Slide 1H.264 EncoderMode decision of H.264 encoderImplication of Hierarchical StructureIntra-PredictionSlide 6Mode DecisionRelated work on Fast Prediction Mode DecisionSlide 9Related work on Fast Inter/Intra Block decisionProposed Algorithm for Block Size DecisionSlide 12Improved Prediction Mode DecisionSlide 14Proposed algorithm for Intra Block DecisionReferencesSlide 17Slide 18Fast Decision of Block size, Prediction Mode and Intra Block for H.264 Intra PredictionEE 5359Gaurav [email protected] EncoderFig. 1. Basic coding structure for H.264/AVC for a macroblock. [2]Mode decision of H.264 encoderFig. 2. Mode decision hierarchy of an H.264 compliant encoder. [4]1. 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.Implication of Hierarchical StructureIntra-PredictionThere 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]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 R-D cost of each individual block.Mode DecisionCan 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. Related work on Fast Prediction Mode DecisionThe 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.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.Related work on Fast Inter/Intra Block decisionBlock 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.Proposed Algorithm for Block Size DecisionFig. 6. Variance-based MB mode decision [4]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. Improved Prediction Mode DecisionInput 2x2 subsampled blockPass through the filters separatelyDetermine the dominant edgeChoose the candidate modesFig. 7. Prediction mode decision [4]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 dˆJ, and based on the above observation, if dˆJ is small both I4MB and I8MB can be skipped. Proposed algorithm for Intra Block Decision[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. 1122-1132, Aug. 2010[5] Y.-W. Huang et al, “Analysis, fast algorithm, and VLSI architecture design for H.264/AVC intra frame coder,” IEEE Trans.
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