Windows Media Video 9

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Windows Media Video 9

• Tarun Bhatia
• tarun79_us_at_yahoo.com
• Multimedia Processing Lab
• University Of Texas at Arlington
• 11/05/04

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Introduction 1
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Introduction 2
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Introduction 3

• Latest video codec which gives state-of-art video
  quality from very low bit rates (160x120 at 10
  Kbps for modem) to very high bit rates (1920x1080
  for HDTV)
• 8-bit , 420 format
• Uses block based transform and motion
  compensation with quantization and entropy
  coding.

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The Block Diagram -
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Block Transforms(Integer DCT)

• 8x8 blocks can be encoded using
• 1_8x8 2_8x4
• 2_4x8 4_4x4
• Frame / Macroblock/Block signaling
• Block level for coarse and fine level
  specification
• Frame level for overhead reduction
• Only 8x8 used for I frames

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8x8 Integer DCT Matrices

• WMV 9

• H.264 HP

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Key features of the Transforms

• The norms of vectors of the ratio 288289299
• The variation in the norm accounted for in the
  encoder itself
• At the decoder
  inverse transform(rows) -gt rounding-gt
  inverse transform (columns) -gtrounding
• (to operate in the 16 bit range)

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Quantization

• Same rule applied to all block sizes
• Both types with (bit savings at low bit rates)
  and without dead zone ( available
• Type used signaled at the frame level to the
  decoder
• At the encoder side automatic switch from uniform
  quantization to dead zone quantization as Q
  parameter increases
• Other factors like noise and rate control can be
  used to control this switch

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Loop Filtering

• Done to remove blocky artifacts and thus quality
  of current frame for future prediction
• Operates on pixels on the border of blocks
• The process involves a discontinuity measurement
• Checks are computationally expensive so done only
  for one set of pixel per boundary

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Motion Estimation and Compensation

• Max resolution of ¼ pixel (i.e. ¼, ½, ¾) allowed
• 16x 16 motion vectors used by default but 8x8
  allowed
• Bicubic filter with 4 taps/ Bilinear filters with
  2 taps to generate subpixel precision.
• 4 combined modes
• 1.Mixed block size (16x16 and 8x8), ¼ p
  ,bicubic
• 2.16x16, ¼ p, bicubic
• 3.16x16, ½ p, bicubic
• 4.16x16, ½ p, bilinear
• Bilinear filters for chroma components

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Advanced entropy coding

• Simple VLC codes
• Multiple code tables for encoding each particular
  alphabet out
• A possible set of code tables is chosen (based on
  frame level quantization parameter) and signaled
  in the bitstream
• Additional information e.g. motion vectors
  resolution coded using bitplane coding

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Interlaced coding

• Supports field and frame coding

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Advanced B frame coding

• B frames- employ bi-directional prediction
• Fractional position definition with respect to
  the reference frames for better scaling of motion
  vectors
• Intra coded B frames for scene changes
• Allow inter field reference

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Overlap smoothing

• The deblocking filter smoothens out the block as
  well as true edges and it may be disabled in less
  complex profiles
• A lapped transform (input spans to pixels from
  other blocks as well) is used at the edges
• Used in spatial domain as pre and post processing
• Used only at low bit rates where blocking
  artifacts are higher
• Signaled at macroblock level so can be turned off
  in smooth regions

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Low rate tools (lt100 Kbps)

• Code frames at multiple resolutions (both in X
  and Y direction)
• A frame can be downscaled at the encoder and then
  upscaled at the decoder for LBR transmission
• The downscaling factor needs to remain same from
  the start of 1 I frame to the start of next I
  frame.
• The frame must be upscaled prior to display
  (upscaling out of scope of the standard).

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Fading compensation

• Large amount of bits required for scenes having
  effects like fade-to-black ,fade-from-black
• Not possible to predict motion using normally
  used techniques.
• Effective fading detection (original refrence
  image- current video imagegtthresholdfading)
• If detected then encoder computes fading
  parameters which specify a pixel-wise first order
  linear transform for the reference image.
• Also signaled to the decoder

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References

• Windows Media Video 9 overview and applications
• Sridhar Srinivasan, Pohsiang (John) Hsu,
  TomHolcom b, Kunal Mukerjee, Shankar L.
  Regunathan, Bruce Lin, Jie Liang, Ming-Chieh Lee,
  Jordi Ribas-Corbera
• Windows Digital Media Division, Microsoft
  Corporation, Redmond, WA 98052, USA
• available online at www.sciencedirect.com

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AVS
Chinas next generation video coding standard
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Introduction

• Streamlined video coder dedicated to coding HDTV
  content (1920x1080 in 420 and 422)
• 4 levels are defined
• Level 4.0 Standard Definition 420
• Level 4.2 Standard Definition 422
• Level 6.0 HD 420
• Level 6.2 HD 422 (HD High
  Definition)
• Designed to provide near optimum performance and
  considerable reduction in complexity (low cost
  implementation)
• Applications include broadcast TV,HD-DVD and
  broadband video networking

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Data Formats

• Progressive scan
• results in significantly less coding
  complexity
• Interlaced scan

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Structure
Have start codes
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Structure
Slice
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Structure
420
422
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Block Diagram (Encoder)
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Block Diagram (Decoder)
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Buffers

• The rate buffer at the encoder side helps in
  converting variable data rate produced by encoder
  to fixed data rate by controlling quantization
  using feedback
• The rate buffer at the decoder side gets the
  fixed rate data and stores it and then passes on
  to the decoder at a rate demanded by decoding of
  each macroblock and frame.