1. COMS 161 Introduction to Computing
Title: Digital Images
Date: February 28, 2005
Lecture Number: 19

2. Announcements
Homework
Due 3/4/05

3. Review
Storing Images
File formats
gif
jpeg

4. Outline Jpeg Gif LZW Image compression experiment
Image compression scheme
LZW

5. Compressing Images Bitmap images typically do not use compression
File size: 270,870 Bytes
Image width: height: 186
Bits per pixel: 24 (RGB) 7

6. Compressing Images Science Use XnView Read in a bmp image Display it
Save it as a jpeg image
Fidelity adjustment is modified 7

7. Compressing Images bmp image File size: 271,235 bytes
jpeg image with 100% quality
File size: 50,235 bytes 7

8. Compressing Images Gray scale difference image
Range of values: 0 – 18 gray scale values 7

9. Compressing Images bmp image File size: 271,235 bytes
jpeg image with 90% quality
File size: 21,207 bytes 7

10. Compressing Images Gray scale difference image
Range of values: 0 – 22 gray scale values 7

11. Compressing Images bmp image File size: 271,235 bytes
jpeg image with 70% quality
File size: 13,079 bytes 7

12. Compressing Images Gray scale difference image
Range of values: 0 – 45 gray scale values 7

13. Compressing Images bmp image File size: 271,235 bytes
jpeg image with 50% quality
File size: 10,335 bytes 7

14. Compressing Images Gray scale difference image
Range of values: 0 – 58 gray scale values 7

15. Compressing Images bmp image File size: 271,235 bytes
jpeg image with 25% quality
File size: 7,522 bytes 7

16. Compressing Images Gray scale difference image
Range of values: 0 – 79 gray scale values 7

17. Compressing Images bmp image File size: 271,235 bytes
jpeg image with 10% quality
File size: 4,986 bytes 7

18. Compressing Images Gray scale difference image
Range of values: 0 – 118 gray scale values 7

19. Compressing Images bmp image File size: 271,235 bytes
jpeg image with 5% quality
File size: 3,765 bytes 7

20. Compressing Images Gray scale difference image
Range of values: 0 – 171 gray scale values 7

21. Compressing Images bmp image File size: 271,235 bytes
jpeg image with 1% quality
File size: 3,151 bytes 7

22. Compressing Images Gray scale difference image
Range of values: 0 – 250 gray scale values 7

23. 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

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

25. 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

26. 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

27. 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

28. 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. 7

29. 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