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FAST BLOCK SIZE AND MODE DECISION ALGORITHM

FOR INTRA PREDICTION IN H.264/AVC

IEEE Transactions on Consumer Electronics, Vol. 58, No. 2, May 2012

Kyungmin Lim, Seongwan Kim, Jaeho Lee, Daehyun Pak and Sangyoun Lee, Member,

IEEE

報告者：劉冠宇

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Outline Introduction Overview Method

A. Block Size Decision using Inner-Block Variance B. MD-based Intra Mode Skip C. Candidate Prediction Modes Reduction in Intra4x4

Experimental Results Conclusion Q & A

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Introduction(1/2)

There are various techniques to improve the performance Spatial Intra Prediction Improved Inter Prediction Motion Estimation Multiple Reference Frame Context-Based Adaptive Binary Arithmetic Coding(CABAC) Rate Distortion Optimization (RDO) Most Probable Mode (MPM) The sum of absolute hadamard transformed differences (SATD)

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Introduction(2/2)

RDO technique compares all possible combinations of modes to find the minimum rate distortion cost.

MPM is selected as the best mode which provides effective mode coding by addressing the spatial correlation of neighboring blocks.

SATD is calculated for each of four chroma prediction modes. The mode which has the minimum SATD is set as the best mode.

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Overview(1/6)

In H.264 intra prediction, intra4x4 and intra16x16 prediction are provided for luma, and 8x8 prediction is supported for chroma.

There are nine prediction modes for intra 4x4 prediction that use reference pixels which are block boundary pixels such as X, A-L.

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Overview(2/6)

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Overview(3/6)

Intra16x16 has three directional prediction modes and one DC mode.

Chroma prediction also has four prediction modes similar to intra16x16.

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Overview(4/6)

The total combination with RDO computation in the MB will be Chr8x8 (16 Intra4x4 + Intra16x16) = 592 RDO ⅹ ⅹcomputations for every MB and identifies the mode with the minimum value as the best prediction mode. [1]

This full search process requires high computational cost.

[1] F. Pan, X. Lin, S. Rahardja, K. P. Lim, Z. G. Li, D. Wu, and S. Wu, “Fast mode decision algorithm for intraprediction in H.264/AVC video coding,” IEEETrans.Circuits Syst. Video Technol., vol. 15, no. 7, pp. 813–822, Jul. 2005.

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Overview(5/6)

Most Probable Mode (MPM) which exploits the correlations between intra modes of neighboring 4x4 blocks.

If the block A and B are coded as intra 4x4 as depicted, the MPM of the current coding block C is set as the minimum prediction modes of A and B.

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Overview(6/6)

Otherwise, MPM is set to DC mode. For each 4x4 block, one flag bit is used for signaling MPM. If the flag is signaled as “1”, the best mode is selected to be MPM. If the flag is “0”, a different mode is set as the best mode.

Therefore, if MPM is selected as the best mode, only one flag bit is used to represent the mode. Otherwise, three additional bits are needed. [2]

[2] I. E. G. Richardson, H.264/MPEG-4 Part 10: Intra Prediction White Paper 2002.

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Method A. Block Size Decision using Inner-Block Variance

B. MD-based Intra Mode Skip

C. Candidate Prediction Modes Reduction in Intra4x4

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Method.A(1/5)

The intra4x4 prediction which is predicted based on each 4x4 block is well suited for coding parts of significant details(complex). On the other hand, the intra16x16 which is predicted for the whole 16x16 block is well suited for smooth regions.

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Method.A(2/5)

In many cases, they found that intra16x16 is selected as the best mode when the border of the MB lies on an edge of the homogeneous region.

We assume that it is well guaranteed whether intra16x16 can be selected as the best block size or not, by observing the homogeneity of the inner-8x8 sub-block.

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Method.A(3/5)

The variance of the inner-8x8 sub-block defined is used as the measurement of texture homogeneity.

Where Y (i, j) is the luminance value of the pixel at (i, j).

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Method.A(4/5) A simulation result of the cumulative hit ratio of

intra16x16 and intra4x4 corresponding to the variance using different QPs.

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Method.A(5/5)

The resulted threshold formula is as follows 456 2

If the sub Var is higher than Th , the examination of intra16x16 block is skipped.

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Method A. Block Size Decision using Inner-Block Variance

B. MD-based Intra Mode Skip

C. Candidate Prediction Modes Reduction in Intra4x4

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Method.B(1/3)

In intra4x4 prediction, we use the mean deviations (MD) to measure the similarities of boundary pixels. It can be obtained as follows

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Where is the upper block boundary pixel, is the left block boundary pixel, and m is the average of the boundary block pixels.

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Method.B(2/3)

The threshold Th , which is depending on QP, is defined as 1

we propose that MPM is the best mode when the is lower than .

RDO selects more intra4x4 blocks with MPM than any other modes. It always needs only one bit for mode coding.

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Method.B(3/3)

This algorithm also can be applied to intra16x16. In this case, DC mode is selected as the best mode when the MD of intra16x16 block boundary pixels is lower than the threshold Th , which is defined as

0

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Method A. Block Size Decision using Inner-Block Variance

B. MD-based Intra Mode Skip

C. Candidate Prediction Modes Reduction in Intra4x4

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Method.C(1/3) As described above, the MD-based intra mode skip can

effectively reduce RDO calculations. But if the MD of block boundary pixels is higher than Th , nine prediction modes will still be calculated using the RDO technique for intra4x4.

If the MD in the upper pixels, which are denoted as pixels A, B, C, and D, is small enough, then prediction mode 0, 3, 4, 5, and 7 will produce similar prediction blocks. The MD of upper pixels is defined as follows

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where is the upper block boundary pixel.

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Method.C(2/3)

Therefore, when the upper pixels are similar to each other, we only search mode 0 instead of adjacent five modes (mode 0, 3, 4, 5, 7). In the same manner, when the left pixels are similar to each other, mode 1 is searched and mode 4, 6, and 8 are skipped..

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Method.C(3/3)

If the MD value of upper and left is lower than Th , we skip the adjacent directional modes which produce similar prediction block. Below shows the skipped modes in both cases.

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Experimental Results Algorithm in [4]

MPM DC Mode skip

Algorithm in [7]

[4] F. Pan, X. Lin, S. Rahardja, K. P. Lim, Z. G. Li, D. Wu, and S. Wu, “Fast mode decision algorithm for intraprediction in H.264/AVC video coding,” IEEETrans.Circuits Syst. Video Technol., vol. 15, no. 7, pp. 813–822, Jul. 2005.

[7] A.C. Tsai, A. Paul, J.C. Wang, and J.F. Wang, “Intensity gradient technique for efficient intra-prediction in H.264/AVC,” IEEE Transactions on Circuits and Systems for Video Technology, vol.18, no.5, pp.694-698, May 2008.

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Experimental Results

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Conclusions This paper presents a fast block size and prediction

mode decision methods for intra prediction. The basic idea which uses the variance of the inner-8x8

sub-block is motivated by the observation. The future work for this research is that effectively

extends our algorithm to the new video coding scheme known as high-efficiency video coding (HEVC).

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Q & A