CS430 Computer Graphics

1
CS430 Computer Graphics

• Introduction

2
Topics

• Introduction
• Applications
• Elementary Output Primitives
• Graphics Output Devices
• Graphics Input Devices
• Graphics Software Standards
• Interactive Computer Graphics

3
Introduction

• What is Computer Graphics?
• History
• Whirlwind Computer
• 1950, MIT
• Computer-driven CRT (Cathode Ray Tube) displays
• Sketchpad drawing system
• 1963, Ivan Sutherland
• Interactive graphics keyboard and light pen
• Data structures
• DAC system
• 1964, GM
• Automobile design (CAD/CAM)

4
Introduction

• History (contd)
• Bitmap graphics
• Apple and PC
• Bitmap
• A ones and zeros representation of the
  rectangular array of points on the screen.
• The points are called pixels or pels (picture
  elements).
• GUIs
• SIGGRAPH www.siggraph.org
• Why computer graphics?

5
Applications

• Art, entertainment, publishing
• Image processing
• Monitoring manufacturing processes
• Displaying simulation
• CAD
• Scientific analysis and visualization

6
Elementary Output Primitives

• Polyline
• Curves
• Text
• Filled region
• Raster image
• Each primitive is associated with a set of
  attributes

7
Polyline

• Connected sequence of straight lines
• Figs. 1.8 and 1.9
• Point (vertex, or dot)
• Degenerated case of polyline
• E.g., drawDot(x, y)
• Line
• Polyline of one line segment
• E.g., drawLine(x1, y1, x2, y2)

8
Polyline

• Polyline
• E.g., drawPolyline(poly), where poly is a list
  containing all endpoints
• Fig. 1.10
• Polygon
• Formed by connecting the first and last points of
  a polyline
• Simple polygon no two edges cross
• Convex polygon
• Concave polygon
• Fig 1.11

9
Attributes of Polyline

• Thickness
• Color
• Style
• Solid, dotted, dashed,
• Joint (Fig 1.13)
• Butt-end
• round end
• mitered
• trimmed mitered

10
Curves

• Circle
• Ellipse
• Parabola
• Hyperbola
• B-spline
• NURBS
• Attributes?

11
Text

• Character string
• Shape defined by (Fig. 1.16)
• Bit map
• Polyline or curve
• E.g., drawString(x, y, string)
• Attributes
• Font or type face (Fig. 1.15)
• Size
• Spacing
• orientation

12
Filled Region (Area)

• Shape filled with color or pattern
• Figs. 17 and 18
• Boundary is usually a polygon
• E.g., fillPolygon(poly, pattern)
• Attributes
• Attributes of enclosing border
• Pattern of filling
• Color of filling

13
Raster Image

• Image made up of pixels (Fig. 1.19)
• Stored as a 2D array of numerical values (Fig.
  1.20)
• Bit map 1 bit per pixel
• Pixel map gt1 bits per pixel
• Formats
• pbm
• ppm

14
Gray-Scale Raster Image

• Pixel depth
• bits needed to represent the gray level of a
  pixel
• 1 bit bi-level, black-and-white,
• monochrome (Fig 1.26)
• 2 bits 4 gray levels
• 4 bits 16 gray levels
• 8 bits 256 gray levels

15
Color Raster Image

• Color value
• Ordered triple (r, g, b) representing intensities
  of red, green, blue, respectively
• Color depth
• bits used to represent the color of a pixel
• E.g., color depth 3
• (0,0,0) black, (1,1,1) white
• (0,0,1) blue, (1,0,0) red, (1,0,1) ?
• Q how many colors can be represented?

16
Color Raster Image

• High-quality true color image
• Color depth 24, 8 bits for each color component
• Q how many colors can be represented?
• Q how many bytes are required to store a 480 x
  640 image
• Q Is the image quality improved with a larger
  color depth?

