Scene Modeling - Rochester Institute of Technology

Scene Modeling

July 1, 1999

July 1, 1999

Motivational Film

✔Grinning Evil Death– MIT Media Lab (1990)

July 1, 1999

Logistics

✔Paper summaries on Scene Modeling– Any takers?

✔Projects– Presentations

• Presentation week…need 2 additional hours(early or late)

• 15 minutes / presentation• signup -- schedule on Web

July 1, 1999

Photography and Light

pho•tog•ra•phy, n., the process or art ofproducing images of objects by the action oflight on a sensitized surface, esp, a film in acamera.

Yeah, we get it…photography,light…whatever..

July 1, 1999

Computer Graphics as Virtual Photography

camera(captureslight)

syntheticimage

cameramodel

(focusessimulatedlighting)

processing

photoprocessing

tonereproduction

realscene

3Dmodels

Photography:

ComputerGraphics:

Photographicprint

July 1, 1999

Today’s Class

✔Scene Modeling– Placing individual object model, lights, etc.

into a single scene

July 1, 1999

Today’s Class

✔Scene Modeling– Coordinate transformations

– Object hierarchy– Scene Description Languages

• OpenGL• Graph Based• Object-Oriented

July 1, 1999

Coordinate Transformations

✔How to specify placement of individualobjects into a scene

✔Two coordinate systems– Object coordinate system

– World coordinate system

July 1, 1999


✔Homogeneous Coordinates– (x,y,z) -> (X, Y, Z, W)

– (x,y,z) = (X/W, Y/W, Z/W, 1)

✔Why?– Easily incorporate projections into

transformation model…more when we getto camera models

July 1, 1999

Coordinate Transformation

✔Transformation Matrix

{ {objecttiontransforma

44434241

34333231

24232221

14131211

world

11

⋅

=

o

o

o

w

w

w

z

y

x

mmmm

mmmm

mmmm

mmmm

z

y

x

4444 34444 21

July 1, 1999


✔Transform operations– Scaling

– Translation– Rotation

July 1, 1999


✔Scaling– Make object bigger or smaller

=

1000

000

000

000

),,(z

y

x

zyx s

s

s

sssS

July 1, 1999


✔Translation– Move object to a new location

=

1000

100

010

001

),,(z

y

x

zyx d

d

d

dddT

July 1, 1999


✔Rotation– rotate object about a given axis

−

=

1000

0100

00cossin

00sincos

)(θθθθ

θzR

July 1, 1999


✔Rotation

−

=

1000

0cossin0

0sincos0

0001

)(θθθθ

θxR

−=

1000

0cos0sin

0010

0sin0cos

)(θθ

θθ

θyR

July 1, 1999


✔Individual operations can be composedinto a single matrix.

xy RSTRM ⋅⋅⋅=

July 1, 1999


✔Beware: Transformation is notcommunitive

TRSRRSTR xyxy ⋅⋅⋅≠⋅⋅⋅

July 1, 1999


✔So how do you transform objects?– On a point by point basis

– Example• Sphere• NURB• Polygonal Mesh

July 1, 1999

Object Hierarchy

✔Wish to rotate wrt p

BA

p

July 1, 1999

Object Hierarchy

✔TBW = transformation of B wrt world✔TAW = transformation of A wrt world✔TBA= transformation of B wrt A

AWBABW TTT ⋅=

✔To rotate about p, change TBA

July 1, 1999

Object Hierarchy

✔Stack of transformation matrices

Arm wrt world

Body wrt world

Finger wrt world

July 1, 1999

Object Heirarchy

✔Enough about transformations….whatabout languages?

✔Any questions?

July 1, 1999

Scene Modeling Languages

✔From the low level to the high level– OpenGL

– Inventor (Graph Based Scene Languages)– True Object Oriented Model Languages.

July 1, 1999


✔Open GL– Originally designed as an API for SGI’s

rendering hardware– Very low level “C” programming library– Includes:

• Scene modeling• Rendering• Animation• Some rudimentary and low level interactivity

July 1, 1999


✔OpenGL - scene modeling– Objects

• only knows about polygons (different kinds ofpolygons, but just polygons)

• Polygons are specified by vertex list• You are responsible for definition of normal

vector.

July 1, 1999


✔OpenGL– Transformations

• Routines for scaling, translation, and rotation• Routines for direct manipulation of

transformation matrix• Maintains a stack of transformation matricies

for support of object hierarchy

July 1, 1999


✔OpenGL– Shading

• Support for flat at smooth shading• Support for ambient, diffuse, and specular

material components• Native support for texture mapping

July 1, 1999


✔OpenGL– Lighting

• support for ambient light• support for multiple individual light sources• support for spotlights

July 1, 1999


✔OpenGL– Models a state machine. Maintains

current:• Color• Transformation (transformation stack)• Material properties

– New polygonal objects will use currentcolor, transforms and materials.

