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Images

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Given a square image with a dimension of 4 inches and given 72 ppi. What file size would be needed just to store the image data (in bits) ... – PowerPoint PPT presentation

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Title: Images


1
Images
  • 01/29/04
  • Resources
  • Yale Web Style Guide
  • The GIF Controversy
  • Unisys - lzw

2
Image Characteristics
  • Size
  • Image Size Dimension in pixels
  • Canvas Size Dimension in inches
  • Screen Resolution (Pixels per inch)
  • 72 96 ppi for a monitor
  • 300-1200 ppi for print

3
Image Characteristics
  • Color
  • True Color
  • 8 bits per Red, Green, Blue
  • Or 24 bits per pixel
  • Given a square image with a dimension of 4 inches
    and given 2 ppi. What file size would be needed
    just to store the image data (in bits)?

4
Image Characteristics
  • Color
  • True Color
  • 8 bits per Red, Green, Blue
  • Or 24 bits per pixel
  • Given a square image with a dimension of 4 inches
    and given 72 ppi. What file size would be needed
    just to store the image data (in bits)?
  • 24bits/pixel 72 pixels/inch 4 inches/scan
    line 6912 bits/scan line
  • 6912 bits/scan line 72 scan lines/inch 4
    inches 2000 K

5
Image Characteristics
  • Given a square image with a dimension of 4 inches
    and given 300 ppi. What file size would be
    needed just to store the raw image data (in bits)?

6
Image Characteristics
  • Given a square image with a dimension of 4 inches
    and given 300 ppi. What file size would be
    needed just to store the raw image data (in
    bits)?
  • 24bits/pixel 300 pixels/inch 4 inches/scan
    line 28800bits/scan line
  • 28800bits/scan line 300 scan lines/inch 4
    inches 34560 K

7
Image Transmission
  • Most Internet users connect at 56K modems. The
    download rate for a 56K modem is actually pretty
    close to 7K.
  • To transmit 34560 K would take one hour
  • To transmit 2000K will take 4.76 minutes
  • Studies show that users get impatient after a 2
    second delay.
  • Implications?

8
Compression What is it?
  • The goal of data compression is to represent an
    information source (e.g. a data file, a speech
    signal, an image, or a video signal) as
    accurately as possible using the fewest number of
    bits.

9
Image Compression Web Formats
  • GIF lossy for more than 256 colors
  • For line drawings, gray scale images and color
    pictures with a palette of 256 colors or fewer
  • JPG always lossy
  • For true color images, especially photographs
  • PNG - lossless
  • Not well supported in IE
  • Replacement format for GIF

10
Compression How does it work?
  • Describe the algorithm.
  • A procedure or formula for solving a problem.
  • An algorithm can be described in
  • Pseudocode
  • An outline of an algorithm, written in a form
    that can easily be converted into a program.
  • There are no formatting or syntax rules
  • Enables the programmer to concentrate on the
    algorithms without worrying about all the
    syntactic details of a particular programming
    language

11
Run Length Encoding
  • RLE is used in TIFF and PDF files
  • For a binary image, given the dimensions of the
    image (r x c) and that (x0, x1, . . . ,
    x((rc)-1) are image pixels with one of two
    possible intensity values (0 or 1), the image
    line can be said to consist of k image segments
    having length li.
  • Assume that the initial image segment has a white
    value or value of 1.

12
Run Length EncodingPseudo Code
  •  
  • initialize RunColor to 1
  • initialize RunLength to 0
  • for each pixel in the image
  • if (pixel is the same as the RunColor)
  • increment RunLength
  • else
  • write RunLength
  • change RunColor to pixel's intensity
  • initialize RunLength to 1
  • write last RunLength
  •  

13
  •  
  • initialize RunColor to 1
  • initialize RunLength to 0
  • for each pixel in the image
  • if (pixel is the same as the RunColor)
  • increment RunLength
  • else
  • write RunLength
  • change RunColor to pixel's intensity
  • initialize RunLength to 1

14
Three Programming Contructs
  • Sequence construct
  • Statements are executed in the order they're
    found in the code.
  • X 3
  • Y 2
  • Z X Y

15
Three Programming Contructs(continued)
  • Selection
  • Controls the flow of execution down one of two or
    more paths, depending on the result of a logical
    test.
  • If then else then
  • If (temperature lt 50)
  • Wear long pants
  • Else
  • Wear shorts

16
Three Programming Contructs(continued)
  • Selection - Switch
  • Switch (temperature / 10)
  • Case 0
  • Case 1
  • Case 2
  • Case 3
  • Case 4
  • Wear long pants
  • break
  • Case 5
  • Wear zip-offs
  • break
  • Default
  • Wear shorts

17
Three Programming Contructs(continued)
  • Iteration
  • A block of code is required to be executed
    continually until a condition is met
  • For
  • While
  • Pre condition
  • Do While
  • Post Condition

18
Lempel Ziv Welch - LZW
  • Used in GIF, TIFF-LZW, PostScript, Portable
    Document Format (PDF), V.42bis, compress
  • U.S. LZW patent (Unisys) expired June 20, 2003.
  • CompuServe released GIF as a free and open
    specification in 1987.
  • the reading and/or writing of GIF images
    requires a license to use Unisys patented Lempel
    Ziv Welch (LZW) data compression and
    decompression technology form Unisys

19
LZW
  • LZW starts with a 4K dictionary, of which entries
    0-255 refer to individual bytes, and entries
    256-4095 refer to substrings.
  • Each time a new code is generated it means a new
    string has been parsed.
  • New strings are generated by appending the
    current character/pixel color K to the end of an
    existing string w.

20
LZW
  • set w
  • loop
  • read a character/pixel K
  • if wK exists in the table
  • w wK
  • else
  • output the code for w
  • add wK to the table
  • w K
  • endloop

21
LZW
  • Input string /WED/WE/WEE/WEB
  • Character Code New code value
  • input Output and associated string
  • /W / 256 /W
  • E W 257 WE
  • D E 258 ED
  • / D 259 D/
  • WE 256 260 /WE
  • / E 261 E/
  • WEE 260 262 /WEE
  • /W 261 263 E/W
  • EB 257 264 WEB
  • ltENDgt B

22
LZW
  • LZW applet
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