1. Data compression

2. lossless – looking for unicolor areas or repeating patterns –Run length encoding –Dictionary compressions Lossy – reduction of colors - approximating methods (JPEG)

3. Grid graphics formats BMP – no compression PCX – lossless compression RLE PNG – lossless dictionary compression LZW GIF – lossless dictionary compression LZW + reduction to 256 color (adaptive palette) JPG – approximating compression JPEG

4. JFIF format (JPEG File Interchange format)‏  sequential, common known  progressive, more effective, for computer net transmissions  lossless, not known and not widely supported  hierarchic, more resolutions in one file, quick preview

5. Sequential JFIF encoding

6. Color model transofmation RGB → Y Cb Cr Y= 0,299*R + 0,587*G + 0,114*B (brightness)‏ Cb = - 0,1687*R - 0,3313*G + 0,5*B + 128 Cr = 0,5*R - 0,4187*G - 0,0813*B + 128 R = Y + 1.402*(Cr-128)‏ G = Y - 0.34414*(Cb-128) - 0.71414*(Cr-128)‏ B = Y + 1.772*(Cb-128)‏

7. Subsampling of Cb,Cr Computing of average value for the block 2x1 pixels (6 bits sample), –6 bits -> 4 bits (compression 67%) or 2x2 pixels (12 bits sample), –12 bits -> 6 bits (compression 50%)

8. DCT transformation

9. Example 139 144 149 153 155 155 155 155 144 151 153 156 159 156 156 156 150 155 160 163 158 156 156 156 159 161 162 160 160 159 159 159 159 160 161 162 162 155 155 155 161 161 161 161 160 157 157 157 162 162 161 163 162 157 157 157 162 162 161 161 163 158 158 158

10. DCT coefficients AC coefficient (= 8 x average brightness

11. Quantization matrix – example for 90% “qality”

12. Quantization matrices Defined by standardization committee JPEG. Separately for brightness and for color components. Defined matrices for quality 10% and 90%. For other values of quality between 10% and 90%obtained by linear interpolation. For values under 10% or over 90% extrapolation can be used but it is not recommended.

13. Coefficients after quantisation

14. AC coefficients Stored separately Not compressed Possibility used for quick preview

15. Huffman encoding

16. Example 0, -2, -1, -1, -1, 0, 0, -1, -1, 0, 0, 0…..

17. Reconstruction of DCT coeffients

18. After inverse DCT transformation

19. Table of differences