CS 360 Computer Graphics

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CS 360 Computer Graphics

• Chapter 1 - Introduction

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Topics

• Introduction
• Applications
• Elementary Output Primitives
• Graphics Output Devices
• Graphics Input Devices

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1.1 Introduction

• What is computer graphics?
• Pictures generated by computer
• Tools used to make the pictures
• Field of study involving the tools and the
  pictures they produce
• Purpose of course show the tools and how to
  apply them, both hardware and software tools.

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OpenGL

• Will be utilizing graphics libraries
• OpenGL is a device-independent library
• Developed by Silicon Graphics in 1992
• Widely used in industry and education

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1.2 Applications

• Art, entertainment, publishing
• Computer graphics image processing
• Monitoring manufacturing processes
• Displaying simulations
• CAD (computer aided design)
• Scientific analysis and visualization

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1.3 Elementary Output Primitives

• Polyline
• Text
• Filled regions
• Raster images
• Each primitive is associated with a set of
  attributes

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Polyline

• A connected sequence of straight lines, can
  appear as a straight curve
• Polygon 1st and last points are connected, if
  no 2 edges cross it is a simple polygon.

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Attributes of Polyline

• Color
• Thickness
• How are edges dashed solid, dashed, dotted,
  etc
• How do edges blend at endpoints butt-end, round
  end, mitered, etc. (Fig 1.13.)

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Text

• There are two display modes
• Text mode
• Graphics mode

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Display modes

• Text mode used for simple input and output,
  uses built in character generator, characters
  only placed in built-in grid
• Graphics mode richer set of character set, and
  can be placed arbitrarily

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Attributes of Text

• Font
• Color
• Size
• Spacing
• Orientation
• Others (less important)

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Filled Regions

• A shape filled with some color or pattern,
  boundary is typically a polygon
• Attributes
• Pattern of filling
• Color of filling
• Attributes of enclosing border

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Raster Image

• Image made up of pixels (Fig 1.19)
• Stored as a 2D array of numerical values, each
  pixel has its own color
• Bit map 1 bit per pixel
• Pixel map gt1 bit per pixel

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How to create raster images.

• Hand designed
• Computed images use an algorithm to calculate
  color of light that falls on each pixel. Raster
  images frequently contain straight lines decide
  which pixels best fit the ideal line between 2
  endpts.
• Scanned images place grid over picture, at each
  grid pt, find closest color, store

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Gray-Scale Raster Image

• Pixel depth
• of bits needed to represent the gray level of a
  pixel
• 1 bit bi-level, black and white, monochrome
• 2 bits 4 gray levels
• 4 bits 16 gray levels
• 8 bits 256 gray levels (acceptably quality in
  scanned images) requires lots of memory.
• N bits 2n values

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Color raster images

• More common as costs come down.
• Each pixel has a color value
• Common technique, a color is represented by an
  ordered triple (amount of red, green, blue)
• Color depth of pixel of bits used to
  represent the color of a pixel, each value in the
  triple is described using a certain number of
  bits.

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For example

• Color depth 3 bits ? 1 bit / color ? 8 possible
  colors
• (0, 1, 1) ? green blue cyan
• (1, 1, 1) ? white
• Q how many colors can be represented?
• Color depth 8 is common, why?

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True color image

• Highest quality
• Color depth 24
• 8 bits/color component
• This is as good as the eye can see.
• Q how many colors can be represented?
• Q how many bytes are required to store a 1080 x
  1024 image?

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1.4 Graphics Display Devices

• Line Drawing Display most early graphics were
  of this type, currently not too common except in
  specialized applications
• Raster Displays common today
• Video monitors PCs
• Flat-panel laptops
• For hard copies laser printers, ink jet,

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Display surface

• 2D display surface, with a built in 2D coordinate
  system.
• Coordinate system is upside down, (or not).
• Position on display is associated with an image
  pixel
• Connected to frame buffer a region of memory
  large enough to hold all pixel values for the
  display
• May be physical memory, or graphics card,

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Process to create display image

• Program is executed line by line
• Program computes values for each pixel loads
  into frame buffer
• Scan controller (not under program control)
  causes frame buffer to send each pixel through a
  converter to the appropriate physical spot on
  display
• Converter takes pixel value converts it to the
  corresponding quantity that produces color on
  display

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Example

• Location on display (149, 262)
• Value of the pixel is held in frame buffer
  mem149262
• Convert value in memory to color intensity

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Video monitors

• Use CRT ( cathode ray tube)
• 3 electron guns one for each color
• Different intensities receive different voltages
• DAC (digital to analog converter) converts
  digital number to voltage level
• The color will fade so must be refreshed (at
  least 60 times/sec).

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• Better monitors have frame buffer that supports
  24 planes of memory 24 bits/pixel.
• Each component gets 8 bits ? 256 levels of red,
  green and blue
• ? 16 million colors
• Monochrome displays single color displayed at
  different intensities

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Another method of associating pixel values with
colors

• LUT (color lookup table)
• Pixel value used as an index into LUT
• Programmable
• Palette set of possible colors that can be
  displayed

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How LUT works

• Fig 1.40 6 bit frame buffer values refer to a
  location in LUT that location specifies the
  color (what the DAC receives).
• So 6 bits of indexes in table 64 values
  LUT0 --- LUT63
• Program will set the values held in LUT
  flexibility in choosing colors, how to choose
  color???

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Why use?

• 64 entries each entry has 15 bits ? each color
  could be one of 215 32,768 colors.
• In this example, could be using 64 colors out of
  a possible 32K colors. (size of palette)
• Contents of LUT can be changed, but not in the
  middle of a scan-out of entire frame buffer.

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Example

• Display has color depth of b bits.
• LUT entry is w bits wide
• System can display 2w colors, any 2b at a time.
• W is typically a multiple of 3, and b lt w
• LUT requires little memory, 2b words of w bits.

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Example 1024 x 1280 display

• 24 bit/pixel frame buffer, no LUT
• 1.3 million pixels, 224 colors
• 8 bits / DAC
• Memory 1024 x 1280 x 24 bits 4 MB
• 8 bit/pixel frame buffer, LUT 24 bits wide
• 256 colors at a time out of 224 possible colors
• Memory 1024 x 1280 x 8 1 MB

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LUTs final comments

• Indexed color displays with LUT much cheaper than
  true color systems!!
• LUT will help inexpensive system
• Becoming less common as cost of memory comes
  down.
• HOMEWORK due next class period
• P 29 1, 2

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Hard copy output devices

• Plotter (flatbed and drum)
• Dot matrix printer
• Laset printer
• Inkjet printer
• Film recorder

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1.5 Graphics Input Primitives Devices

• Input graphic primitives (each device produces a
  primitive)
• String
• Choice
• Valuator (0 1.0)
• Locator (x, y)
• pick

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Physical Input Devices

• Keyboard
• Buttons
• Mouse
• Tables
• Joystick and trackball
• Knobs
• Space ball and data glove