July 1, 1999


✔Open GL– Summary

• Low level “C” interface• Polygonal model• State machine• Transformation Stack

July 1, 1999


✔Open GL - Further Reading– Fosner, OpenGL Programming for

Windows95 and WindowsNT– http://www.sgi.com/Technology/openGL

July 1, 1999


✔Graph Based Languages (Inventor)– Object oriented wrapper around GL

• Not a truly Object Oriented scenerepresentation.

– Scene is represented by a tree structure(scene graph)

– Scene graph is passed to a viewer which isresponsible for rendering

– Basis for VRML and Java3D

July 1, 1999


✔Graph Based Languages (Inventor)– Object oriented wrapper around GL

• Not a truly Object Oriented scenerepresentation.

– Implemented as C++ library withassociated file format.

July 1, 1999


✔Inventor– Scene is represented by a tree structure

(scene graph)– Scene graph is passed to a viewer which is

responsible for rendering

– Basis for VRML and Java3D– Max will be talking about VRML in a couple

of weeks.

July 1, 1999


✔Inventor -- Graph Nodes– Shape nodes

• represent physical objects• cone, cylinder, sphere, 3Dtext, triangleMesh,

NURB surface

– Lighting Nodes• introduces lights to a scene• directional, point, spot

July 1, 1999


✔Inventor– Camera Nodes

• Introduces a camera model• Orthographic/Perspective camera

– Property nodes• Appearance nodes (texture, color, material)• Transformations nodes (scale, rotate, translate)• Other (environment, normal, draw style)

July 1, 1999


✔Inventor– Group nodes

• Support for definition of “objects”• Locally groups subgraphs• Switch node (switch between 2 subgraphs)

July 1, 1999

Scene Modeling Language

✔Inventor

July 1, 1999


✔Inventor -- Node Kits

July 1, 1999


✔Graph Actions– Traversal of scene graph for a particular

purpose.– Examples

• Rendering• Searching• Compute Bounding Box• Interactivity / Picking• Write to file

July 1, 1999


✔Inventor - Rendering– Like OpenGL, Inventor assumes a state

machine.– Unlike OpenGL, entire state can be placed

on a stack.

– Visiting a shape node introduces a newobject with “current” properties

– Visiting a property node is equivalent tomaking a given material current.

July 1, 1999


✔Inventor - Rendering– Entering a group node pushes current

state on stack– Leaving a group node pops current state

off stack.

July 1, 1999


✔Inventor -- Interactive aspects:– Event model

• Ability to define and respond to events

– Sensors• Tracks changes in scene and responds• Performs node change at given time intervals

– Manipulators• Interface to input devices (e.g. mouse,

trackball)

July 1, 1999


✔Inventor - summary– “Object oriented” wrapper to OpenGL

– Scene represented as graph– Actions performed on graphs for specific

purposes (like rendering)

– Supports events and interactivity– Precursor to VRML and Java3D

July 1, 1999


✔Inventor - further reading– [Strauss92]

– Strauss/Carey, The Inventor Mentor

July 1, 1999


✔Inventor - not a truly OO scene model– Separation between object and materials

– Still based on state machine model.– Created for CG user. Still doesn’t fit real

world paradigm• e.g. We usually aren’t concerned

transformation matricies

July 1, 1999


✔GROOP– truly object oriented scene description

– transformations, material, geometry, andcolor are all associated with an object

– uses a camera/actor/stage paradigm.

July 1, 1999


✔GROOP

July 1, 1999


✔GROOP– Scene object is a collection of objects in a

scene.– Objects are introduced by literally “Adding”

them to a scene.

– Unlike Inventor, order of addition is notimportant.

July 1, 1999


✔GROOP– Extensibility achieved by use of C++

inheritance.

July 1, 1999


✔GROOP - summary– example of a truly object oriented scene

description mechanism– intuitive– extensible

– Alas, not used….never caught on.

July 1, 1999

Summary

✔Scene Modeling– means of assembling objects into a scene

– Coordinate Transformations– Scene Modeling Languages

• Open GL• Inventor• GROOP

July 1, 1999

Next Class

✔Switch from modeling to rendering✔Rendering Equation

– basic theory behind rendering algorithms

July 1, 1999

Remember

✔Class Web Site:– http://www.cs.rit.edu/~jmg/virtualPhoto

✔Any questions?

Documents

Scene Modeling - Rochester Institute of Technology