An Early Block Type Decision Method for Intra Prediction in H.264/AVC Jungho Do, Sangkwon Na and Chong-Min Kyung VLSI Systems Lab. Korea Advanced Institute of Science and Technology (KAIST)

Table of contents Introduction Intra prediction Early block type decision – Block type decision point – Rate-distortion cost Experimental results Conclusion Reference 2SiPS 2009

Introduction Two prediction methods in H.264/AVC – Inter prediction Using temporal correlation between the earlier frame and the current frame – Intra prediction Using spatial correlation between neighboring blocks in the current frame 9 prediction modes 3SiPS 2009

Introduction Why do we focus on Intra prediction (IP)? – Rate-distortion optimization (RDO) in Intra prediction also improves an overall rate-distortion performance. – More block types and prediction modes in Intra prediction than inter prediction Constraint: 560 cycles for (133MHz) Total: 640 cycles 4SiPS 2009

Introduction Why do we focus on Intra prediction (IP)? – Rate-distortion optimization (RDO) in Intra prediction also improves an overall rate-distortion performance. – More block types and prediction modes in Intra prediction than inter prediction Constraint: 560 cycles for (133MHz) Total: 640 cycles 5SiPS 2009 IPDCTQQ -1 IDCT Reco n. IPDCTQQ -1 IDCT Reco n. IPDCTQQ -1 IDCT Reco n. Pipeline for one prediction mode

Introduction Rate-distortion cost function for RDO [9] M.G. Sarwer and L.M. Po, “Bit rate estimation for cost function of 4x4 intra mode decision of h.264/avc,” ICME, 2007, pp – SiPS 2009

Intra prediction Nine prediction modes – Associated with the direction of prediction 7SiPS 2009

Early block type decision Motivation – Spatial correlation between sub-blocks and macroblock (MB) The prediction mode of neighboring blocks have the similar direction. – We assume that R-D cost of neighboring blocks also have the similar cost based on the above hypothesis. R-D cost of macroblock can be substituted by R-D cost of sub-blocks for mode decision. 8SiPS 2009

Encoding order Conventional encoding order – 8x8 blocks are performed after all 4x4 blocks are performed. 9 4x4 C8x8 4x44x44x44x44x44x44x44x44x44x44x44x44x44x44x4 8x8 16x16 16 SiPS 2009

Encoding order Proposed encoding order – To use spatial correlation, the prediction of 8x8 block is performed after the predictions of the corresponding four 4x4 blocks are finished. Three possible decision points 10SiPS 2009

1 st block type culling 16x16 block type – Probability of 16x16 block to be decided as the best mode when 4x4 block is selected by proposed early block type decision – 16x16 block type is bypassed when 4x4 block type is selected. 11 QP Probability(%) SiPS 2009

2 nd block type culling Block type decision at 1/4MB point – If J RD,4x4(1/4) < J RD,8x8(1/4) Prediction of only 4x4 block type is performed evaluated after 1/4MB. – Otherwise 8x8 block type is taken x16 block SiPS x4 8x8

2 nd block type culling Block type decision at 1/4MB point – If J RD,4x4(1/4) < J RD,8x8(1/4) Prediction of only 4x4 block type is performed evaluated after 1/4MB. – Otherwise 8x8 block type is taken x16 block SiPS x4 8x8

2 nd block type culling Block type decision at 2/4MB and 3/4MB 14SiPS x4 8x8 4x4 8x8

Block type decision point Which block type decision point is the best? – In terms of R-D performance and the number of reconstruction loops according to the positions of early block type decision method. 15 1/4MB2/4MB3/4MB R-D performance ∆ PSNR (dB) ∆ BR (%) Cycle counts of reconstruction loops 4x4 block type x8 block type SiPS 2009

Block type decision point Which block type decision point is the best? – In terms of R-D performance and the number of reconstruction loops according to the positions of early block type decision method. 16SiPS 2009 Constraint

Block type decision point Proposed early block type decision – 2/4 MB point is effective in terms of distortion, bit- rate and computational complexity (the number of cycles for MB process). 17 SiPS 2009 Constraint

R-D cost function Original cost function 18SiPS 2009

R-D cost function Proposed R-D cost for sub-block – Assumption: the best mode of chroma block is decided independently regardless of luma mode. 19SiPS 2009 The best mode of luma block is decided independently. The difference of R CBP between 4x4 block type and 8x8 block type is frequently very small. The value of R MB is fixed as ‘intra’ for all block types.

R CBP Coded_block_pattern (CBP) variable 20 Coded_block_pattern indicates which 8x8 blocks(6) in the MB contain nonzero coefficients 8 LumaChroma SiPS 2009

R CBP Comparison of R CBP 21SiPS 2009

R-D cost function Simplified R-D cost function – Proposed R-D cost function enables evaluation of R header to be performed at the early decision point (2/4MB) because R Header can be derived from R L mode. 22SiPS 2009

Experimental results R-D performance comparison 23SiPS 2009 Full search Proposed

Conclusion Computational complexity problem in RDO- enabled Intra prediction An early decision at 2/4MB point for RDO- enabled Intra prediction – Block type decision based on spatial correlation – R-D cost computation is reduced by 90.1% with 0.93% bit-rate increase and dB PSNR decrease compared to full search. 24SiPS 2009

Thank you SiPS

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Appendix SiPS

Introduction H.264/AVC coding flow 28 Entropy Coding Scaling & Inv. Transform Motion- Compensation Control Data Quant. Transf. coeffs Motion Data Intra/Inter Coder Control Decoder Motion Estimation Transform/ Scal./Quant. - Input Video Signal Split into Macroblocks 16x16 pixels Intra-frame Prediction De-blocking Filter Output Video Signal SiPS 2009

Introduction Why we focus on Intra prediction – Computational bottleneck in rate-distortion (R-D) optimized encoder due to many block types and prediction modes Constraint: 560 cycles for (133MHz) Total: 640 cycles (= 15* * ) – One 4x4 block: 15 cycles – One 8x8 block: 60 cycles – One 16x16 block: 160 cycles 29SiPS 2009

Early block type decision Flow 30 SiPS 2009