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

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Open Scene GraphVisualization IIMSIM 842, CS
795/895

• Instructor
• Jessica Crouch

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Implementation work

• Time constraints and algorithmic complexity make
  it impractical to implement very many of the
  algorithms well discuss
• Well introduce OpenSceneGraph and do a little
  programming
• Its a useful skill
• Good to learn after OpenGL
• Well do enough to prepare you to pursue it
  further on your own
• Our primary focus in class will remain on the
  readings

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VTK Alternative

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

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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

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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
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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

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Scene graph example
scene graph
circlesBVs
root
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Culling Overview

• Hierarchical view-frustum culling
• Use bounding volumes
• Detail culling
• Choose resolution of rendering based on objects
  distace to camera

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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 BVs and their
  containing geometry

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Example of Hierarchical View Frustum Culling
camera
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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

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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

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Example of Detail Culling
Images courtesy of ABB Robotics Products, created
by Ulf Assarsson
detail culling OFF
detail culling ON

• Not much difference, but 80-400 faster
• Good when moving

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Level-of-Detail Rendering

• Use different levels of detail at different
  distances from the viewer
• More triangles closer to the viewer

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LOD rendering

• Not much visual difference, but a lot faster

• Use area of projection of BV to select
  appropriate LOD

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Scene graph with LODs
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Scene GraphsState 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

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Scene GraphsState 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?

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Scene GraphsState 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

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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

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Open Scene Graph

• Supports
• View frustum culling
• Occlusion culling
• Small feature culling
• Level Of Detail nodes
• State sorting
• Many other features

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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

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Core OSG
osgNode - Base class for all nodes in the scene
graph.
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osgGroup - General group node which maintains a
list of children.
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Transform Nodes
osgTransform - 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.
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Geometry Nodes
osgGeode - 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.
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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.
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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

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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/
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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

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References

• www.openscenegraph.org
• http//www.nps.navy.mil/cs/sullivan/osgTutorials/S
  ceneGraphOverview.htm
• http//www.realityprime.com/scenegraph.php

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Your To-Do List

1. Email me (jrcrouch_at_cs.odu.edu) your paper
   preferences by Sunday night.
2. Download and build OSG. See instructions on
   course web site.
3. 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.
4. Read the two assigned papers for the next class.