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MPEG1 - City University of Hong Kong City Moving Picture...

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City University of Hong Kong Moving Picture Expert Group - Established in 1988 by the Joint ISO/IEC Technical Committee on IT. Mission - To develop standards for coded representation of motion pictures and audio at a bit rate of up to 1.5Mb/s. MPEG-1 was issued in 1992. MPEG-2 (1994) - higher quality (not lower than NTSC and PAL) with bit rates between 2-10Mb/s. Applications - Digital CATV and Terrestrial digital broadcasting distribution, Video recording and retrieval.
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City University of Hong Kong Lossy compression Trade off image quality with bit rate according to objective or subjective criteria Video sequences usually contains large statistical redundancies in both temporal and spatial directions Intraframe coding Interframe coding Subsampling of Chrominance - Human eye is more sensitive to luminance than chrominance
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City University of Hong Kong Encoding of a single picture Similar to JPEG Discrete Cosine Transform- Converts spatial to frequency domain Quantization of spectral coefficients DPCM to encode DC terms Zigzag scan to group zeros into long sequences, followed by run-length coding Lossless, Variable Length Coding to encode AC coefficients
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City University of Hong Kong Remove temporal redundancies between frames Use extensively in MPEG-1 and MPEG-2 Based on estimation of motion between video frames Use of motion vectors to describe displacement of pixels from one frame to the next Spatial correlation between motion vectors are high One motion vector can represent the motion of a block of pixels.
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City University of Hong Kong Current frame Previous frame Figure 1 For each image block in the current frame, Find its nearest counterpart in the previous frame. Record the displacement vector
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City University of Hong Kong Figure 2 mv Frame N-1 Frame N Search Window Previous Block Location Current Block Location Only the prediction error (residual) images are transmitted Good prediction reduces information content in residual images
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Partition the previous and the current images into non- overlapping square blocks of size N x N Previous frame Current frame
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Previous frame Current frame e.g., N =8 Partition the previous and the current images into non- overlapping square blocks of size N x N
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Represent each block with a 2D matrix: f ( x,y ) for previous frame g ( x,y ) for current frame Previous frame Current frame Partition the previous and the current images into non- overlapping square blocks of size N x N
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Represent each block with a 2D matrix: f ( x,y ) for previous frame g ( x,y ) for current frame ( 29 ( 29 ( 29 [ ] ∑ ∑ = = - = N Row N Col y x g y x f N g f D 0 0 2 2 , , 1 , Previous frame Current frame Partition the previous and the current images into non-overlapping square blocks of size N x N Difference between any two blocks is given by
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( 29 ( 29 ( 29 [ ] ∑ ∑ = = - = N Row N Col y x g y x f N g f D 0 0 2 2 , , 1 , Previous frame Current frame Partition the previous and the current images into non-overlapping square blocks of size N x N The lower the difference, the more similar is the pair of blocks Represent each block with a 2D matrix: f ( x,y ) for previous frame g ( x,y ) for current frame
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A motion vector is computed for ‘EVERY’ blocks in the current frame. HOW?
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