1. Open Scene Graph Visualization II MSIM 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
Objects close to each other in the virtual world are close in the scene graph

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

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
Happens early (on CPU) in the pipeline == GOOD

10. Example of Hierarchical View Frustum Culling
root
Assuming that testing a sphere against the view frustum is cheaper than rendering the object
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
Also, 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 well

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

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

15. LOD rendering Not much visual difference, but a lot faster
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 we’ve shown here are called “Discrete Geometry LODs”
Read more in the book about Alpha LODs, Geomorph LODS, and more sophisticated LOD
selection critera.
Speedups: usually at least a factor of 2
Use area of projection of BV to select appropriate LOD

16. Scene graph with LODs
Car chair

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:

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

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

32. Your To-Do List
me 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.