Dr. Jeffrey Huang, Assistant Professor

1
N420 / D379 Multimedia Project DevelopmentFall
2000
Topic 4
3D Geometric Graphics

• Dr. Jeffrey Huang, Assistant Professor
• The School of Informatics, New Media Program,
  IUPUI
• e-mail huang_at_cs.iupui.edu

2
3D Graphics

• 3D Computer Programs
• Rapidly create, view, and change 3D forms without
  using physical 3D materials
• Easily construct abstract 3D spaces building
  and combining objects
• Experimenting different material properties
• Reflectiveness, transparency, and texture
• Controlling lighting effects
• Creating 3D models and looking at them from any
  point of view
• Frustration
• Difficult to work with 3D information on a 2D
  screen with 2D input device
• Unfamililiar technical terms Primitives,
  polygonal meshes, sweeping a profile, Gouraud
  shading, ray tracing .

3
3D Concepts

• Use all concepts and methods in 2D graphics
• 2D geometry ? 2D shapes ? continuous 2D geometric
  description ? reaterized ? raster-based screens
  and printers
• 3D geometry ? 2D 3D shapes ? continuous 2D
  geometric description ? reaterized ? raster-based
  screens and printers
• Challenges added dimension and effects of
  lighting on object surfaces
• Modeling
• Create 3D objects (similar to 2D in digital
  design and layout program)
• Rendering
• Rasterize the models (more complex than in 2D)
• Project 3D information onto a 2D plane for
  display
• Calculate lighting effects on object surfaces
  (hardware acceleration)

4
Modeling

• Modeling is coping with complexity (van Dam,
  1994)
• Creating the objects, concept, or phenomenon that
  presents the important features
• However, only presenting some part of a scene
  (feeling, experience, salient features), while
  leaving other parts out
• 3D geometric modeling
• Need not be based on physical geometric forms
• Can be Scientific Visualization (data
  exploration) creation of images based on very
  large sets of data. Ex Wind Flow, NASA,
• Depth
• X - and y - axes on Cartesian coordinate system
  z -axis

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Modeling (cont.)

• 3D geometric modeling
• Create simple model elements
• Assemble the elements into more complex objects
• Arrange objects in a 3D scene
• Choose materials
• Set up lights
• Choose view points and rendering model
• Behavioral modeling
• Model objects behaviors
• Animate articulated characters, facial
  expression, realistic skin deformation. Ex
  deformable model, particle system,
• Real 3D world accuracy

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Modeling (cont.)

• 3D geometry-based applications vs. sample-based
  programs
• Voxel (volume element)
• A point in a 3D space with associated values.
• Volume visualization
• The modeling of objects interiors.
• ex sample-based data from medical
• imaging equipment such as CT or MRI
• Medicine, engineering, and scientific fields.
• Graphic programming language (ex OpenGL)
• Volumetric sculpting
• 3D paint-brushes (painting in 3D), sculpting away
• material, using 3D imaging techniques
• Voxel-based modeling
• Computers need more memory
• Faster algorithms
• Better I/O devices. ex force-feedback devices
  for the feel of the material

7
3D Touch Creating Building Blocks

• 3D primitives
• Primary building blocks
• Sphere, cube, cone, cylinder, torus (donut-like),
  banana, fig.7.9
• Sweeps
• Profile 2D geometric shape
• Sweeping profile through space to describe a 3D
  form
• Extrusion
• Swept along a straight line or curved path
• Extrusion
• Curve extrusion
• Scaling extrusion
• Swept around an axis
• Revolve
• Produce symmetric result. ex human-made 3D
  objects

Scale Extrusion
Revolve
Curve Extrusion
Extrusion
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3D Touch Creating Building Blocks (cont.)

• More than one profile in process
• Lofting
• Object presentation
• Linear approximation
• Polygon ? polygonal mesh
• ex http//www.bk.isy.liu.se/candide/candemo.html
  
• Resolution ? polygonal representation ? curve
  model
• Curve
• Spline-based models or NURBS-based models fig.
  7.19
• More realistic-looking and accurately detailed
  renderings
• Boolean Operations
• 3D sculpting
• Polygon vertices
• Spline patches 2D surfaces bounded by four
  splines and controlled by the control vertices
  (CV points)

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Assembling the building blocks

• Sketch
• Decomposition
• Composition
• Based on base operations translation, rotation,
  and scaling
• Hierarchy fig. 7.26
• Tree diagrams root, node, leaves
• Relationship between nodes parent/child
• Joints
• Pin Joint, hinge
• Ball and socket joint
• Skeleton structure
• Maser-Instance Relationships
• In designing complex scenes in which identically
  shaped items are used repeatedly but may need to
  be redesigned globally
• Apply different models will change all at once

Cyclops
root
Upper-body
neck
Lower-body
nodes
head
hair
arm
Leg_l
Leg_r
nose
eye
palm
Feet
mouth
leaves
fingers
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Algorithmic Form generation

• Modeling Nature
• Fractals
• Self-similarity the structure of small section
  resembles the structure of the whole object.
• Tree, fire, mountain, hair, skin texture
• Fractal dimension and iterations
• Growth simulations
• Genetic and Evolutionary Art, A-Life
• Genetic Algorithms Darwinian nature selection
• Bleed Mutation, Crossover of genes, and
  selection
• Biological form by the design of mathematician
  computer science
• Particle systems
• Algorithmically controlled of masses of
  individual shapes

11
Rendering

• Rendering
• Rasterize the models
• Project 3D onto 2D plane
• Calculate lighting effects on object surface
• Surface Properties
• Complexity of color in 3D
• Color picker is not quite helpful because 3D CG
  model almost never appears to be a single color
• Gradients of color value
• Reflection and shadow from surrounding objects
  and lights make color change
• Surface normal
• Outward-facing direction of the face
• Vector
• A line with direction (arrow)

normal
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Surface Properties

• Surface reflection properties
• Diffuse reflection
• angle of the light shining on surface
• Light bounces equally in all directions
• Specular reflection
• mirror-like reflection
• highlights
• Material dependent, view dependent
• Light bounces off only at an angle equal and
• opposite to that of the incoming light
• Non-perfect reflector specular highlight is
  evident over a small angle and loses its
  coherence
• Combination of diffuse and specular reflection
  Diffuse Specular

Incoming light
Diffuse
Perfect specular
Imperfect specular
Diffuse Specular
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Surface Properties (cont.)

• Surface Transmission properties
• Light is absorbed
• Light is transmitted
• Transparency light is not reflected
• Refracted light when light passes through
  different media it is bent
• Refraction index water ? 1.3, air ? 1, diamond ?
  2.4
• Texture
• 3D paint-type images applied to 3D objects
• 3D painting