Computer Graphics

1
Computer Graphics
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Lecture Outline

• 2D vs. 3D graphics
• Basics of 3D graphics
• Graphics hardware
• Application Program Interfaces
• Comparisons and benchmarks
• Computer Graphics Applications

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The Emergence of 3-D Graphics

• 2-D graphics has been used for years for
  illustration, diagrams, 2-D CAD, etc.
• 3-D graphics has until recently been available
  only on high-end workstations
• 3-D graphics is now available on standard PCs

4
Applications of 3-D Graphics

• CAD
• Scientific visualization
• virtual reality
• simulation
• games

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Pixel Graphics Vs. Vector Graphics

• Pixel graphics
• paint programs
• eg., Windows Paintbrush
• Vector graphics
• lines and polygons
• eg., Microsoft Draw

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2-D Versus 3-D Graphics
Two-dimensional
Three-dimensional
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3-D Wireframe Graphics

• Three dimensional objects are drawn as meshes of
  connected points
• Interaction and animation requires geometric
  calculations only
• The resulting image is often ambiguous

or
?
wireframe image
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3-D Shaded Solids

• Three dimensional objects are drawn as smoothly
  shaded solid objects
• More calculations are needed than for wireframe
• Slower than wireframe, but more realistic

shaded
wireframe
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Typical Graphics Functions

• Draw
• Pick and Select
• Transform
• Shade and Render

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Draw

• Draw lines or polygons connecting specified points

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Pick and Select

• Identify the graphic object under the cursor
• Identify the graphic objects inside a region

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Transform

• Rotate, scale, translate

Transformed
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3-D Transform
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Shade and Render

• Colour and shade surfaces, and display the image
  on the screen

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Graphics generation
Geometry and colour calculation
Frame buffer and display
User application
Rasteriz- ation
Graphics pipeline
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User Application
Z
Y
X
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Geometry Calculation
Z
(xw, yw, zw)
Y
X
World coordinates
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Colour Calculation
Colour is calculated based on surface
properties, light direction and colour, using
lighting models.
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Rasterization
Transform polygon to pixels
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Hidden Surface Removal

• When several overlapping polygons are drawn on
  the screen, which one is on top?

Which is right?
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Z-buffering
This pixel is drawn on screen

• A fast hardware solution is the Z-buffer
• The depth of each pixel relative to the screen
  is calculated and saved in a buffer
• The pixel with the smallest depth is the one that
  is displayed
• The other pixels are on surfaces that are hidden

Screen
Polygons
Z
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Lighting Models and Shading

• For visual realism, lighting models have been
  developed to illuminate the surfaces of solid
  models
• These models incorporate
• ambient lighting and illumination
• diffuse reflection of directional lighting
• specular reflection of directional lighting

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Lighting Types
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Ambient Illumination

• Ambient light comes from many directions, as a
  result of multiple reflections and emission from
  many sources
• The resulting surface illumination is uniform
• Depends only on object material

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Diffuse Reflection

• Dull, matte surfaces reflect directional light
  equally in all directions

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Specular Reflection

• Shiny surfaces reflect light in one direction
• The reflection is the colour of the incident
  light, not the colour of the object
• Specular reflection is the source of highlights

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Dull and Shiny Surfaces
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Lighting Models

• Real objects are illuminated by a combination of
  these three illumination types.
• CAD systems like IDEAS allow the user to define
• colour and location of lights
• types and intensities of lights (point, diffuse,
  etc.)
• object material properties, including colour,
  specularity, reflection coefficients

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Example
Powerpoint can vary the lighting and material
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Lighting Models

• There are several popular shading algorithms
• Flat shading
• each polygon has a uniform colour
• the surface appears faceted
• Phong or Gouraud shading
• the colour is varied smoothly across the polygons
• the result is a surface that appears to be smooth

Flat
Gouraud
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Flat Shading

• The simplest form of shading is flat shading
• Each polygon is uniformly coloured, depending on
  its orientation relative to the light source

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Flat Shading
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Flat Shading
n
?
d
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Gouraud and Phong Shading

• Flat shading is simple, but gives a faceted
  appearance
• Gouraud and Phong developed techniques to
  smoothly interpolate the illumination across
  polygons
• The resulting shading is smooth and continuous

