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Media Compression

Media Compression. You are Here. Encoder. Decoder. Middlebox. Receiver. Sender. Network. Why compress?. “Bandwidth Not Enough” “Disk Space Not Enough” Size of Uncompressed DVD Movie =. Why compress?. “Bandwidth Not Enough” “Disk Space Not Enough”

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Media Compression

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  1. Media Compression

  2. You are Here Encoder Decoder Middlebox Receiver Sender Network

  3. Why compress? • “Bandwidth Not Enough” • “Disk Space Not Enough” • Size of Uncompressed DVD Movie =

  4. Why compress? • “Bandwidth Not Enough” • “Disk Space Not Enough” • Size of Uncompressed DVD Movie = 720 x 576 x 3 x 25 x 60 x 120 =208.6GB

  5. JPEG Compression

  6. Original Image (1153KB) 1:1

  7. Original Image (1153KB) 3.5:1

  8. Original Image (1153KB) 17:1

  9. Original Image (1153KB) 27:1

  10. Original Image (1153KB) 72:1

  11. Original Image (1153KB) 192:1

  12. Compression Ratio

  13. Magic of JPEG • Throw away information we cannot see • Color information • “High frequency signals” • Rearrange data for good compression • Use standard compression

  14. Discard color information Y V U

  15. Color Sub-sampling 4:4:4 4:2:0 4:2:2 4:1:1

  16. 4:2:2 Sub-sampling Y V U

  17. Original Image (1153KB) 4:2:0

  18. Original Image (1153KB) “4:1:0”

  19. Discrete Cosine Transform Demo

  20. Quantization DC 242 65 23 5 8 8 8 8 30 8 2 0 -54 -10 -4 -2 8 8 8 16 -6 -1 0 0 / = 13 6 3 5 8 8 16 32 1 0 0 0 2 1 -1 -2 8 16 32 64 0 0 0 0 Quantization Table AC

  21. -5 30 30 27 2 25 8 3 3 3 3 8 2 1 1 1 1 2 0 0 0 0 0 0 2 6 6 2 2 2 1 1 -1 -1 1 1 0 0 0 0 0 0 0 0 0 0 0 0 1 4 4 4 4 1 0 0 0 0 0 0 1 1 0 1 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 Differential Coding

  22. 27 3 1 0 2 1 0 0 4 0 1 0 0 0 0 0 Zig-zag ordering 27, 3, 2, 4, 1, 1, 0, 0, 0, 0, 0, 1, 0, 0, 0, 0

  23. 27 3 1 0 2 1 0 0 4 0 1 0 0 0 0 0 Run-Length Encoding 27, 3, 2, 4, 1, 1, 0, 0, 0, 0, 0, 1, 0, 0, 0, 0 (27, 1) (3, 1) (2, 1), (4, 1), (1, 2), (0, 5), (1, 1), (0, 4)

  24. Idea: Motion JPEG • Compress every frame in a video as JPEG • DVD-quality video = 208.6GB • Reduction ratio = 27:1 • Final size = 7.7GB

  25. Video Compression

  26. Temporal Redundancy

  27. Motion Estimation

  28. Bi-directional Prediction

  29. Watch for Motion Vectors Demo

  30. H.261 P-Frame I-Frame

  31. MPEG-1 B-Frame

  32. MPEG Frame Pattern • Example display sequence: • IBBPBBP … • Example encoding sequence: • IPBBPBB

  33. Compression Ratio

  34. Sequence • sequence header: • width • height • frame rate • bit rate • :

  35. GOP: Group of Picture • gop header: • time • :

  36. Picture • pic header: • number • type (I,P,B) • :

  37. Picture

  38. Slice

  39. Macroblock

  40. Y Y Y Y Block U 1 Macroblock = V

  41. For I-Frame • Every macroblock is encoded independently (“I-macroblock”)

  42. For P-Frame • Every macroblock is either • I-macroblock • a motion vector + error terms wrt a prev I/P-frame (“P-macroblock”)

  43. For B-Frame • Every macroblock is either • I-macroblock • P-macroblock • a motion vector + error terms wrt a future I/P-frame • 2 motion vectors + error terms wrt a prev/future I/P-frame

  44. Review: MPEG structure • Sequence • GOP • Picture • Slice • Macroblock • Block

  45. MPEG Decoding (I-Frame) Entropy Decoding Dequantize 101000101 IDCT

  46. MPEG Decoding (P-Frame) Entropy Decoding Dequantize 101000101 IDCT Prev Frame +

  47. MPEG Decoding (B-Frame) Entropy Decoding Dequantize 101000101 Future Frame IDCT AVG Prev Frame +

  48. There are more.. • Half-pel Motion Prediction • Skipped Macroblock • etc.

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