1. Multimedia- and Web-based Information Systems Lecture 6

2. Picture Compression OverviewThe JPEG method

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

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

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

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

7. 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

8. JPEG Overview • Different Modes • Overview of the different possibilities Picture Preparation Picture Processing Quantisation Entropy encoding Pixel Prediction Run time encoding Block, MCU FDCT Huffman

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

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

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

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

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

14. DCT-based mode • Lossy and sequential • 8 bits per Pixel 8x8 Blocks FDCT Quantisation Tables Entropy encoding Tables uncompressed picture compressed picture

15. Transformation of blocks

16. Forward Discrete Cosine Transformation (FDCT) Forward Discrete Cosine Transformation (FDCT): , for u,v=0; else cu,cv=1 with: • Formula applied to each block for all 0 ≤ u, v ≤ 7: • Blocks with 8x8 pixel result in 64 DCT coefficients: • 1 DC-coefficient S00: basic color of the block • 63 AC-coefficients: (likely) zero or near-zero values

17. Example

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

19. Entropy encoding • Quantisated DC-coefficient • Zig-zag

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

21. Progressive image composition • Image gets clearer through multiple steps

22. 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

23. Perl

24. 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

25. 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

26. How to get & use it • http://www.perl.com • ActiveState makes port for Microsoft Windows • Current Version is 5.6.1 • Comprehensive documentation included • C:\Perl\html\index.html • Perldoc • Running perl scripts • perl –w script.pl • #!/usr/bin/perl + chmod (Unix)

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

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

29. Hashes • Multivalued Variable • Lookup by name • List Assignments • Accessing %longday = ( "Sun" => "Sunday", "Mon" => "Monday", "Tue" => "Tuesday" ); @list = %longday $longday{"Sun"}, $longday{"Mon"}

30. 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”;