Fast Decision of Block size Prediction Mode and Intra Block for H 264 Intra Prediction EE 5359 Gaurav Hansda 1000721849 gaurav hansda mavs uta edu H 264 Encoder Fig 1 Basic coding structure for H 264 AVC for a macroblock 2 Mode decision of H 264 encoder Fig 2 Mode decision hierarchy of an H 264 compliant encoder 4 Implication of Hierarchical Structure To ensure the correctness of the decision at upper layer 2 To ensure early termination is executed accurately and as early as possible Most fast mode decision algorithms developed so far only deal with a single stage of the mode decision hierarchy 5 14 and fail to achieve the best possible complexity reduction We will concentrate on left branch of the mode decision hierarchy namely intra intra block decision of inter frames block size decision of intra blocks and the prediction mode decision of intra blocks 1 Intra Prediction There are 3 macroblock MB modes for intra prediction of luma pixels intra4x4 I4MB intra8x8 I8MB and intra16x16 I16MB Intra4MB and Intra8MB have 9 prediction modes as shown in Fig 3 a Intra16MB has only 4 prediction modes as shown in Fig 3 b Fig 3 Prediction modes for a Intra4MB and b Intra16MB 4 Fig 4 Prediction flow diagram 18 Fig 5 Intra prediction 18 Mode Decision To achieve a better tradeoff between bit rate and distortion H 264 encoder adopts the rate distortion R D optimization framework and the Lagrangian technique for mode decision 2 For intra frames the best prediction mode of a block is defined as the mode that among all prediction modes of the block gives rise to the minimum R D cost The R D cost of an MB mode is the sum of the minimum RD cost of each individual block Related work on Fast Prediction Mode Decision Can be classified into two categories non filter based and filter based Non filter based algorithms attempt to reduce the computation of the R D cost and improve the R D performance by modifying the matching criterion Most such algorithms like that of developed by Huang et al 5 and Tseng et al 7 deal with the low complexity mode The best prediction mode of a block is highly correlated with its dominant edge direction Filter based algorithms determine the dominant edge direction of each block and then select the associated prediction mode the neighboring two prediction modes and the DC mode as candidate modes Always include the DC mode as a candidate mode Algorithms developed by Pan et al 6 Tsai et al 9 Li et al 10 and Bharanitharan et al 12 all exploits various edge detection Due to limited prediction accuracy filter based algorithms introduce more the 2 increase in bit rate Related work on Fast Inter Intra Block decision The goal of fast inter intra block decision is to efficiently determine which coding type between inter coding and intra coding is more suitable for coding an MB of an inter frame Basic idea is to measure the correlation between spatially and temporally neighboring MBs If the spatial correlation is larger than the temporal correlation the block under consideration is more likely to be an intra block Algorithms developed by Choi et al 13 and Kim et al 14 have high computational overhead Proposed Algorithm for Block Size Decision Block size is highly correlated with texture complexity Variance of block corresponds to the total energy of the AC coefficients of the block hence it is good measurement of the texture complexity Thus variance based classification of texture complexity is used 16 If variance is above the threshold Intra4MB and Intra8MB is selected otherwise Intra8MB and Intra16MB is chosen This is simple way to skip the examination of Intra4MB mode Fig 6 Variance based MB mode decision 4 Improved Prediction Mode Decision Earlier algorithms only consider the edge information of the current block and do not take correlation between blocks into account Hence the Most Probable Mode MPM is used The MPM which takes advantage of the spatial correlation of the prediction modes between the neighboring blocks and the current block for coding is defined as the prediction mode of the left or the upper neighbor whichever has the smaller prediction mode number Input 2x2 subsampled block Pass through the filters separately Determine the dominant edge Choose the candidate modes Fig 7 Prediction mode decision 4 Proposed algorithm for Intra Block Decision Intra block decision for inter frames occupies a considerable percentage of the total computations of inter frame coding Intra16MB takes much less computation time than the other modes Hence scaled R D cost is used 4 An MB is less probable to be intra coded if the R D cost difference between best inter mode and Intra16MB is small Denoting the scaled R D cost differences between Intra16MB and the inter MB mode by d J and based on the above observation if d J is small both I4MB and I8MB can be skipped References 1 Draft ITU T Recommendation and Final Draft International Standard of Joint Video Specification document JVT G050 doc ITU T Rec H 264 and ISO IEC 14496 10 AVC 2003 2 T Wiegand et al Overview of H 264 video coding standard IEEE Trans Circuits Syst Video Technol vol 13 no 7 pp 560 576 Jul 2003 3 G J Sullivan and T Wiegand Rate distortion optimization for video compression IEEE Signal Process Mag vol 15 no 6 pp 74 90 Nov 1998 4 Y W Huang T Ou and H Chen Fast decision of block size prediction mode and intra block for H 264 intra prediction I EEE Trans Circuits Syst Video Technol vol 20 no 8 pp 11221132 Aug 2010 5 Y W Huang et al Analysis fast algorithm and VLSI architecture design for H 264 AVC intra frame coder IEEE Trans Circuits Syst Video Technol vol 15 no 3 pp 378 401 Mar 2005 6 F Pan et al Fast mode decision algorithm for intra prediction in H 264 AVC video coding IEEE Trans Circuits Syst Video Technol vol 15 no 7 pp 813 822 Jul 2005 7 C Tseng H Wang and J Yang Enhanced intra 4x4 mode decision for H 264 AVC coder IEEE Trans Circuits Syst Video Technol vol 16 no 8 pp 1027 1032 Aug 2006 8 J C Wang J F Wang and J T Chen A fast mode decision algorithm and its VLSI design for H 264 AVC intra prediction IEEE Trans Circuits Syst Video Technol vol 17 no 10 pp 1414 1422 Oct 2007 9 A C Tsai et al Intensity gradient technique for efficient intra prediction in H 264 AVC IEEE Trans Circuits Syst Video Technol vol 18 no 5 pp 694 698 May 2008 10 H Li K Ngan and Z Wei Fast and efficient method for block edge classification and its application in H 264 AVC video coding IEEE Trans Circuits Syst Video Technol vol 18 no 6 pp 756 768 Jun 2008
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