Image compression

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Image compression

  1. 1. Image Compression Techniques Presented by PARTHA PRATIM DEB MTECH(CSE),1st YEAR
  2. 2. Outline Image Compression Fundamentals ◦ Why Compress ◦ How is compression possible? ◦ Redundancy ◦ The Flow of Image Compression ◦ General Image Storage System Lossy compression: Reduce correlation between pixels ◦ Karhunen-Loeve Transform ◦ Discrete Cosine Transform ◦ Discrete Wavelet Transform ◦ Differential Pulse Code Modulation ◦ Differential Coding Lossless compression: Quantization and Source Coding ◦ Huffman Coding ◦ Arithmetic Coding ◦ Run Length Coding ◦ Lempel Ziv 77 Algorithm ◦ Lempel Ziv 78 Algorithm Overview of Image Compression Algorithms ◦ JPEG ◦ JPEG 2000 ◦ Shape-Adaptive Image Compression
  3. 3. OutlineImage Compression FundamentalsReduce correlation between pixelsQuantization and Source CodingOverview of Image Compression Algorithms
  4. 4. The Flow of Image Compression What is the so-called image compression coding? ◦ To store the image into bit-stream as compact as possible and to display the decoded image in the monitor as exact as possible Flow of compression ◦ The image file is converted into a series of binary data, which is called the bit- stream ◦ The decoder receives the encoded bit-stream and decodes it to reconstruct the image ◦ The total data quantity of the bit-stream is less than the total data quantity of the original image
  5. 5. Basic concept of compression
  6. 6. spatial redundancy Elements that are duplicated within a structure, such as pixels in a still image and bit patterns in a file. Exploiting spatial redundancy is how compression is performed.Intraframe codingCompressing redundant areas within a single video frame.Interframe codingInterframe coding often provides substantial compression because in many motion sequences,only a small percentage of the pixels are actually different from one frame to another.
  7. 7. General Image Storage System
  8. 8. Coding Java Syntax: private int[] capture() throws InterruptedException { dim = Toolkit.getDefaultToolkit().getScreenSize(); Image tmp = robot.createScreenCapture(new Rectangle(dim.width,dim.height)).getScaledInstance(800,600,BufferedImage.SCALE_SMOOTH); int pix[] = new int[IMG_WIDTH*IMG_HEIGHT]; PixelGrabber pg = new PixelGrabber(tmp,0,0,IMG_WIDTH,IMG_HEIGHT,pix,0,IMG_WIDTH); pg.grabPixels(); return pix; }
  9. 9. Outline ImageCompression Fundamentals Reduce correlation between pixels(lossy) Quantization and Source Coding Overview of Image Compression Algorithms
  10. 10. Reduce the Correlation between PixelsLossy compression: Orthogonal Transform Coding ◦ KLT (Karhunen-Loeve Transform)  Maximal Decorrelation Process ◦ DCT (Discrete Cosine Transform)  JPEG is a DCT-based image compression standard, which is a lossy coding method and may result in some loss of details and unrecoverable distortion. Subband Coding ◦ DWT (Discrete Wavelet Transform)  To divide the spectrum of an image into the lowpass and the highpass components, DWT is a famous example.  JPEG 2000 is a 2-dimension DWT based image compression standard. Predictive Coding ◦ DPCM  To remove mutual redundancy between successive pixels and encode only the new information
  11. 11. Outline ImageCompression Fundamentals Reduce correlation between pixels(lossy) Quantization and Source Coding(lossless) Overview of Image Compression Algorithms
  12. 12. Quantization and Source Coding Lossless compression techniques:  Quantization ◦ The objective of quantization is to reduce the precision and to achieve higher compression ratio ◦ Lossy operation, which will result in loss of precision and unrecoverable distortion  Source Coding ◦ To achieve less average length of bits per pixel of the image. ◦ Assigns short descriptions to the more frequent outcomes and long descriptions to the less frequent outcomes ◦ Entropy Coding Methods  Huffman Coding  Arithmetic Coding  Run Length Coding  Dictionary Codes  Lempel-Ziv77  Lempel-Ziv 78