17
Graphics Output Devices

• Vector Raster
• Display Hard-copy

18
Vector Display

• Electron beam deflected from endpoint to endpoint
• One line at a time, similar to pen plotter
• Dictated by display commands
• Random Scan, refresh required
• Display processor must cycle through display list
  to refresh CRT's phosphor at least 60 times per
  second (60Hz) to avoid flicker.
• Image changed by changing display commands in the
  list

19
Raster Output Device

• 2D display surface
• Built-in 2D coordinate system, either upside-down
  or not (Fig. 1.35)
• A position on the display surface is associated
  with an image pixel
• Frame buffer
• A region of memory to hold all of the pixel
  values for the display surface
• Where is it in a computer?
• Block diagram (Fig. 1.36)

20
Raster Output Device

• Scanning process (Fig. 1.37)
• Sequential scan
• One line at a time, from left to right
• From top to bottom
• Back to top (refresh, required for CRT monitor)
• Refresh rate must be gt60 times per second to
  prevent flicker

21
Raster Display (Monitor)

• Display primitives stored in a frame buffer in
  terms of primitives' component pixels
• Images formed from the raster, a set of
  horizontal scan lines
• Scan line a row of individual pixels
• Raster is stored as a matrix of pixels
  representing entire screen area
• Memory issue
• Sequential scan and refresh

22
Raster Display (Monitor)
23
Raster Display (Monitor)

• Beam's intensity set to reflect pixel's intensity
  at each pixel
• Color Systems
• Three beams, RED, GREEN, and BLUE, are controlled
• Two types
• True color display system
• Indexed color display system

24
Comparison of Vector and Raster

• Raster Displays
• Advantages
• Lower cost
• Ability to display filled area
• Disadvantages
• Discrete nature of pixel presentation
• Aliasing Jaggies or staircasing caused by
  approximation

25
Comparison of Vector and Raster
26
True-color Display System

• Operation (Fig. 1.38)

27
Indexed-Color Display System

• Pseudo-color display system
• Color lookup table (LUT)
• Pixel value used as an index into LUT
• Programmable
• Palette set of possible colors can be displayed
• If color depthb and LUT widthw
• 2w colors can be displayed, any 2b at one time
• Example (Fig1.40) 6 bpp, 15-bit color
• 32K colors can be displayed, 64 at one time
• The display has a palette of 32K colors

28
Comparisons of Color Systems

• Example (Fig 1.41)
• 1024x1280-pixel-display
• True color system, color depth 24
• ? Memory requirement
• 1024 x 1280 x 24 ? 4MB
• Indexed color system, color depth 8, width of
  LUT 24 bits
• ? Memory requirement
• 1024 x 1280 x 8 256 x 3 ? 1MB
• Indexed color displays with LUT is much cheaper
  than true color systems!!

29
Hard-Copy Output Devices

• Plotter
• Flatbed plotter
• Drum plotter
• Dot matrix printer
• Laser printer
• Inkjet printer
• Film recorder
• Which are vector and which are raster?

30
Graphics Input Devices

• Logical input devices (primitives)
• String
• Choice
• Valuator
• Locator
• Pick

31
Graphics Input Devices

• Physical input devices
• Keyboard
• Buttons
• Mouse
• Tablet
• Joystick and trackball
• Knobs
• Space ball and data glove
• Digitizing 3D objects and capturing motion
• Which physical input devices are suitable for a
  particular logical input device?

32
Graphics Software Standards

• Core
• 3D Core graphics System
• Produced by an ACM SIGGRAPH committee
• GKS
• Graphics Kernel System
• 2D
• GKS-3D, 1988
• Groups of primitives arranged as segments, but
  not nested

33
Graphics Software Standards

• PHIGS
• 1988 (Programmers Hierarchical Interactive
  Graphics System)
• Nested structures
• Structure databases
• PHIGS PLUS includes shading and lighting
• Complicated, large
• de facto Standards
• PEX (PHIGS Extension to X)
• PostScript
• OpenGL

34
Interactive Computer Graphics

• Framework

35
Interactive Computer Graphics

• Interactive handling schema
• Generate initial display
• do
• enable selection of commands or objects
• wait for user selection
• switch (on selection)
• process selection to complete command
  or process completed command
• updating model and screen as needed
• while (!quit)