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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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Presentation Transcript
slide2

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
slide3

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.

slide4

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].

slide5

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

……

slide6

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

slide7

2.5 dB

3.3 dB

Motion-Estimation Results

slide8

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

slide9

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.

slide10

Rate Control Results

Avg Hash bitrate

= 0.175 bpp

Avg WZ bitrate

= 1.27 bpp

slide11

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.

slide12

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