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Media Compression
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You are Here Encoder Decoder Middlebox Sender Receiver Network
NUS.SOC.CS5248 Ooi Wei Tsang
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Why compress? “Bandwidth Not Enough” “Disk Space Not Enough”
Size of Uncompressed DVD Movie = NUS.SOC.CS5248 Ooi Wei Tsang
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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 NUS.SOC.CS5248 Ooi Wei Tsang
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JPEG Compression
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Original Image (1153KB) 1:1
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Original Image (1153KB) 3.5:1
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Original Image (1153KB) 17:1
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Original Image (1153KB) 27:1
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Original Image (1153KB) 72:1
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Original Image (1153KB) 192:1
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Compression Ratio Quality Size Ratio Raw TIFF 1153KB 1:1 Zipped TIFF
1.2:1 Q=100 331KB 3.5:1 Q=70 67KB 17:1 Q=40 43KB 27:1 Q=10 16KB 72:1 Q=1 6KB 192:1 NUS.SOC.CS5248 Ooi Wei Tsang
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Magic of JPEG Throw away information we cannot see
Color information “High frequency signals” Rearrange data for good compression Use standard compression NUS.SOC.CS5248 Ooi Wei Tsang
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Discard color information
Y YUV is also known as LUV or YCbCr V U NUS.SOC.CS5248 Ooi Wei Tsang
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Color Sub-sampling 4:4:4 4:2:0 4:2:2 4:1:1
420 is used by MPEG. 411 is used by DV. 4:2:2 4:1:1 NUS.SOC.CS5248 Ooi Wei Tsang
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4:2:2 Sub-sampling Y V U YUV is also known as LUV or YCbCr
NUS.SOC.CS5248 Ooi Wei Tsang
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Original Image (1153KB) 4:2:0
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Original Image (1153KB) “4:1:0”
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Discrete Cosine Transform
Demo NUS.SOC.CS5248 Ooi Wei Tsang
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Quantization DC 242 65 23 5 8 8 8 8 30 8 2 -54 -10 -4 -2 8 8 8 16 -6 -1 / = 13 6 3 5 8 8 16 32 1 2 1 -1 -2 8 16 32 64 Quantization Table AC NUS.SOC.CS5248 Ooi Wei Tsang
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Differential Coding 30 8 6 -1 2 1 25 3 2 1 4 27 3 2 1 4 30 8 6 -1 2 1
1 25 3 2 1 4 27 3 2 1 4 30 8 6 -1 2 1 -5 3 2 1 4 2 3 1 4 NUS.SOC.CS5248 Ooi Wei Tsang
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Zig-zag ordering 27 3 2 1 4 27, 3, 2, 4, 1, 1, 0, 0, 0, 0, 0, 1, 0, 0, 0, 0 NUS.SOC.CS5248 Ooi Wei Tsang
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Run-Length Encoding 27 3 2 1 4 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) NUS.SOC.CS5248 Ooi Wei Tsang
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Idea: Motion JPEG Compress every frame in a video as JPEG
DVD-quality video = 208.6GB Reduction ratio = 27:1 Final size = 7.7GB NUS.SOC.CS5248 Ooi Wei Tsang
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Video Compression
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Temporal Redundancy NUS.SOC.CS5248 Ooi Wei Tsang
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Motion Estimation NUS.SOC.CS5248 Ooi Wei Tsang
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Bi-directional Prediction
NUS.SOC.CS5248 Ooi Wei Tsang
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Watch for Motion Vectors
Demo NUS.SOC.CS5248 Ooi Wei Tsang
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H.261 P-Frame I-Frame NUS.SOC.CS5248 Ooi Wei Tsang
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MPEG-1 B-Frame NUS.SOC.CS5248 Ooi Wei Tsang
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MPEG Frame Pattern Example display sequence:
IBBPBBP … Example encoding sequence: IPBBPBB NUS.SOC.CS5248 Ooi Wei Tsang
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Compression Ratio Frame Type Typical Ratio I 10:1 P 20:1 B 50:1
NUS.SOC.CS5248 Ooi Wei Tsang
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Sequence sequence header: width height frame rate bit rate :
NUS.SOC.CS5248 Ooi Wei Tsang
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GOP: Group of Picture gop header: time : NUS.SOC.CS5248 Ooi Wei Tsang
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Picture pic header: number type (I,P,B) : NUS.SOC.CS5248 Ooi Wei Tsang
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Picture NUS.SOC.CS5248 Ooi Wei Tsang
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Slice NUS.SOC.CS5248 Ooi Wei Tsang
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Macroblock NUS.SOC.CS5248 Ooi Wei Tsang
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Block Y U 1 Macroblock = V NUS.SOC.CS5248 Ooi Wei Tsang
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For I-Frame Every macroblock is encoded independently (“I-macroblock”)
NUS.SOC.CS5248 Ooi Wei Tsang
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For P-Frame Every macroblock is either I-macroblock
a motion vector + error terms wrt a prev I/P-frame (“P-macroblock”) NUS.SOC.CS5248 Ooi Wei Tsang
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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 NUS.SOC.CS5248 Ooi Wei Tsang
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Review: MPEG structure
Sequence GOP Picture Slice Macroblock Block NUS.SOC.CS5248 Ooi Wei Tsang
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MPEG Decoding (I-Frame)
Entropy Decoding Dequantize IDCT NUS.SOC.CS5248 Ooi Wei Tsang
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MPEG Decoding (P-Frame)
Entropy Decoding Dequantize IDCT Prev Frame + NUS.SOC.CS5248 Ooi Wei Tsang
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MPEG Decoding (B-Frame)
Entropy Decoding Dequantize Future Frame IDCT AVG Prev Frame + NUS.SOC.CS5248 Ooi Wei Tsang
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There are more.. Half-pel Motion Prediction Skipped Macroblock etc.
NUS.SOC.CS5248 Ooi Wei Tsang
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MPEG in daily life MPEG Standards Bit-rate Usage MPEG-1 1.5Mbps VCD
DVD, SVCD, HDTV MPEG-4 Scalable QuickTime, DivX;) NUS.SOC.CS5248 Ooi Wei Tsang
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The Future: H.264 Same quality at half the rate How:
Variable block size Multiple reference frame ... NUS.SOC.CS5248 Ooi Wei Tsang
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Hands-On Download source code, compile and play with
ffmpeg mpeg_stat Try different MPEG-1 encoding parameter NUS.SOC.CS5248 Ooi Wei Tsang
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Impact on Systems Design
How to package data into packets? How to deal with packet loss? How to deal with bursty traffic? How to predict decoding time? : NUS.SOC.CS5248 Ooi Wei Tsang
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