1. COMS 161 Introduction to Computing
Title: HTML
Date: March 2, 2005
Lecture Number: 20

2. Announcements
Homework
Due 3/04/05

3. Review
Storing Images
RGB
Color index
Image Compression
jpeg

4. Outline
Image Compression
GIF
LZW

5. Compressing Images GIF (Graphic Interchange Format) Codec
Invented by CompuServe as a universal format for graphics
GIF compression algorithm owned by Unisys
Producers of GIF images need a license
GIFs are good for
buttons, banners, text (in graphic form), and simplistic images 7

6. Compressing Images Two flavors of GIF images
GIF87a
GIF89a
Supports animation
GIF images allow only 256 colors
Allow for a transparent color 7

7. Compressing Images GIF compression algorithm
Why are the sized different?
Has to do with the compression algorithm
File size: 4814 bytes
File size: 5652 bytes 7

8. Compressing Images GIF compression algorithm Horizontal image
Row 1: A big long red line
Row 2: A big long red line
Row 3: A big long red line
Row 4: A big long yellow line
Vertical image
Row 1: a few green pixels, a few carolina blue pixels, a few red pixels …
Row 2: a few green pixels, a few carolina blue pixels, a few red pixels … 7

9. Compressing Images GIF compression algorithm
Vertical image requires more information per row
Leads to a larger file
Good compression for images with nearly uniform background 7

10. Compressing Images Lempel Ziv Welch compression algorithm
Relies on reoccurrence of byte sequences (strings)
A table maps input strings to associated output codes
The table initially contains mappings for all possible strings of length one
Input is taken one byte at a time to find the longest initial string present in the table
The code for that string is output and then the string is extended with one more input byte, b.
LZW link 7

11. Compressing Images Lempel Ziv Welch compression algorithm
A new entry is added to the table mapping the extended string to the next unused code (obtained by incrementing a counter)
The process repeats, starting from byte b
The number of bits in an output code, and hence the maximum number of entries in the table is usually fixed and once this limit is reached, no more entries are added 7

12. LZW Compression
Given a stream of text from an alphabet of two letters a and b
Rule: Separate the stream of characters into pieces of text so that the shortest piece of data is the string of characters that we have not seen so far
aaababbbaaabaaaaaaabaabb
aaababbbaaabaaaaaaabaabb 7

13. LZW Compression 0aaababbbaaabaaaaaaabaabb
Insert a 0 at the beginning
Assign each section of letters with an increasing number
0aaababbbaaabaaaaaaabaabb
0aaababbbaaabaaaaaaabaabb 1 2 3 4 5 6 7 8 9 10 7

14. LZW Compression Use the indices to number the data
The empty string is labeled 0 a
The piece indexed with the 1 is a
a with the empty string is labeled 0a aa
Is labeled 1a since it contains the string whose index is 1 followed by a new a b
In terms of what came before is labeled 0b 7

15. LZW Compression
Continue labeling all the pieces in terms of previous labels
0aaababbbaaabaaaaaaabaabb 1 2 3 4 5 6 7 8 9 10
0a1a0b1b3b2a3a6a2b9b 1 2 3 4 5 6 7 8 9 10 7