Presentation given in the Seminar of B.Tech 6th Semester during session 2009-10 By Paramjeet Singh Jamwal, Poonam Kanyal, Rittitka Mittal and Surabhi Tyagi.
3. What is Image Compression ?
Application of data compression on digital images
Reduce redundancy of the image data
Benefits of Image Compression
Store data efficiently
Transmit data efficiently
4. Lossy Compression
Decompression retrieves data different
from the original
Used to compress multimedia data
Streaming media and internet telephony
Methods
JPEG
TIFF
MNG
PGF
8. Lossless Compression
Exact reconstruction of original data
Executable programs and source codes
Data loss cant be tolerated
Methods
JPEG
2000
GIF
PNG
TIFF
12. What is JPEG ?
Stands for Joint Photographics Experts
Group
Lossy compression method
Mostly used by digital cameras & web
usage
Not suited for drawing , textual and iconic
graphics
13. Basics of JPEG Compression
Human vision is insensitive to high spatial frequencies
JPEG Takes advantage of this by compressing high frequencies more
coarsely and storing image as frequency data
Losslessly compressed
image,
150KB
JPEG compressed,
14KB
14. The JPEG Compression Algorithm
Divide image into 8x8 pixel blocks
Apply 2D Fourier Discrete Cosine Transform
(FDCT) Transform
Apply coarse quantization to high spatial
frequency components
Compress resulting data losslessly and store
8x8
pixel
blocks
FDCT
Frequency
Dependent
quantization
Quantization
Table
Zig-zag
scan
RLE
Huffman
Encoding
output
15. The JPEG File Structure
Short name
SOI
Bytes
0xFFD8
Size
none
Name
Start Of Image
SOF0
0xFFC0
variable size
Start Of Frame (Baseline
DCT)
SOF2
0xFFC2
variable size
Start Of Frame
(Progressive DCT)
DHT
0xFFC4
variable size
DQT
0xFFDB
variable size
DRI
0xFFDD
2 bytes
SOS
0xFFDA
variable size
RSTn
0xFFD0 … 0xFFD7
none
APPn
0xFFEn
variable size
Application-specific
COM
EOI
0xFFFE
0xFFD9
variable size
none
Comment
End Of Image
Define Huffman Table(s)
Define Quantization
Table(s)
Define Restart Interval
Start Of Scan
Restart
20. 3/7 : Downsampling ( optional )
Y is taken every pixel , and Cb,Cr are taken for a block of 2x2 pixels
MCU(minimu
m coded unit) :
The smallest
group of data
units that is
coded.
Data size
reduces to a
half
immediately
22. 4/7 : Apply DCT [ Discrete Cosine Transformation ]
Shift operations
From [0, 255]
To [-128, 127]
DCT
Result
23. 5/7 : Quantization
Luminance Quantization Matrix
Chrominance Quantization Matrix
Each DCT coefficient F(u, v) is divided by the corresponding quantizer
step-size parameter Q(u, v) in the quantization matrix and rounded
to the nearest integer as
24. 5/7 : Quantization [ Quality Factor ]
Quality of the reconstructed image and the achieved
compression can be controlled by a user by selecting a
quality factor [ Q_JPEG ] :
Q_JPEG ranges between 1 to 100
When Q_JPEG is used, the entries in tables in previous slide is
scaled by the factor alpha (α), defined as :
Q_JPEG is 100 for best reproduction
28. JPEG Compression Ratio
500KB image,
minimum
compression
40KB image,
half
compression
11KB
image, max
compression
29. Effects of varying JPEG Compression Ratio
Uncompressed image
Half compression,
Blurring around sharp
edges
Max compression,
8-pixel blocks
apparent, large distortion
in high-frequency areas
30. DWT v/s DCT
Images containing sharp
edges/continuous curves
Uses more optimal set
of functions to represent
sharp edges
Wavelets are finite in
extent
Different families of wavelets
32. Applications of Image Processing
Computer Vision
Optical Sorting
Face Detection
Feature Detection
Augmented Reality
Remote Sensing
Medical Image Processing
33. Advantages/Disadvantages of Image Processing
Post-processing
High cost
Easy Sharing
Easy Retrieval
Environment
Friendly
Multiple Use
Disadvantages
Advantages
Easy Storage
Extra Knowledge
High Maintenance
Standardization
Shape/Size of
detectors