  13. 13. Huffman CodingCodeword Codeword X Probability length 01 00 1 0 2 01 1 0.25 0.3 0.45 0.55 1 10 01 00 1 2 10 2 0.25 0.25 0.3 0.45 11 10 01 3 11 3 0.2 0.25 0.25 000 11 3 000 4 0.15 0.2 001 3 001 5 0.15
  14. 14. Arithmetic Coding (1/3) Arithmetic Coding: a direct extension of Shannon-Fano-Elias coding calculate the probability mass function p(x n) and the cumulative distribution function F(xn) for the source sequence xn ◦ Lossless compression technique ◦ Treate multiple symbols as a single data unit Arithmetic Coding Algorithm Input symbol is l Previouslow is the lower bound for the old interval Previoushigh is the upper bound for the old interval Range is Previoushigh - Previouslow Let Previouslow= 0, Previoushigh = 1, Range = Previoushigh – Previouslow =1 WHILE (input symbol != EOF) get input symbol l Range = Previoushigh - Previouslow New Previouslow = Previouslow + Range* intervallow of l New Previoushigh = Previouslow + Range* intervalhigh of l END
  15. 15. Arithmetic Coding (2/3) Symbol Probability Sub-interval Input String : l l u u r e ? k 0.05 [0.00,0.05) l l u u re ? l 0.2 [0.05,0.25) u 0.1 [0.20,0.35) 0.0713348389 w 0.05 [0.35,0.40) =2-4+2-7+2-10+2-15+2-16 e 0.3 [0.40,0.70) r 0.2 [0.70,0.90) Codeword : 0001001001000011 ? 0.2 [0.90,1.00) 1 0.25 0.10 0.074 0.0714 0.07136 0.071336 0.0713360 1.00 0.90 ? ? ? ? ? ? ? ? r r r r r r r r 0.70 e e e e e e e e 0.40 0.35 w w w w w w w w 0.25 u u u u u u u u 0.05 l l l l l l l l 0 k k k k k k k k 0 0.05 0.06 0.070 0.0710 0.07128 0.07132 0.0713336
  16. 16. Arithmetic Coding (3/3) Symbol Probability Huffman codeword Input String : l l u u r e ? k 0.05 10101 Huffman Coding 18 bits l 0.2 01 Codeword : u 0.1 100 01,01,100,100,00,11,1101 w 0.05 10100 e 0.3 11 Arithmetic Coding 16 bits r 0.2 00 Codeword : 0001001001000011 ? 0.2 1011 Arithmetic coding yields better compression because it encodes a message as a whole new symbol instead of separable symbols Most of the computations in arithmetic coding use floating-point arithmetic. However, most hardware can only support finite precision While a new symbol is coded, the precision required to present the range grows There is a potential for overflow and underflow If the fractional values are not scaled appropriately, the error of encoding occurs
  17. 17. Zero-Run-Length Coding-JPEG (1/2) The notation (L,F) ◦ L zeros in front of the nonzero value F EOB (End of Block) ◦ A special coded value means that the rest elements are all zeros ◦ If the last element of the vector is not zero, then the EOB marker will not be added An Example: 1.57, 45, 0, 0, 0, 0, 23, 0, -30, -16, 0, 0, 1, 0, 0, 0, 0, 0, 0, 0, ..., 0 2. (0,57) ; (0,45) ; (4,23) ; (1,-30) ; (0,-16) ; (2,1) ; EOB 3. (0,57) ; (0,45) ; (4,23) ; (1,-30) ; (0,-16) ; (2,1) ; (0,0) 4. (0,6,111001);(0,6,101101);(4,5,10111);(1,5,00001);(0,4,0111);(2,1,1);(0,0) 5.1111000 1111001 , 111000 101101 , 1111111110011000 10111 , 11111110110 00001 , 1011 0111 , 11100 1 , 1010
  18. 18. Dictionary Codes Dictionary based data compression algorithms are based on the idea of substituting a repeated pattern with a shorter token Dictionary codes are compression codes that dynamically construct their own coding and decoding tables “on the fly” by looking at the data stream itself It is not necessary for us to know the symbol probabilities beforehand. These codes take advantage of the fact that, quite often, certain strings of symbols are “frequently repeated” and these strings can be assigned code words that represent the “entire string of symbols” Two series ◦ Lempel-Ziv 77: LZ77, LZSS, LZBW ◦ Lempel-Ziv 78: LZ78, LZW, LZMW
  19. 19. OutlineImage Compression FundamentalsReduce correlation between pixelsQuantization and Source CodingOverview of Image Compression Algorithms
  20. 20. Outline ImageCompression Fundamentals Reduce correlation between pixels(lossy) Quantization and Source Coding(lossless) Overview of Image Compression Algorithms
  21. 21. Joint Photographic Experts Group (JPEG) The JPEG Encoder The JPEG Decoder
  22. 22. ReferenceDigital image processing, by Rafael C.Gonzalez and Richard E.Woods page no:421-423,440-456,459-486.
  23. 23. THANK UFOR UR PATIENCE

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