1 L Feb-15-04 SMD159, Object-Oriented Modeling Overview - - PDF document

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Feb-15-04 SMD159, Object-Oriented Modeling 1 L

INSTITUTIONEN FÖR SYSTEMTEKNIK

Object-Oriented Modeling

David Carr Fundamentals of Computer Graphics Spring 2004

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Overview

• Motivation
• Hierarchical models, trees, and DAGs
• Scene graphs
• Jazz, an example scene graph system
• Other graphical trees

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INSTITUTIONEN FÖR SYSTEMTEKNIK

Motivation

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Why Object Oriented?

• Strong tie between object-oriented design and graphics
• Often hand-in-hand
• Concentration on drawing has drawbacks
• Interactivity is awkward
• Independent movement (animation) is awkward
• Pruning the display list is difficult
• Reuse is difficult

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Advantages of Object-Oriented Models

• Can define objects and create many instances
• E.g. a car has 4 wheels

+ Define the wheels once + Create 4 instances in different locations

• Separates location from structure
• Simplifies Animation

Wheels rotate independently of the car

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INSTITUTIONEN FÖR SYSTEMTEKNIK

Hierarchical Models, Trees, and DAGs

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Object Models

• Consider the car example
• We would want:
• Wheels made of tires, rims, and hub caps
• Windows made of glass
• Body panels made of metal and painted
• Lens for lights made of different colors of plastic
• Lights with different intensities
• …
• Note there are many common components

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Simplified Car Object

• Easily represented as a

tree

• But, requires 4 copies of

the wheel

• Better structure, DAG
• Directed Acyclic Graph

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Trees versus DAGs

• Logically the same
• Tree requires unique nodes, DAG can share
• DAG representations of trees are loop free
• For object models:
• DAGs require separation of common properties from specific

properties

• I.E., the values of parameters must be separated from

parameter existence.

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Storing Trees

• Binary trees are simple
• Left and right children
• But, arbitrary trees don’t

easily fit

• Left child, Right sibling
• List representation
• Natural for pre-order traversal

using recursive traversal

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Traversal and OpenGL

• Consider a hierarchical model
• Each level is defined relative to the one above (its container)
• We need a uniform way to handle changes
• If we use recursive traversal
• Associated with each left-child there is:

+ A new sub-model + A new graphical context

• Associated with each right-sibling

+ The sub-model context is unchanged + But, the sibling may have its own graphical context

• So:
• A push-draw node/draw sub-model-pop applies to the graphics
• The program can’t do this for us, we must do it explicitly

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Pseudo Code

• Method DrawTree
• glPushMatrix, glPushParameters, …
• Modify to create map local coordinates, parameters
• DrawTree on left child
• Draw self
• DrawTree on right sibling
• glPopMatrix, glPopParameters, …
• Return

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Example, a Humanoid Figure

• Head and limbs attached to

torso as children.

• Top figure shows block

diagram

• Bottom figure is tree
• Matrix traversal if M is

transform for torso

• M fi MMh head fi M torso fi
• MMlua fi MMluaMlla left-lower

arm fi

• MMlua Left-upper arm fi M …

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Graphical Objects

• How should we implement an object-oriented graphics

system?

• What are the characteristics of objects?
• Topology
• Geometry
• Surface material
• What other things do we need?
• Lights
• Viewers (more than one per scene?)
• Operations on objects such as rendering

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Simple Example, a Cube (Topology)

• 6 faces, with 8 vertices
• Same length sides
• Right angles in each corner
• Canonical cube
• Front, lower-left corner at (0,0,0)
• Unit sides
• Vertices (0,0,0), (1,0,0), (1,1,0), (0,1,0), repeat z=1
• Faces (0,1,2,3), (1,5,6,2), …
• Note, could define based on center!

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Cube (Other attributes)

• Geometry
• Location in world or relative

to parent

• Front, lower-left corner (center)
• Size
• Length of side
• Orientation
• Direction of front, bottom edge
• Direction from center to a

vertex

• Materials and/or Color
• By face
• By entire cube
• Only light properties
• Methods
• Render

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INSTITUTIONEN FÖR SYSTEMTEKNIK

Scene Graphs

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Motivation

• Need a structure to construct hierarchical

representations of a scene

• Would like to describe the scene and then modify only

small parts

• Would like to avoid (re)writing the code to display the

hierarchical representation

• Reuse
• Reduced chance of error
• Would like to save the scene in a file for reuse

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Scene Graphs

• We’ve see how trees (DAGs) can represent objects
• Nodes are the objects
• Links are their relationship
• What if we extend the idea? We could
• Have nodes for transformations
• Represent cameras as nodes

fiScene Graphs

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Scene Graph Basics

• Different types of nodes for:
• Transformations
• Object aggregation
• Material properties
• Cameras

+ Limited parts of the hierarchy + Different views + Special properties

• Types are application dependent
• Links for relationships
• Graph compiler
• API or language
• Rendering engine

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Scene Graph Systems

• Open Inventor
• SGI, API
• Based on GL (now OpenGL)
• VRML
• Text description language
• Based on Open Inventor
• X3D
• Extension of VRML adding XML syntax, semantics
• Java 3D
• Java-based API
• Based on OpenGL
• Jazz
• Java-based, 2D API for Zooming User Interfaces (ZUIs)
• Piccolo
• Simplified version of Jazz

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INSTITUTIONEN FÖR SYSTEMTEKNIK

Jazz, an Example Scene Graph System

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Zooming User Interfaces

• Problems with Overlapping

Windows

• Significant time managing

windows.

