Images

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Images
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Why use Images?

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

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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

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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

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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

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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 8010
• Can also have range of 0.0 1.0

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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?

Macintosh 256 Color palette
Color Cube
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Alpha

• RGBA - Use an extra 8 bits to represent the
  visibility level of an image (total 32-bits)
• Can be done with .gifs
• Cant be done with .jpgs

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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)

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GIF Compression

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

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The Math

• If you have 320x240 image with standard 32-bit
  RGBA color information, and you get 101
  compression, how big is the file?
• Answer 320x240x32/10

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Vector Graphics

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

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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

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Acquiring Images

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

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