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 pixel i e 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 p bicubic 2 16x16 p bicubic 3 16x16 p bicubic 4 16x16 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
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