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Interframe Video Compression

Interframe Encoder. Interframe Decoder. X i. X i ’. Standard codec. X’ i-1. Interframe Video Compression. Interframe predictive coding for compression Encoder is 5-10 times more complex than decoder. X i-1 ’. Interframe Decoder. Intraframe Encoder. X i. X i ’. Side Information.

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Interframe Video Compression

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  1. Interframe Encoder Interframe Decoder Xi Xi’ Standard codec X’i-1 Interframe Video Compression • Interframe predictive coding for compression • Encoder is 5-10 times more complex than decoder

  2. Xi-1’ Interframe Decoder Intraframe Encoder Xi Xi’ Side Information Low-complexity Video Coding • Low-complexity encoding, possibly more complex decoding • Applications: video sensors for surveillance, wireless PC cameras, mobile camera-phones Wyner-Ziv Coding Lossy source coding with decoder side information Imagine dependence channel between source and side-info. WZ coding = sending parity symbols to correct errors in the dependence channel.

  3. Outline • How to improve side-information at decoder w/o access to current frame?  Hash-Aided Motion Estimation • How to decide encoding bit-rate needed to correct errors in dependence channel?  Hash-Aided Rate Control Project motivated by Wyner-Ziv video codec architecture described in [Aaron et al., VCIP ’04, ICIP ’03].

  4. Generating the Hash • Visual hash originally used in digital watermarking [Fridrich, 1997] • E.g., Generate a 16 bit hash for an image-block of size NxN. • Generate 16 NxN matrices with elements i.i.d. ~ U[0,1]. • Low-pass filter each matrix. (e.g., repeated 2x2 averaging.) • Write out image block as vector • Write out each low-pass pattern as vector • Find inner products ……

  5. Unavailable Available Frame n Frame n-1 B=00110110received from encoder as helper information A=00111110 Side info Hash-aided Motion Estimation Projecting onto low frequency patterns is equivalent to finding the low-frequency DCT coefficients. Similar looking blocks have almost equal low-frequency DCT coefficients and produce almost similar hashes. Motion Compensated Side info

  6. 2.5 dB 3.3 dB Motion-Estimation Results

  7. Motion-Estimation Results Foreman QCIF Frames 301 to 350 Error between current frame and side information With previous frame As side info With hash-aided motion estimation with excess bit-rate = 0.14 bpp

  8. Hash-Aided Rate Control • Encoder can perform motion-estimation at low-complexity, with small hash store. • Experimentally found that there is a high correlation between MSE of MC prediction and parity bitrate, which the encoder needs to transmit. • For given code parameters, can characterize the required bitrate as a function of MSE of MC prediction.

  9. Rate Control Results Avg Hash bitrate = 0.175 bpp Avg WZ bitrate = 1.27 bpp

  10. Conclusions • Hash-aided motion estimation can be used at the decoder to improve the quality of the side-information at very low bit-rate overhead. This reduces the overall Wyner-Ziv bit-rate. • With a small complexity addition, motion estimation can be performed at the encoder. This enables the encoder to control the Wyner-Ziv bit-rate depending upon the quality of the MC prediction.

  11. Frames encoded independently Frames decoded conditionally Input current video frame Decoded Frame Wyner-Ziv Encoder Wyner-Ziv Decoder Motion compensated prediction (extrapolation) Hash-aided rate control Hash-aided motion estimation Side Info: previous decoded frame(s) + Helper information from encoder

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