View
214
Download
0
Category
Tags:
Preview:
Citation preview
Multimedia Specification Design and Production
2012 / Semester 1 / L3Lecturer: Dr. Nikos Gazepidisgazepidis@ist.edu.gr
2
Introduction
Compress methods are key enabling techniques for multimedia applications.
Raw media takes much storage and bandwidth
A raw video with 30 frame/sec, resolution of 640x480, 24bit color
One second of video 30 * 640 * 480 * 3 = 27.6480 Mbytes
fps width height 3*8bit (True Colour)
One hour video is about 100Gbytes
Lossless Compression
3
Lossless vs. Lossy (Audio file example)
Lossless Compression
4
Some Terms
Lossless Compression
Encoder(compression)
Storage ornetworks
Decoder(decompression)
Data Input(a sequence of symbols from an alphabet)
Recovered data sequenceCode (a sequence of codewords)
Informationsource
Lossless compression: The recovered data is exactly the same as the input
Lossy compression: The recovered data approximates the input data
Compression ratio = bits used to represent the input data
bits of the code
5
Properties of Huffman Coding
Lossless Compression
…in Computer Science and information theory, Huffman coding is an entropy encoding algorithm used for lossless data compression.
Huffman coding uses longer codewords for symbols with smaller probabilities and shorter codewords for symbols that often occur.
The two longest codewords differ only in the last bit.
Huffman coding is not effective for cases when there are small number of symbols
6
Properties of Huffman Coding
Lossless Compression
“this is an example of a huffman tree”
Char Freq
Code
x 1 10010
u 1 00111
r 1 11000
p 1 10011
o 1 00110
l 1 11001
s 2 1011
t 2 0110
m 2 0111
n 2 0010
i 2 1000
h 2 1010
f 3 1101
e 4 000
a 4 010
space 7 111
parent node
leaves
‘1’‘0’
7
Dictionary Based
Lossless Compression
Dictionary based method is another way to capture the correlation of symbols.
Static dictionary Good when the data to be compressed is specific in some application. For instance, to compress a student database, the world “Name”, “Student ID” will often appear.
Static dictionary method does not work well if the source characteristics change.
Adaptive Dictionary
8
Compress Multimedia Data Losslessly
Lossless Compression
Model the media (audio, image, graphics data, or video as) as a source that generates symbols.
The naive way of choosing symbols: Pixels for images and videos Samples for audios Characters for text messages
Apply the lossless compression methods to the string of symbols.
9
PNG (Portable Network Graphics)
Lossless Compression
PNG is a lossless image compressing method based on LZ77
PNG supports three main image types: true color, grayscale and palette-based ("8-bit") PNG supports alpha channel
10
PNG (cont.)
Lossless Compression
PNG also supports interlaced coding and decoding
1 6 4 6 2 6 4 67 7 7 7 7 7 7 75 6 5 6 5 6 5 67 7 7 7 7 7 7 73 6 4 6 3 6 4 67 7 7 7 7 7 7 75 6 5 6 5 6 5 67 7 7 7 7 7 7 7
The scanning pattern in a 8x8 block. The whole image is partitioned into 8x8 blocks and scanned based on the pattern in each block.
11
File Compression - LZ Algorithm (Lempel & Ziv)
Lossless Compression
ZIP compression. This compression system is a very handy invention, especially for Web users, because it let users reduce the overall number of bits and bytes in a file so it can be transmitted faster over slower Internet connections, or take up less space on a disk.
Compression up to certain %Original files size
12
File Compression - LZ Algorithm
Lossless Compression
“Ask not what your country can do for you - ask what you can do for your country.” Quote by J.F. Kennedy, 1961
The above quote has 17 words made up of:
1. 61 letters2. 16 spaces3. 1 dash (-)4. 1 period (.)
Total: 79 units
Words Frequency
ask 2
what 2
your 2
country 2
can 2
do 2
for 2
you 2ask not wha
tyou
rcountr
ycan do for you
Unique Words
13
LZ Algorithm - Dictionary
Lossless Compression
“Ask not what your country can do for you - ask what you can do for your country.”
Dictionary(words+numbers)
1. ask 2. what 3. your 4. country 5. can 6. do 7. for 8. you
According to our dictionary the above quote can be:"1 not 2 3 4 5 6 7 8 - 1 2 8 5 6 7 3 4"
79units 3
7units
Quote ‘before’ compression
Quote ‘after’ compression
Dictionary
37
units 74units
14
LZ Algorithm - Searching for Patterns
Lossless Compression
“Ask not what your country can do for you - ask what you can do for your country.”
Possible patterns in a dictionary "t" followed by a space “ou” which appears in both "your" and "country“ “can do for you” which appears 2 times - 14 units
Dictionary
1. ask_2. what_ 3. you4. r_country 5. _can_do_for_you
"1not_2345_-_12354"
79units 4
1units
Quote ‘before’ compression
Quote ‘after’ compression
Dictionary
18
units 59units
15
LZ Algorithm - Searching for Patterns
Lossless Compression
“Ask not what your country can do for you - ask what you can do for your country.”
79units
59units
79units
74units
≈ -7.5% ≈ -25%
Pattern A Pattern B
16
GIF (Graphics Interchange Format)
Lossless Compression
GIF was devised by UNISYS and Compuserve.
GIF is based on LZW lossless compression.
GIF supports 8bit (256) color images only. Each image can have its own color table.
It supports transparency layer and simple animation functions.
It also supports interlaced coding and decoding.
17
Audio Compression
Lossless Compression
Audio can also be compressed in a similar way to image data.
For lossless audio compression, prediction is usually applied first. Simple prediction. Adaptive prediction.
Stereo decorrelation.
Entropy coding.
18
Audio Examples
Lossless Compression
FLAC (free lossless audio codec) Polynomial fitting prediction or linear prediction. Rice coding.
MPEG4-ALS Lossless audio coding standard in MPEG4. Adaptive Linear Prediction. Supports up to 65535 channels. Fast random access. Rice coding.
Apple lossless audio coding (ALAC)
19
MPEG-4 ALS System Diagram
Lossless Compression
Recommended