Introduction to Information Technology

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Introduction to Information Technology

• Image and Video
• Lecture Notes - 4

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FROM THE REAL WORLD TO IMAGES AND VIDEO
Chapter 5 of text.

• Binary digits can be used to represent images.

A picture is worth a thousand words
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CAMERAS AND IMAGE FORMATION

• Basic image formation process
• Essential components
• Object/scene
• Lens
• Image recording medium
• Display device may correct the inverted condition
• Projection 3D to 2D

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HUMAN VISUAL DISCRIMINATION AND ACUITY

• Contrast sensitivity is 2 of full brightness
• 50 levels of gray
• lines per degree of visual arc
• Image brought closer to the eye, we can resolve
  more detail
• Humans can resolve 60 lines per degree of visual
  arc or 120 pixels per subtended degree of visual
  arc
• Line requires two strings of pixels one black,
  one white
• Pixel - An image can be broken down to small
  regions

Visual Arc
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OTHER TYPES OF IMAGE FORMATION

• Radar, Sonar, X-rays, CAT scan
• Differ from traditional camera
• Type of energy used to form image
• Geometry of the system that relates the location
  of the objects in the real world to the image
  world

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RADAR

• Radar Radio Detection and Ranging
• TV weather forecasts
• Sends a narrow beam of radio waves in a
  particular direction and waits for reception of
  some reflected energy
• Distance and angle of transmission (polar
  coordinate system)

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CONVERTING IMAGES TO BITS

• Digitization representing information in binary
  form.
• Convert information from continuous form to
  discrete form - analog to digital conversion.
• How do you digitize and store a black-and-white
  photograph?
• Image contains an infinite amount of points and
  the brightness may be infinite.
• Must represent the image in a finite number of
  small picture elements pixels.
• Must convert brightness level corresponding to
  each pixel into a code representing the
  approximate gray level.

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PIXELS

• Image represented in a finite number of small
  picture elements
• Represent a single intensity (brightness) level
• Usually arranged in rectangular grid

13 x 13 grid 169 pixels
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PIXELIZED IMAGE

• Process of breaking a continuous image into a
  grid of pixels is called pixelization, sampling,
  scanning or spatial quantization.

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HOW MANY PIXELS SHOULD BE USED

• If too few pixels used, image appears coarse

16 x16 (256 pixels)

64 x 64 (4096 pixels)
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SHADES OF GRAY

• Each pixel must be converted to binary data.
• Suppose 8 bits are used to represent the
  brightness at each pixel.
• How many brightness levels?
• 28 256 brightness levels (black gt white)
• Quantization process of rounding off continuous
  values so they can be represented by a fixed
  number of binary digits.

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SHADES OF GRAY
A 3-bit (8 gray levels) image
A 6-bit (64 gray levels) image
A 1-bit (black and white only) image
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HOW MUCH STORAGE IS NEEDED?

• Total number of bits required for storage total
  number of pixels number of bits used per pixel
• For example Black and white photo
• 64 x 64 pixels
• Use 32 gray levels (5 bits)
• 64 x 64 x 5 20,480 bits 2560/1024 bytes
  2.5KB
• Remember data storage is in bytes
• KB represents 210 or 1024 bytes

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ANOTHER EXAMPLE

• Black and White photo
• 256 x 256 pixel
• 6 bits (64 gray levels)
• How much storage is needed?
• 256 x 256 x 6 393,216 bits
• 393,216/8 49,152 bytes
• 49,152/1024 48 KB

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COLOR REPRESENTATION
RGB system

• How can we represent color images in binary form?
• Color is represented by three numbers indicating
  the amount of red, green and blue

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DIGITIZING A COLOR IMAGE

• Must represent the image in a finite number of
  pixels.
• Determine RGB representation for each pixel.
• Amount of red, green, blue
• Digitize three numbers.
• For example
• 3 bits for each color value 9 bit total
• 23 8 different color intensities
• 8 x 8 x 8 512 different possible color
  combinations

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HUMAN VISUAL PERSISTENCE

• Visual Persistence or Visual Latency Our visual
  system has a slow response to change in stimulus
• No flashing evident above 50 flashes per second
• TV
• 30 new images per second
• Images presented twice per frame
• 60 frames per second

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COMPUTER DISPLAYS

• Computer displays
• CRTs Cathode Ray Tubes
• Cathode glass tube emits beams of electrons
  causes fluorescent chemical to light up on its
  face
• TVs
• Continually refreshing at a rate of at least 50
  frames/sec
• LCD Liquid Crystal Display
• Chemicals are made to become transparent at each
  pixel location
• Each pixel remains the same until state is
  changed
• No flickering noticed

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DISPLAY DEVICE FORMATS

• PC Monitor
• VGA (Video Graphics Array)
• 640 x 400 pixels
• 16 possible color values for each pixel
• 640 x 400 x 4 1,024,000 bits
• 1,024,000/8 128,000 bytes 125KB
• Super VGA
• 640 x 480 pixels
• 256 colors
• 640 x 480 x 8 2,457,600 bits
• 2,457,600/8 307,200 bytes 300KB
• Higher resolution and more colors

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ADDING UP THE BITS

• Color TV Screen 512 x 512 pixels
• 3 bits per color per pixel 9 bits/pixel
• Scene changes 60 frames/second
• 512 x 512 x 9 x 60 x 3600 500 billion bits/hour
• American Wedding requires 191 GB storage ????
• COMPRESSION is used.

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In-Class Problems