3D Graphics

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3D Graphics

• 3D Graphics Pipeline
• Modelling
• TUTORIAL3D Graphics with POVRay

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Three-dimensional Graphics

• 3D Graphics is characterised more by the 3D
  nature of the Models (input into rendering)
  rather than the final display (output) modeling
  world
• In most cases, our output devices are limited to
  2D, so we need to utilise techniques to render
  these objects with the illusion of the third
  dimension of depth

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Three Dimensional Concepts

• 3D viewing positions
• projection
• depth cueing
• visible line and visible surface identification
• surface rendering
• three-dimensional views/ stereoscopic views

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3D GRAPHICS PIPELINE
WORLD/SCENE/OBJECT/CONCEPT
Modelling coordinates - world coordinate system,
- object coordinate system
3D MODELLING
VIEWING
3D CLIPPING
Camera coordinates
PROJECTION
Screen/Window coordinates
RASTERIZATION
Device coordinates
2D PIXELMAP DISPLAY
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Modelling
Implicit
3D Modelling
Polygonal
Volumetric
Parametric
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Viewing and Projection
3d models
camera setup
viewport
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Camera Positioning
Different views of a 3D model
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Projection
Representing three dimensions on two dimensional
media by Projection
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Albrecht Dürer (1471-1528).
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Vermeer
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Depth Cues
Shadows as depth cues
Perspective and Depth of Field
Highlight/Shadow
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Surface Rendering
Light and shadow colour of diferent parts of an
object suggest its three -dimensional nature
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Rasterization
Although the images are flattened to 2D, there is
some depth knowledge required on how to order them
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Modelling
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Goals

• Technical Goals
• Storage on digital memory
• Aim for a concise, unambiguous description of
  concept
• In graphics, optimise model for visual output
• Aesthetic Goals
• Look appealing
• Look Real? (maybe)
• Look the way we want it to

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Modelling Objects

• Basic requirements The shape of an object
• Additionally we may require
• Topology (interconnections, normals)
• Material Properties
• Colour
• Shininess
• Transparency
• Behavioural Information
• e.g. Density, Importance,
• Modeling decisions based heavily on the target
  application the rendering system

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Rendering

• What do rendering systems require?
• Consider
• Speed Do we require Real-time, Interactive or
  Offline rendering
• Space limitations
• Re-use?
• Rendering limitations (software/hardware)

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Wire frame rendering
Largely, only edge/surface data is required
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Filled Polygons
Colour/Weight/Name is required for each region
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Basic Shading
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Shading
The vectors N, L, V, R need to be stored or
evaluated
We need to know material colour, texture and
finish
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Phong Shading
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Reflectance Models
How different materials reflect incoming light
determined by roughness or finish
Basic Shading (Perfect Diffuse)
Perfect Mirror
Phong Highlight
Diffuse
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Advanced Illumination

• Offline methods employ more detailed simulations
  of light transfer
• Some of these now possible in real-time due to
  high-end graphics hardware

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Ray tracing
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Ray Tracing
The colours of light reflected from one point are
also dependent on indirect light bouncing off
other objects in the scene.
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Texture Mapping
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Texture Mapping
l
n


v
r
Lighting calculations as per usual but with
material properties (or colour values) varying
across the polygon.
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Bump Mapping
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Bump Mapping
A real bump distorts the directions of the
normals (this effects calculations of light
reflectance)
Fake bumps created by distorting the normals
although the geometry is still flat.
A bump-map texture applied to a flat polygon.
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Radiosity
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Modelling Shape

• Boundary Representation Surface Models
• Spatial Partitioning Volume Models
• Mathematical Models
• Discrete Models

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Surface Representations

• Polygon Mesh a.k.a. Wire frame

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Mathematical Models

• Parametric/Implicit
• Evaluators

Implicit
Parametric
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Discretization
300 polygons
120 polygons
1000 polygons
48 polygons
In most cases, mathematical models need to be
converted to discrete representations before we
use them in scenes. We need to find an
appropriate resolution for this.
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Volumetric Modules
Constructive Solid Geometry
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Volumetric Models

• Voxels (Volume elements/Volumetric pixels)