• Replacement causes a disruptive

context switch

• Even a “rooms” metaphor causes

the context switch

• Scrolling is not an appropriate

browsing method + For many applications + But, it’s all you usually get.

• ZUI an Alternate Paradigm
• Infinite canvas, infinitely

stretchable

• No icons, overlapping windows
• Navigation by zoom and pan
• Smooth context switch maintains

spatial continuity

• Small windows effectively icons
• Rooms are groups of windows
• Semantic zooming
• Navigation aids and shortcuts
• Windows may be views of the

canvas

• Windows may be pasted to the

display instead of the canvas

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Why a Special Scene Graph System?

• ZUIs have unique requirements. They must support:
• Custom non-rectangular or transparent graphics
• Large numbers of objects
• Arbitrary transforms and hierarchical grouping
• Continuous and smooth zooms and pans
• Rendering objects differently in different (simultaneous)

contexts

• Multiple views
• Event handlers for individual elements and groups of objects
• Standard tool kits don’t met these requirements

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Jazz Architecture

• Nodes, visual components, and cameras
• Nodes
• Hierarchy, which represents relationships between objects
• No visual appearance
• Based on light-weight ZNode object
• Specify where something is in the visual hierarchy (structure)
• Visual Components
• Provide appearance
• Attached to visual leaf nodes and visual group nodes
• Specify what something looks like (content)

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Jazz Architecture

• Cameras
• Views of the scene graph
• Implement a drawing surface that is a Swing

component

• Can also be interior to a scene graph, provide an

embedded view of the graph (portals)

• Contains a list of layers

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Jazz Events

• Follow the Java model
• Listeners attached to scene graph nodes
• Multiple listeners allowed
• May be associated with any scene graph node
• Two kinds
• Input, mouse events, etc.
• Object, result from modifications of the scene graph
• Events are passed up the tree to ancestors by default
• Are dispatched in the “event thread”
• Jazz is not thread safe
• Same restrictions as Swing and AWT apply

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Relation Between Jazz and Swing

• Jazz’s ZCanvas is a Swing object
• A Jazz scene graph can be displayed in a Swing frame

(JFrame)

• Jazz provides ZSwing to embed Swing widgets
• N.B.! JMenu and JComboBox don’t work

+ Use ZMenu and ZComboBox instead

• JPopupMenu is always potentially heavyweight

+ Can extend outside its parent window + Manage at the level of ZCanvas

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Overview of Classes

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Visual Components

• ZShape
• Abstract class
• Meant to be extended by visual

components that wrap the standard java.awt.Shapes.

• ZShape sub classes and their

underlying shape model

• Zarc, uses Arc2D
• ZCubicCurve, uses CubicCurve2D
• ZQuadCurve, uses QuadCurve2D
• ZLine, uses Line2D
• ZRectangle, used for rectangles and

squares

• ZRoundedRectangle, uses

RoundedRectangle2D

• ZShape sub classes, continuted
• ZPolyline, set of line segments that

connect a set of points.

• ZPolygon, fills in the polygon between

a set of points.

• ZEllipse, hard-cornered or rounded

ellipse.

• ZImage, an image.
• ZLabel, lightweight visual component

to support a label containing one line of text.

• ZText, paints a text string.
• ZCamera, view onto a portion of the

scenegraph.

• ZSwing wraps a standard Java Swing

widget.

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Transforms

• Standard 2D affine transforms
• Affine transforms preserve straightness and parallelism of

lines

• Based on ZTransformGroup
• All nodes contained in a ZTransformGroup are transformed
• Implements: scale, rotate, and translate

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INSTITUTIONEN FÖR SYSTEMTEKNIK

Other Graphical Trees

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Constructive Solid Geometry

• Problem:
• Polygons represent surfaces
• Difficult to compute properties of solids (volume, center-of-

mass)

• Ambiguous representations
• Solution:
• Use solid primitives
• With set operations », «, & -
• Construct expression trees
• Use post-order traversal to render

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CSG Tree, Example

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Shade Trees

• Remember Phong shading
• Simple shading equation
• Computation of the reflection
• Renderman uses a similar

structure to express custom renderers r = 2 (l · n ) n - l I =kd Id l · n + ks Is (v · r )a + ka Ia

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BSP Trees

• Binary Spatial Partition trees
• Insert polygons front to back
• Backward in order transversal

+ Right branch first

• Avoids resorting by depth
• If the user moves
• Change order of traversal
• Prune

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Quadtrees and Octrees

• Recursive division of a plane into 4 parts
• Stops when a subdivision is 1 color
• Octrees extend to volume

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Questions?