EE 5359 PROJECT REPORT Topic Complexity Reduction Algorithm for Intra Mode selection in H 264 AVC Video Coding By Amruta Kulkarni Student ID 1000666836 Under the guidance of Dr K R Rao TABLE OF ACRONYMS AVC advanced video coding CABAC DCT context adaptive binary arithmetic coding discrete cosine transform I frame JM intra frame joint model MSE mean square error PSNR peak signal to noise ratio SSIM structural similarity index metric VLC variable length coding RDO rate distortion optimization MPEG moving picture experts group VCEG video coding experts group FMO flexible macro block ordering ASO arbitrary slice ordering RS redundant slices SATD sum of absolute transformed differences Implementation of Complexity reduction algorithm for intra mode selection in H 264 AVC Video Coding Objective It is proposed to implement a complexity reduction algorithm for intra mode selection in H 264 AVC video coding 5 Need for the same arises as the mode decision algorithm used for H 264 encoding is inherently complex The encoding time required for a single macro block depends on the number of computations The mode decision process can be made simpler by reducing the number of computations this algorithm proposes to do the same Introduction H 264 is an Advanced Video Compression standard developed by ITU T Video Coding Experts Group together with ISO IEC Moving Picture Experts Group 1 It is the widely used video codec in mobile applications internet YouTube flash players set top box TV etc A H 264 encoder converts the video into a compressed format H 264 and a decoder convert s compressed video back into an uncompressed format A H 264 video encoder carries out prediction transform and encoding processes to produce a compressed H 264 bit stream The H 264 encoder block diagram is shown in Fig 1 1 Fig 1 H 264 encoder block diagram 1 A decoder carries out a complementary process by decoding inverse transform and reconstruction to output a decoded video sequence Fig 2 shows the basic building blocks of H 264 decoder Fig 2 H 264 decoder block diagram 1 The H 264 encoder forms a prediction of the current macro block One based on the current frame using intra prediction spatial prediction technique Intra prediction is an important technique in image and video compression to exploit spatial correlation within one picture Intra prediction supports the following block sizes 8 a 16x16 for Luma H 264 Intra 16x16 prediction modes are shown in Fig 2a Mode 0 vertical extrapolation from upper samples H Mode 1 horizontal extrapolation from left samples V Mode 2 DC mean of upper and left hand samples H V Mode 3 Plane a linear plane function is fitted to the upper and left hand samples H and V This works well in areas of smoothly varying luminance Fig 2a 16x16 intra prediction modes 9 b 8x8 for Chroma Mode 0 DC mean of upper and left hand samples H V Mode 1 horizontal extrapolation from left samples V Mode 2 vertical extrapolation from upper samples H Mode 3 Plane a linear plane function is fitted to the upper and left hand samples H and V This works well in areas of smoothly varying luminance b 4x4 for Luma H 264 Intra 4x4 prediction modes are shown in Fig 2b The intra 4x4 prediction has 9 directional modes as listed below Mode 0 Vertical Mode 1 Horizontal Mode 2 DC Mode 3 Diagonal down left Mode 4 Diagonal down right Mode 5 Vertical Right Mode 6 Horizontal down Mode 7 Vertical left Mode 8 Horizontal up Fig 2b Intra prediction 4x4 modes 2 A H they are previously coded pixels of the upper macro block and are available both at encoder decoder I L they are previously coded pixels of the left macro block and are available both at encoder decoder M this is previously coded pixel of the upper left macro block Inter Prediction in H 264 2 Inter prediction is the process of predicting a block of luma and chroma samples from a picture that has previously been coded and transmitted a reference picture This involves selecting a prediction region generating a prediction block and subtracting this from the original block of samples to form a residual that is then transformed coded and transmitted The block size can range from 16x16 to 4x4 luma and corresponding chroma samples as shown in Fig 4 and Fig 5 Fig 4 Macro block partitions 8x8 4x8 8x4 4x4 9 Fig 5 Macro block sub partitions 8x8 4x8 8x4 4x4 9 Rate Distortion Optimization 6 The H 264 AVC intra prediction is conducted for all types of blocks such as 4x4 luma blocks 16x16 luma blocks and 8x8 chroma blocks The residual between the current block and its prediction is then transformed quantized and entropy coded To obtain the best mode among these modes the H 264 AVC encoder performs the rate distortion optimization RDO technique for each macro block Set macro block parameters QP quantization parameter and Lagrangian multiplier MODE Calculate MODE 0 85x 2 QP 12 3 Then calculate cost which determines the best mode Cost D MODE x R D Distortion R Bit rate with given QP Distortion D is obtained by SSD sum of squared differences between the original macro block and its reconstructed block Bit rate R includes the number of bits for the mode information and transforms coefficients for macro block Considering the RDO procedure for intra mode selection in H 264 AVC the number of mode combinations in one macro block is N8x 16xN4 N16 N8 number of modes of an 8x8 chroma block N4 number of modes of an 4x4 luma block N16 number of modes of an 16x16 luma block Computing best mode for one macro block N8x 16xN4 N16 4 x 16 x 9 4 592 Thus to select the best mode for one Macro block in the intra prediction the H 264 AVC encoder carries out 592 RDO calculations As a result the complexity of the encoder increases extremely Goal This project uses the baseline profile as it provides simplicity in implementation The profiles supported by H 264 AVC are shown in Fig 6 Baseline profile Main profile High profile Extended profile Fig 6 the specific coding parts of the profiles in H 264 10 The important features are a I and P slice coding b Enhanced error resilience such as FMO Flexible macro block ordering and Arbitrary slice ordering ASO and redundant slices RS c Context adaptive variable length coding CAVLC Baseline profile is primarily used for low cost applications for data loss robustness The joint model JM 17 2 implementation of the H 264 encoder is used in this project 7 This project has implemented the complexity reduction algorithm for all the 3 block sizes 1 16x16 luma 2 4x4 luma 3 8x8 chroma The
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