No-Reference Metrics For Video Streaming Applications

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No-Reference Metrics For Video Streaming
Applications

• International Packet Video Workshop (PV 2004)
• Presented by Bhavana
• CPSC 538
• February 21, 2004

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Video Quality Assessment

• Whats Quality ?
• - Implies Comparison gt reference
• Three Techniques
• - Full-Reference eg. MSE, PSNR
• - Reduced Reference
• - No Reference

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What is a No-Reference metric ?

• Estimating end-users QoE of a multimedia stream
  without using an original stream as a reference.
• In other words
• Quantify quality via blind distortion
  measurement

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Purpose

• To evaluate two types of distortions in
  streaming of compressed video over
  packet-switched networks
• - Compression related block-edge impairment
• - Transmission related packet-loss
  impairment

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Where Can It Be Used ?

• For real time monitoring .
• Reference unavailable or expensive to send
• Feedback to Streaming Server .
• Evaluation of Compression Algorithms

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What are Block-Based Codecs ?

• Process several pixels of video together in
  blocks
• At high compression rates, strong
  discontinuities called block edges come up.
• Whats blockiness ?
• Distortion of image characterized by
  appearance of underlying block encoding structure

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Block based Distortion

• Idea A block-edge gradient can be masked by a
  region of high spatial activity around it .
• Measure two things
• - spatial activity around block edges s
• - block-edge gradient ?

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Calculation of NR Blockiness Metric

• For each 8 x 8 Block Bij ,
• For each edge Ik of Bij ,
• divide edge into 3 segments akl
• For each segment of akl
• calculate skl
• calculate ?kl

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E1
I1
An 8 x 8 block and its edges
E2
I2
Bij
I4
E4
I3
E3
a3
a2
a1
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Three segments akl of a
block edge
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NR Blockiness Metric contd

• CB No. of Blocks for which at least one edge
  satisfies
• skl lt e where e 0.1
• ?kl gt t where t 2.0
• e min. spatial activity required to mask
  gradient
• t max. gradient which is imperceivable.
• ßF CB / Total no. of blocks in the frame

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Simulation Setup For NR Blockiness Metric

• Aim to measure how well the NR Blockiness
  metric conveys QoE
• Codec MPEG -4 , GOP 30 frames
• Bit Rate gt compression level

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NR Packet Loss Metric

• Error Concealment Replace damaged/lost
  macroblock with corresponding macroblock from
  previous frame.
• Idea Use length of artifact to estimate amount
  of distortion caused by packet loss

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Calculation of NR Packet Loss Metric

• For a m x n frame
• For each 16 x 16 macroblock
• Calculate
• Êj strength vector across macroblock
• edge
• Ê?j strength vector within macroblock
• near the edge

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Macroblock 1
Macroblock 2
Figure Calculating Strength vector across and
within a macroblock
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• Convert strength vectors into binary vectors
• Ej(k) 1 if Êj gt t
• 0 otherwise
• E?j(k) 1 if Ê?j gt t
• 0 otherwise
• where t 15

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• If the sum of differences between the two binary
  edge vectors is substantial , then there is
  distortion
• Packet loss metric for jth macroblock
• Hj ? Ej(k) - E?j(k) if ? Ej(k)
  - E?j(k) gt ?
• 0 otherwise
• where ? 10 of frame width (n)
• Packet loss metric for whole frame
• F ? Hj2

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Simulation Setup for NR Packet Loss Metric

• Bit Rate 1.5 Mbps
• Frame Rate 30 fps
• Frame Size 352 x 240
• Used NTT DoCoMo packet loss generating software .

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Limitations Of NR-metrics

• Blockiness metric might fail in the presence of
  strong de-blocking filters which might otherwise
  introduce blur
• Metric predictions lose meaning in presence of
  other distortions like blur, noise etc.

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Future Directions

• VQEG standardization efforts
• HVS based approaches
• Statistical models for natural scenes
• NR QA schemes for
• - Non-block based compression schemes such
  Wavelet-based
• -Targeting full range of artifacts

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References

• No Reference Image and Video Quality Assessment
  http//live.ece.utexas.edu/research/quality/nrqa.h
  tm
• Objective video Quality Assessment
  http//www.cns.nyu.edu/zwang/files/papers/QA_hvd_
  bookchapter.pdf
• Perceptual Video Quality and Blockiness Metrics
  for Multimedia Streaming Applications
• www.stefan.winkler.net/Publications/wpmc2001.pdf