Windows Media Video 9

Tarun Bhatia

tarun79_us@yahoo.com

Multimedia Processing Lab

University Of Texas at Arlington

11/05/04

Introduction 1

Introduction 2

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 , 4:2:0 format• Uses block based transform and motion

compensation with quantization and entropy coding.

The Block Diagram :-

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

8x8 Integer DCT Matrices

• WMV 9 • H.264 HP

Key features of the Transforms

• The norms of vectors of the ratio 288:289:299

• The variation in the norm accounted for in the encoder itself

• At the decoder inverse transform(rows) -> rounding-> inverse transform (columns) ->rounding

(to operate in the 16 bit range)

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

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

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

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

Interlaced coding

• Supports field and frame coding

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

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

Low rate tools (<100 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).

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 image>threshold=fading)

• 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

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

AVS China’s next generation video

coding standard

Introduction

• Streamlined video coder dedicated to coding HDTV content (1920x1080 in 4:2:0 and 4:2:2)

• 4 levels are defined

Level 4.0 : Standard Definition 4:2:0

Level 4.2 : Standard Definition 4:2:2

Level 6.0 : HD 4:2:0

Level 6.2 : HD 4:2:2 (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

Data Formats

• Progressive scan

results in significantly less coding complexity

• Interlaced scan

Structure

Have

start codes

Structure

Slice

Structure

4:2:0

4:2:2

Block Diagram (Encoder)

Block Diagram (Decoder)

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.