Getting the most out of H.264

Getting the most out of H.264 17 th November 2010 Iain Richardson

H.264 : what’s essential and what’s optional?

slices macroblocks frames or fields

I slice P slice B slice I MB I P Skip I P Skip B

Intra prediction choices

Inter prediction choices

Quantization Parameter Transform + quantization choices

Quantized coefficients Motion vectors Macroblock headers … .. Coded bitstream Exp-Golomb / CAVLC encoder CABAC encoder Entropy coding choices

What are the trade-offs?

Quality (or Distortion) Rate Computation

Example: Group of Pictures structure

Encoder choices are based on: Decoder capabilities Video content Computing resources Profile + Level Current + previous statistics Time to encode

What’s important? Bitrate Quality Delay Real-time or offline Flexibility

H.264 mode decisions

MB header and vectors Coded residual Coded macroblock

I slice

P slice

B slice

Making the best decisions

Cost of coding Quality (or distortion) Bitrate Computation

Bitrate PSNR Rate-Distortion Curve for a video clip BETTER WORSE

Rate PSNR R-D points for one MB

Each MB mode has a D “cost” and an R “cost”
Calculate a combined cost J:
J = D + λR
R D Choose the mode with the smallest value of J

Rate PSNR λ1 λ2

For every macroblock: 1. Try out every coding mode, measure D and R 2. Choose mode with minimum J = D + λR  Rate Distortion Optimised (RDO) Mode Selection

Rate cost Distortion cost Headers Residual Skip MB Intra MB P MB B MB Distortion J

Bitrate PSNR Few modes, no RDO Multiple modes, no RDO Multiple modes, with RDO

Computational overload?

Practical strategy 1: Reduce the number of modes

Practical strategy 2: Approximate cost measures
Sub-sample number of search points
Use a less sophisticated comparison metric
Etc.

What’s next?

HD content everywhere & not enough bandwidth?

Computation increasing on every platform

More computation More coding options High Efficiency Video Coding Flexible Coding

Summary

Questions? onecodec.com www.dcktn.org.uk vcodex.com Email me: iain @ onecodec.com Follow me on Twitter: @onecodec elecard.com

Getting the most out of H.264

by Iain Richardson

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