Open Scene Graph Visualization II MSIM 842, CS 795/895 Instructor: Jessica Crouch

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

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

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…

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

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

Scene graph example scene graph circles=BVs root Objects close to each other in the virtual world are close in the scene graph

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

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

Example of Hierarchical View Frustum Culling root Assuming that testing a sphere against the view frustum is cheaper than rendering the object camera

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

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

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

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,

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

Scene graph with LODs Car chair

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

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?

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

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

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

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

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

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

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.

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.

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.

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

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 http://osgedit.sourceforge.net/

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

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

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.