsFront WheelT
Scene Graphs & Bounding VolumesBasic idea:Augment scene graphs with bounding volume data (spheres or blocks) at each nodeSometimes 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
Scene graph examplecircles=BVsscene graphroot
Hierarchical view-frustum cullingUse bounding volumes
Detail cullingChoose resolution of rendering based on objects distace to camera
View-Frustum CullingIf 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 BVs and their containing geometry
Example of Hierarchical View Frustum Cullingcamera
Scene Graphs & Detail ControlIn 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
Detail CullingIdea: 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
Example of Detail CullingNot much difference, but 80-400% fasterGood when movingdetail culling OFFdetail culling ONImages courtesy of ABB Robotics Products, created by Ulf Assarsson
Level-of-Detail RenderingUse different levels of detail at different distances from the viewerMore triangles closer to the viewer
LOD renderingNot much visual difference, but a lot faster
Use area of projection of BV to select appropriate LOD
Scene graph with LODs
Scene Graphs:State ChangesState changes affect the way the graphics pipeline executeRecall pipelining from your architecture classA change in state may require flushing the pipelineflushing the cache (on the graphics card)
Binding to a texture is one of the most expensive 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 stateManually?Automatically?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 graphBranchings represent state changesState tends to be static
Scene Graphs:State Changes
Open Scene GraphLibrary for scene graphsBuilt on top of OpenGLC++ APIObject orientedOpen sourceGrowing popularity in graphics communityCommercially used (Boeing, NASA, )Web site:http://www.openscenegraph.org
Open Scene GraphSupports:View frustum cullingOcclusion cullingSmall feature cullingLevel Of Detail nodesState sortingMany other features
OSG Distribution ContainsCore 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
Core OSGosg::Node - Base class for all nodes in the scene graph.
osg::Group - General group node which maintains a list of children.
Transform Nodesosg::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.
Geometry Nodesosg::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.
DrawablesPure 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.
Plugins for file I/OHas 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
Helps you compose scenes for OSG
Import models created with other programs, arrange your tree structure and transformshttp://osgedit.sourceforge.net/
Open Scene GraphOSG documentation is incompleteNo textbookDoxygen code documentationSome online tutorials and example programs
Takes time to learn your way around
Useful As-Is, development work continues
Your To-Do ListEmail me (firstname.lastname@example.org) 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.
Objects close to each other in the virtual world are close in the scene graphNext slides shows some examplesHappens early (on CPU) in the pipeline == GOODAssuming that testing a sphere against the view frustum is cheaper than rendering the objectAlso, called SCREEN-SIZE CULLING
Bullet 2: This implies that this is an approximation.forts slide
Bullet 3: higher speed if larger threshold (N)
Often combined with Hierarchical VF CULL,that is, DONE in a hierarchical fashion as wellIDEA: Does not make sense to use 10,000 triangles when the object cover 50 pixels on-screen
Different levels of detail == different number of polygons
BECAUSE, when an object is far away, Other criteria to select LOD: distance to the object, rendering budget (I.e., do we have time to draw a more complex LOD?)Many different types of LODs : what weve shown here are called Discrete Geometry LODsRead more in the book about Alpha LODs, Geomorph LODS, and more sophisticated LODselection critera.
Speedups: usually at least a factor of 2