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