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Gouraud Shading

• The surface normal at the vertices is calculated
  by averaging the adjoining face normals
• The vertex intensity is then calculated
• The intensity is interpolated across the polygon
  based on the vertices

vertex normal
face normals
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Phong Shading

• Rather than interpolating the lighting, Phong
  shading interpolates the surface normal across
  the polygons
• The intensity is then calculated based on the
  interpolated normal
• This gives more realistic specular lighting

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Ray Tracing and Radiosity

• Ray tracing and radiosity are two techniques for
  achieving the ultimate in visual realism
• These techniques work well with curved surfaces

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Ray Tracing

• Ray tracing traces a ray of light from the eye to
  a light source
• Along the way, the light ray may be occluded (in
  shadow), reflected or refracted
• Ray tracing realistically renders scenes with
  shiny and transparent objects

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Radiosity

• In many scenes, illumination is primarily diffuse
• Diffuse illumination results from the absorption
  and reflection of diffuse light from many objects
  in the scene
• Radiosity uses thermal models of emission and
  reflection of radiation to accurately calculate
  diffuse lighting
• Radiosity is very good at rendering architectural
  interiors

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Texture Mapping

• For realism, photographic textures are mapped
  onto the surfaces of objects
• Example textures
• woodgrain
• concrete
• grass
• marble
• Texture mapping is very computationally intensive

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Other Graphics Effects

• Effects that are fairly easy to achieve include
• Fog
• Transparency

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Example - Wireframe
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Example - Flat Shaded
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Example - Smooth Shaded
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Example - Texture Mapped
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Example - Shadows
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Example - Ray Traced
Green glass
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Examples
texture mapping
reflections
shadows
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Graphics Standards

• Graphics functions are incorporated into a C,
  FORTRAN, or Pascal program by using a library of
  functions or subroutines
• This library of functions is known as an
  Application Program Interface (API)
• There is no single standard, but several APIs are
  in wide use.

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A Selection of Graphics APIs

• GKS
• 2-D graphics only
• Windows GDI (Graphics Device Interface)
• 2-D only
• PHIGS (Programmers Hierarchical Interactive
  Graphics Standard)
• Xlib
• X-windows graphics commands
• PEX
• PHIGS Extension to X

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Emerging Standards in APIs

• OpenGL (developed by Silicon Graphics)
• open standard
• available on all platforms
• intended for professional-level graphics like CAD
• Direct3D
• developed by Microsoft
• Windows only
• intended for games

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Double Buffering

• For smooth animations, some graphics cards
  support double buffering
• A new image is drawn in one buffer while the
  other buffer is displayed on the screen
• When the update is finished, the buffers swap
  so that change on the screen is instantaneous
• This prevents flicker as the image is redrawn.

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Frame Buffers

• A generic VGA frame buffer typically stores 8
  bits per pixel (256 colours)
• Graphics cards with more memory use 24 bit frame
  buffers
• 16 million colours
• For most PCs, this is as good as it gets

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Frame Buffers

• High end graphics hardware supports frame buffers
  with over 100 bits per pixel!
• two 24 bit images
• 24 bit Z-buffer (depth)
• overlay bits
• alpha bits for transparency
• and other special purpose bits
• These bits are sometimes called bit planes
• This requires much more graphics memory than in
  common in PCs

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Stereo vision using 3-D glasses

• One frame buffer contains the left eye view, the
  other contains the right eye view
• The views are rapidly alternated on the screen
• Glasses with liquid crystal shutters synchronize
  the left and right views
• The results are amazing!

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Graphics Generation
Geometry and colour calculation
Frame buffer and display
User application
Rasteriz- ation
Graphics pipeline
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Graphics Designs

• Software pipeline
• all processing done in software
• slow, inexpensive
• example PC with 2D graphics card
• Software geometry, hardware rasterization
• good compromize
• example Matrox Millenium graphics card
• Hardware pipeline
• all processing done in hardware
• very fast, expensive
• high-end Silicon Graphics workstations

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Graphics Performance Comparison
Source OPC CDRS results
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Price Comparison
Source OPC CDRS results
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Price/Performance Comparison
Source OPC CDRS results
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Summary

• 3D graphics important for engineering
  applications
• 3D graphics is now available on standard PCs

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