Multimedia- and Web-based Information Systems Lecture 6

Picture Compression Overview The JPEG method

Important steps during the compression of single pictures Picture Preparation – Generation of a suitable digital representation – Blocks of 8x8 pixels, fixed number of bits per pixel, planes, color model Picture preparation uncompressed picture Picture processing Quanti- sation Entropy- coding compressed picture

Important steps during the compression of single pictures Picture Processing – Mathematically exact – Transformations Time domain to frequency domain Cosine Transformation (DCT) Picture preparation uncompressed picture Picture processing Quanti- sation Entropy- coding compressed picture

Important steps during the compression of single pictures Quantisation – Representation of Values (Losses) – Importance of information – Characteristic curve (Companding) Picture preparation uncompressed picture Picture processing Quanti- sation Entropy- coding compressed picture

Important steps during the compression of single pictures Entropy encoding – Lossless compression – Linear data stream Picture preparation uncompressed picture Picture processing Quanti- sation Entropy- coding compressed picture

JPEG and Motion JPEG Demands Independent of the size of a picture Arbitrary Width/Height-relation Independent of color model and color variety Contents of a picture may have arbitrary complexity State of the art regarding compression ratio Sequential and progressive picture composition

JPEG Overview Different Modes – Overview of the different possibilities Picture Preparation Pixel Block, MCU Picture Processing Prediction FDCT QuantisationEntropy encoding Run time encoding Huffman

JPEG Modes Lossy, sequential DCT-based Mode (base mode, baseline process) Extended lossy DCT-based Mode Lossless Mode Hierarchical Mode

Baseline Process Uses – Planes – Blocks (8x8) – Minimum Coded Unit (MCU) – FDCT (Forward Discrete Cosine Transformation) – Run length encoding – Huffmann

Picture Preparation Picture consists of multiple planes (max. 255) – RGB, YUV or YIQ – Alpha channel

Planes with different resolution 2nd and 3rd plane have half the resolution in X-direction (e.g. YUV)

Minimum Coded Unit (MCU) Assembly of data units from multiple different planes

DCT-based mode Lossy and sequential 8 bits per Pixel 8x8 BlocksFDCT QuantisationTables Entropy encoding Tables uncompressed picture compressed picture

Transformation of blocks

Forward Discrete Cosine Transformation (FDCT) Forward Discrete Cosine Transformation (FDCT): with: Formula applied to each block for all 0 ≤ u, v ≤ 7:  Blocks with 8x8 pixel result in 64 DCT coefficients:  1 DC-coefficient S 00 : basic color of the block  63 AC-coefficients: (likely) zero or near-zero values, for u,v=0; else cu,cv=1

Example

Quantisation 64 DCT-Coefficients Table of coefficients for the quantisation S q (v,u) = round (S(v,u) / Q(v,u)) R(v,u) = S q (v,u) * Q(v,u) Trough Q(v,u), particular areas can be emphasized / neglected

Entropy encoding Quantisated DC-coefficient Zig-zag

Sequential image composition In one step (encoded / decoded) Top to bottom

Progressive image composition Image gets clearer through multiple steps

Summary JPEG – State of the Art for the compression of single pictures – Variety of degrees of freedom E.g. Resolution – Lossless mode almost reaches a 2:1 ratio

Perl

What Perl is Merger of Unix tools – Very popular under UNIX – shell, sed, awk Programming language – C syntax Scripting language – Ability to change everything during runtime – Fast to employ, one-liners possible – Slower than C

What Perl is Easy to learn – Learning curve similar to human language – More difficult things possible Tell it to be more strict Object orientation Esp. suited for the web & text processing – Regular expressions

How to get & use it – ActiveState makes port for Microsoft Windows Current Version is Comprehensive documentation included – C:\Perl\html\index.html – Perldoc Running perl scripts – perl –w script.pl – #!/usr/bin/perl + chmod (Unix)

Variables Scalars$ Hash% No need to declare variables Automatic conversion of numbers and strings $camels = ‘123‘; print $camels + 1, “\n“;

Arrays Multivalued Variable Lookup by number List Assignments = ("couch", "chair", "table", "stove") ($one, $two, $three, $four) ($one, $two) = ($two, $one) $home[0] - $home[3]

Hashes Multivalued Variable Lookup by name List Assignments Accessing %longday = ("Sun" => "Sunday", "Mon" => "Monday", "Tue" => "Tuesday" = %longday $longday{"Sun"}, $longday{"Mon"}

Quoting in Perl Double Quotes ““ interprete variables and backslashes Single Quotes ‘‘ don‘t $one=“two”; $two=“four”; print ‘$one + $one is $two \n’; print “$one + $one is $two \n”;