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 Outline Introduction to H 264 Current algorithms for intra prediction Proposed algorithms Implementation results Conclusions Overview of H 264 H 264 is an industry standard It defines a format for compressed video data It provides a set of tools that can be used in a variety of ways to compress and communicate visual information Purpose of a standard Define a coded representation or syntax that describes visual data in a compressed form and method of decoding the syntax to reconstruct visual information compliant encoders and decoders can successfully interoperate with each other H 264 Profiles Extended Profile SP frames SI frames FMO Redundant slices Main Profile Baseline Profile I frames P frames CAVLC B frames Interlace CABAC Fig 1 H 264 profiles 2 Applications Broadcast television Streaming video Video storage and playback Videoconferencing Mobile video Studio distribution Coding Process The image is divided into macroblocks 16x16 pixels The macroblocks are grouped into slice groups which are divided into slices Each slice is coded either as an I P or B slice There are also types called SI and SP In an I slice all blocks are coded as I blocks In a P slice blocks are coded as I or P blocks In a B slice blocks are coded as I P or B blocks Fig 2 Typical H 264 encoder 18 Fig 3 Typical H 264 decoder 18 Mode decision of H 264 encoder Fig 4 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 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 5 a Intra16MB has only 4 prediction modes as shown in Fig 5 b Fig 5 Prediction modes for a Intra4MB and b Intra16MB 4 Fig 6 Prediction flow diagram 18 Fig 7 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 Proposed Algorithm for Block Size Decision Block size is highly correlated with texture complexity Variance of a 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 8 Variance based MB mode decision 4 Improved Prediction Mode Decision Earlier algorithms only consider the edge information of the current block The correlation between blocks was not considered 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 Decision Each original image block is evenly divided into four subblocks first Each subblock is represented by the average pixel magnitude of its pixels Fig 9 Formation of subsampled block for a block of a Intra4MB b Intra8MB and c Intra16MB 4 Apply the following filters Fig 10 Five sets of filter coefficients for dominant edge detection 4 Determine the dominant edge Input 2x2 subsampled block Pass through the filters separately Determine the dominant edge Choose the candidate modes Fig 11 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 Joint Model JM Reference implementation standardized in ISO IEC JTC1 SC29 WG11 Decoder implements almost all the features Encoder Exercises most of the important coding tools Provides an elaborate list of control parameters Offers a rate distortion optimized implementation Offers several fast computation options Serves as a reference for what is best quality possible using H 264 Good description of the reference algorithms exists Currently at v17 2 Can be downloaded from http iphome hhi de suehring tml download Sequences used Hall CIF and QCIF Container CIF and QCIF Mobile CIF JM software implementation The JM reference software version 17 2 is implemented 17 The conditions of the experiment are as follows 1 2 3 4 5 Run on a PC with Intel Core i3 2 27GHz processor and 3 00 GB RAM Set the QP values to 16 20 24 and 28 Number of frames to be coded 100 Enable the R D optimization Choose context adaptive binary arithmetic coding CABAC as the entropy coding method Performance QP PSNR dB Bit rate kbits s Time s SSIM 28 38 685 697 64 20 022 0 9766 24 41 378 961 59 30 875 0 9831 20 44 118 1359 14 34 094 0 9883 16 47 085 1930 5 37 767 0 9934 Table I Performance of Hall QCIF in JM reference software version 17 2 Variance based block size decision Sequence Decrease in PSNR dB Increase in bitrate Decrease in encoding time Container QCIF 0 099 0 49 23 7 Hall QCIF 0 051 0 27 11 5 Container CIF 0 045 0 26 25 4 Hall CIF 0 054 0 43 24 2 Mobile CIF 0 019 0 07 7 41 Table II Performance of variance based block size decision in JM reference software version 17 2 compared to using JM v17 2 directly Prediction mode decision 8 Sequence Decrease in PSNR dB Increase in bitrate Decrease in
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