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					Images
              Why use Images?

•   Provide Facts
•   Explain a Process
•   Set a Mood
•   Pinpoint a location
•   Show relationships (hierarchy, flow chart…)
•   Compare (before and after)
                Digitization

• Images come in (scanned) as a string of
  pixels
• Stored as numbers on disk
• Called a “bitmap”
• Also have “vector” graphics (later)
• Different file formats
               Color Theory

• Colors are comprised of three primary colors:
  – Red
  – Green
  – Blue
• All colors can be made from these 3 colors
• Number of colors available depends on the
  number of bits used to store the color
• Example: 24 bits = 8R + 8G + 8B
                  Color Theory

• With 8 bits, you have a range of 0 – 255
• Common colors:
  –   Red 255, 0, 0
  –   Green 0, 255, 0
  –   Blue: 0, 0, 255
  –   Purple: 255, 0, 255
  –   Black: 0, 0, 0
  –   White: 255, 255, 255
  –   Gray: 100, 100, 100
      Hexadecimal Representation

•   Sometimes, colors are represented in hex
•   Range of 0 – F (0 – 15)
•   Example: #FF 00 50
•   To covert to base 10, multiply first number by
    16!
•   Example:
     FF16 = (16xF) + F = (16x15) + 15 = 25510
     5016 = (16x5) + 0 = (80) + 0 = 80 10
•   Can also have range of 0.0 – 1.0
                   Color Depth
                     (per pixel)

•   8 bits: 256 colors (3-3-2 RGB)
•   16 bits: 65K colors (5-6-5 RGB)
•   24 bits: 16M colors (8-8-8 RGB)
•   Would you want 32-bits for color alone?




      Color Cube          Macintosh 256 Color palette
                    Alpha

• RGBA - Use an extra 8 bits to represent the
  visibility level of an image (total 32-bits)
• Can be done with .gifs
• Can’t be done with .jpgs
              JPEG Compression

• JPEG – Joint Photographic Experts Group
• Best for photographic images
• Algorithm
   – Store 255 levels of color, and 255 levels of brightness
   – Group pixels into block (of 4 or 16)
      • Find average color
      • Store differences from average of each pixel
   – Quantize the blocks (look at adjacent blocks and
     calculate difference from average of all the blocks)
             GIF Compression

• GIF – Graphics Interchange Format
• Best for line images
• Algorithm:
  – Reduce color depth to 8 bits or less (256 colors)
  – Look for repetition
                 The Math

• If you have 320x240 image with standard 32-
  bit RGBA color information, and you get 10:1
  compression, how big is the file?

• Answer: 320x240x32/10
             Vector Graphics

• Made up of mathematical equations for lines
  and arcs
• Completely scalable (no resolution loss if
  stretched)
• PNG file
             Animated Images

• Animated GIFs
  – Simply a series of gifs with timing information
  – File size depends on size of image, and number
    of frames
• Vector animation
  – Animator creates image
  – Creates a (mathematical) path for image to
    follow
  – No series of frames is needed
               Acquiring Images

•   Scanner
•   Digital Camera
•   Drawn from Scratch
•   CD-ROM collections
•   Stills from Video Capture

				
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