Scene Representation How does one describe the objects in a Scene - PDF document

Scene Representation  How does one describe the objects in a Scene Graphs 3D scene?  Scene Graphs State Machine Model API Scene Modeling Languages / API  From the low level to the high level  State Machine Model  State Machine Model -- OpenGL  Current “state” of rendering is maintained in global variable or stack.  Graph Based Scene Languages  Material properties  Inventor  Object transformation matrix  VRML  Lights / Camera parameters  True Object Oriented Model Languages  GROOP  Much like a “graphicsContext” State Machine Model API State Machine Model API  Open GL  OpenGL - scene modeling  Originally designed as an API for SGI’s  Objects rendering hardware  only knows about polygons (different kinds of  Very low level “C” programming library polygons, but just polygons)  Includes:  Polygons are specified by vertex list  Scene modeling  You are responsible for definition of normal  Rendering vector.  Animation  Some rudimentary and low level interactivity 1

State Machine Model API State Machine Model API  OpenGL  OpenGL  Models a state machine. Maintains current:  Transformations  Color  Routines for scaling, translation, and rotation  Transformation (transformation stack)  Routines for direct manipulation of transformation matrix  Material properties  Maintains a stack of transformation matricies  New polygonal objects will use current color, for support of object hierarchy transforms and materials. State Machine Model API Scene Graphs  Open GL  Scene is represented by a tree structure (scene graph)  Summary  Low level “C” interface  Scene graph is passed to a viewer  Polygonal model which is responsible for rendering  State machine  Basis for VRML and Java3D  Transformation Stack  Other State Model specs  RenderMan (RIB) Scene Graphs Scene Graphs  Inventor  Inventor  Object oriented wrapper around GL  Scene is represented by a tree structure (scene graph)  Not a truly Object Oriented scene representation.  Scene graph is passed to a viewer which is  Originator of the scene graph responsible for rendering  Implemented as C++ library with  Basis for VRML and Java3D associated file format. 2

Scene Graphs Scene Graphs  Inventor -- Graph Nodes  Inventor  Shape nodes  Camera Nodes  represent physical objects  Introduces a camera model  cone, cylinder, sphere, 3Dtext, triangleMesh,  Orthographic/Perspective camera NURB surface  Property nodes  Lighting Nodes  Appearance nodes (texture, color, material)  introduces lights to a scene  Transformations nodes (scale, rotate, translate)  directional, point, spot  Other (environment, normal, draw style) Scene Graphs Scene Graphs  Inventor  Inventor  Group nodes  Support for definition of “objects”  Locally groups subgraphs  Switch node (switch between 2 subgraphs) Scene Graphs Scene Graphs  Graph Actions  Inventor - Rendering  Traversal of scene graph for a particular  Like OpenGL, Inventor assumes a state purpose. machine.  Examples  Unlike OpenGL, entire state can be placed on a stack.  Rendering  Visiting a shape node introduces a new  Searching object with “current” properties  Compute Bounding Box  Interactivity / Picking  Visiting a property node is equivalent to  Write to file making a given material current. 3

Scene Graphs Scene Modeling Language  Inventor - Rendering  Inventor - summary  Entering a group node pushes current state  “Object oriented” wrapper to OpenGL on stack  Scene represented as graph  Leaving a group node pops current state  Actions performed on graphs for specific off stack. purposes (like rendering)  Precursor to VRML and Java3D Scene Modeling Language - Scene Modeling Languages VRML  VRML  Inventor - further reading  Virtual Reality Modeling Language  [Strauss92]  “ an open standard for 3D multimedia and shared virtual worlds on the Internet .”  Strauss/Carey, The Inventor Mentor  Text-based scene description language  Need plug-in/applet for viewing VRML scenes  ISO Standard  Originally designed to create virtual worlds as described in books like Snowcrash.  VRML is a Scene Description Language NOT an API  Based on Inventor file format Scene Modeling Language - Scene Modeling Language - VRML VRML  Nodes  Have data fields & children Transform { Translation = Xform translation 1.0 1.0 6.5  Node Types (1.0, 1.0, 6.5) children [ Shape {  Geometry appearance Appearance { material Material {  Appearance diffuseColor 1.0 0.0 0.0 Shape }  Transformation } appearance geometry geometry Sphere { radius 1.0 }  Sound }  Grouping ] Sphere Material }  Viewpoint Radius = 1.0 Color = red 4

Scene Modeling Language - Scene Modeling Language - VRML VRML Define Seat (location, mycolor)  VRML PROTO Seat [ exposedField SFVec3f location 0.0 0.0 0.0 Translation = exposedField SFColor mycolor 1.0 1.0 1.0  Supports Encapsulation and Reuse Xform location ] Transform {  Ability to define new node types translation IS location children [ Shape {  PROTO Statement appearance Appearance { Shape material Material { diffuseColor IS myColor appearance } geometry } geometry Sphere { radius 1.0 } } ] Sphere Material } Seat { location 1.0 1.0 6.5 Radius = 1.0 Color = mycolor mycolor 1.0 0.0 0.0} Scene Modeling Language - Scene Modeling Language - VRML VRML PROTO Seat [ exposedField SFVec3f location 0 0 0  Inventor and VRML not a truly OO scene exposedField SFColor mycolor 1.0 1.0 1.0 Seat3 ] … model DEF Seat1 Seat  Separation between object and materials { location 1.0 1.0 6.5 mycolor 1.0 0.0 0.0} DEF Seat2 Seat  Still based on state machine model. { location 2.0 1.0 6.5 Seat2 Seat4 mycolor 1.0 0.0 0.0}  Created for CG user. Still doesn’t fit real world DEF Seat3 Seat { location 3.0 1.0 6.5 paradigm mycolor 1.0 0.0 0.0} DEF Seat4 Seat  e.g. We usually aren’t concerned transformation { location 4.0 1.0 6.5 matricies Seat5 mycolor 1.0 0.0 0.0} Seat1 DEF Seat5 Seat  Scene Graphs are excellent for specifying object { location 5.0 1.0 6.5 mycolor 1.0 0.0 0.0} heirarchies Color / Location of each seat can be modified by sending messages Scene Modeling Language Scene Modeling Languages  GROOP  GROOP  truly object oriented scene description  transformations, material, geometry, and color are all associated with an object  uses a camera/actor/stage paradigm. 5

Scene Modeling Language Scene Modeling Languages  GROOP  GROOP  Scene object is a collection of objects in a  Extensibility achieved by use of C++ scene. inheritance.  Objects are introduced by literally “Adding” them to a scene.  Unlike Inventor, order of addition is not important. Scene Modeling Languages Summary  GROOP - summary  On the road to scene graphs  example of a truly object oriented scene  Scene Modeling Languages description mechanism  State Model  Scene Graphs  intuitive  True Object Oriented Descriptions  extensible  Questions?  Alas, not used….never caught on. Paper Preview -- Week 2 Paper Preview - Week 2  Procedural Models (Mon)  Light / Ray Tracing (Wed)  [Catmul/Clark78] -- Original B-Spline paper  [Whitted80] - The original ray tracing  [Doo/Sabin78] -- Splines to polys. paper  [Musgrave, etal89] -- Fractal landscapes  [Parker, et al99] - Interactive ray tracing  [Fournier, et al82] -- Fractals and procedural  [Woop, et al 2005] -- RPU: Ray tracing in models hardware  [Reeves83] -- 1st particle system paper  [Perlin/Hoffert89] -- Hypertexture 6