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Modelling Scenes
Modelling complex scenes made up many objects is
a two step process of modelling individual
objects and then placing these in the scene
(using transforms)
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Object Transformations
scaling
translation
rotation
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Scene Graphs
TRINITY
Individual object geometries are first modelled.
These are then linked using positional/orientation
al relationships (transforms)
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A Scene Graph
Body
Hood Trunk Front Bumper Rear Bumper Left Door
Left Door Glass Right Door Right Door
Glass Left Front Wheel Right Front Wheel Left
Rear Wheel Right Rear Wheel
There can be multiple levels of transforms e.g.
the door glass is linked to the door which is
linked to the body
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Sources of Visual Data

• Automated Acquisition
• Scanning
• Digital Cameras (Images and Video)
• Mathematical Models
• Mathematical curves and shapes
• Procedural Modelling
• Experimental Data
• Hand-coded Models (OR Hand Coded Scenes made up
  of Models that have been acquired from elsewhere)
• More on acquisition and Mathematical Modelling
  later

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3D Studio Max
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Sources of 3D Data

• 3D Scanners
• Digitizers

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3D scanner data usually very detailed often
requires some downsampling (simplification)
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Level of Detail
7,809 tris
488 tris
975 tris
1,951 tris
3,905 tris
Select the right number of polygons/triangles for
a specific purpose (e.g. for distant views,
low-end machines use less triangles)
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POVRay

• Basic Tutorial

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POVRay

• A popular free High-quality tool for 3D graphics
• Available for Windows, Mac, Linux
• Get it at http//www.povray.org
• Scenes are stored in a simple text file with a
  .POV extension

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Optional Libraries

• A number of pre-defined high-level functions are
  provided in the optional libraries.
• These are included as required using the include
  directive
• e.g.
• include colors.inc
• include shapes.inc
• include textures.inc

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Vectors

• Colours, Positions and Directions are represented
  using vectors e.g.
• rgb lt1, 1, 1gt
• rgbf lt0, 0, 1, 0.5gt
• translate lt0, 1, 1gt
• colors.inc includes some named colors e.g.
  Blue, White, Red. If you want to use this in your
  model, insert the following at the start of the
  file
• include colors.inc

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Comments

• Comments can be included in a similar way to java
  script e.g.
• texture
• pigment //this is a single line comment
• color Blue
• / this is a multi-line comment
• Still a comment
• Last line of comment /

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Camera Positioning

• The following is a simple set-up you can use for
  now
• camera
• Perspective
• location lt0, 0, -10gt
• direction lt0, 0, 1gt
• up lt0, 1, 0gt
• There are other options you can use for modelling
  a camera. e.g. instead of a direction, you can
  specify a target point (or look_at point) that
  the camera always points to.
• We will discuss camera modelling in more detail
  in later lectures

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Light Source

• For any object to be rendered visible, we require
  at least one light source or emissive object
• light_source
• lt5, 5, 5gt // position
• rgb lt1, 1, 1gt //colour

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Simple Objects

• A few basic shapes are provided in the shapes.inc
  library e.g. the sphere is declared with a vector
  specifying the location of its center and a
  radius
• sphere lt0, 0, 0gt, 2
• Material properties and transformations can also
  be included within the object description e.g.
• sphere
• lt0, 0, 0gt, 2
• pigment rgb lt1, 0, 0gt
• rotate lt4, 0, 0gt

r
c
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Simple Objects Planes

• Infinite planes are defined by a normal direction
  and a distance from the origin e.g.
• plane
• lt1, 1, 0gt, 2
• pigment Blue

n
d
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Simple Objects Cuboids

• Boxes are defined by a the positions of two
  opposite corners e.g.
• box
• lt-1,-1,-1gt, lt1, 1, 1gt
• rotate 30lt0, 1, 0gt
• pigment White

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Texture pigment

• Objects material properties are defined in the
  texture command which can be inserted into the
  object declaration
• The first part of the texture specifies the
  object colour
• texture
• pigment
• rgb lt1, 0, 0gt
• POVRay allows you to take a shortcut and leave
  out the texture part if you wish to just
  declare a simple pigment
• At the very least you need to specify a pigment
  for each object you declare

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Texture finish

• The finish command specifies the objects material
  properties such as shininess/gloss (more on this
  next week)
• texture
• pigment
• Red
• finish
• reflection 0.5
• brilliance