JPEG --Still Image Data Compression Standard.ppt

JPEGStill Image pression Standard
JPEG Introduction - The background
JPEG stands for Joint Photographic Expert Group
A standard pression method is needed to enable interoperability of equipment from different manufacturer
It is the first international digital pression standard for continuous-tone images (grayscale or color)
pression is needed?
Ex) VGA(640x480)  640x480x8x3=7,372,800bits
pression  200,000bits without any visual degradation
JPEG Introduction – what’s the objective?
“very good” or “excellent” compression rate, reconstructed image quality, transmission rate
be applicable to practically any kind of continuous-tone digital source image
plexity
have the following modes of operations:
sequential encoding
Progressive encoding
lossless encoding
JPEG Overview
encoder
statistical
model
entropy
encoder
Encoder
model
Source
image data
compressed
image data
descriptors
symbols
model
tables
entropy
coding tables
The basic parts of an JPEG encoder
JPEG Baseline System
JPEG Baseline System
JPEG Baseline system posed of:
Sequential DCT-based mode
Huffman coding
The basic architecture of JPEG Baseline system
Source
image data
quantizer
entropy
encoder
compressed
image data
table
specification
table
specification
88 blocks
DCT-based encoder
statistical
model
FDCT
Frequency sensitivity of Human Visual System
JPEG Baseline System
– Why does it work?
Lossy encoding
HVS is generally more sensitive to low frequencies
Natural images
The mathematical representation of FDCT (2-D):
Where
f(x,y): 2-D sample value
F(u,v): 2-D DCT coefficient
The Baseline System – DCT
The Discrete Cosine Transform (DCT) separates the frequencies contained in an image.
The original data could be reconstructed by Inverse DCT.
Basis of DCT transform
The Baseline System-DCT (cont.)
Before DCT (image data)
After DCT (coefficients)
An example of 1-D DCT position
The 8 basic functions for 1-D DCT