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An Efficient Mode Decision Algorithm for H.264/AVC Encoding Optimization

IEEE TRANSACTION ON MULTIMEDIAHanli Wang, Student Member, IEEE, Sam Kwong, Senior Member, IEEE, and Chi-Wah Kok, Senior Member, IEEE

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Outline

Introduction Proposed Early Termination of Motion

Estimation and Mode Decision Proposed Algorithm for H.264 Mode

Decision Experimental Results Conclusion

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Introduction

The computational complexity in H.264 is increased due to mode decision where variable block size ME is employed

In this paper, we propose a more efficient and robust algorithm than *Variable Block size Best Motion Detection (VBBMD) to early terminate ME and mode decision.

*L.Yang, K. Yu, J. Li, and S. Li, “An effective variable block-size early termination algorithm for H.264 video coding,” IEEE Trans. Circuits Syst. Video Technol., vol 15, no. 6, pp. 784-788, Jun. 2005.

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Proposed Early Termination of Motion Estimation and Mode Decision

In video coding, it’s common that all DCT coefficients in a block are quantized to zeros.

ME can be early terminated if an all-zero block is detected.

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Proposed Early Termination of Motion Estimation and Mode Decision

4x4 DCT

C(n)= , for n=0, C(n)=1, otherwise denotes to round x to the nearest

integer Quantization

f is

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A. Sufficient Condition to Detect All-Zero Blocks

From(2), the sufficient condition for to be quantized to zero is

From(1)

indicates each element of is

multiplied by the element in the same position in matrix

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A. Sufficient Condition to Detect All-Zero Blocks

The case of u=v=1 is firstly analyzed. A 4x4 block is divided into four regions ,

The sum of absolute difference , for each region

The sum of absolute difference SAD for a 4x4 block

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A. Sufficient Condition to Detect All-Zero Blocks

Considering (4)-(7)

Form(3)(4), the sufficient condition to detect zero quantized DCT coefficient at (u,v) is . Thus

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A. Sufficient Condition to Detect All-Zero Blocks

According to the three categories in Table I, three threshold are derived as

If , then all the DCT coefficients in the ith category will be quantized to zeros.

The sufficient condition to detect all-zero block

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A. Sufficient Condition to Detect All-Zero Blocks

False rejection rate (FRR) The smaller FRR is, the more efficiency all-zero DCT

blocks can be detected. False acceptance rate (FAR)

The smaller FAR is, the less the video quality degrades.

[10] Y.H. Moon, G. Y. Kim, and J. H. Kim, “An improved early detection algorithm for all-zero blocks in H.264 video coding,” IEEE Trans. Circuits Syst. Video Technol., vol. 15, no. 8, pp. 1053-1057, Aug. 2005.

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B. Early Termination of Motion Estimation and Mode Decision

Early Motion Estimation Termination We can early terminate ME if all the 4x4 blocks

within current block are determined as all-zero blocks under the condition(13).

In the assumption on spatial-temporal homogeneity, we may not get the best motion vector…

Early Mode Decision Termination If EMET is true for all the partitioned blocks in a

MB, we can consider this MB is an all-zero MB and skip checking the other modes.

The additional overhead of calculation (10)–(13)… =>we can early stop checking EMET if one of the

4x4 blocks does not satisfy the condition in (13).

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Proposed Algorithm for H.264 Mode Decision

We have executed a number of experiments and observed that 1. SKIP and 16x16 have a large proportion

in MB partitions. 2. 8x8 occupies a large proportion in sub-

MB partitions. 3. INTRA4 and INTRA16 have a very small

proportion in inter coding. M16 : the best mode of SKIP and 16x16

: RD cost of M16 : RD cost of 8x8

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A. Temporal-Spatial Checking

M(n,x,y) : a MB which is in the nth frame with top left pixel in (x,y)

The temporal set is defined as

The spatial set is defined as

If( nonboundary MBs in and were encoded as SKIP or 16x16)

=> skip 8x4, 4x8, 4x4, INTRA4 and INTRA16 If( )=> skip 16x8 and 8x16

Additional computation : few comparison operations

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B. Thresholds Based Prediction

We define two thresholds and If( ) => skip 8x4, 4x8, 4x4, INTRA4, and

INTRA16 If( ) => skip 16x8 and 8x16 From our exhaustive experiments, we

found good results can be achieved when and are set to 50 and 200 respectively.

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C. Monotonic Error Surface Based Prediction

: RD cost of 4x4 If( ) => skip 16x8 and 8x16 If( ) => only check 16x8 and 8x16 and

skip all the other unchecked modes

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D. Overall Algorithm

EMDT C16

EMDT C8

False, record C8

TSC and C16<C8

false

TSC

Done

Done

Done

True

True

True

Set 8x4,4x8,4x4,I4, and I16 to inactive

True

C8-C16>Th16

false

True

C16-C8>Th8

false

Set 16x8 and 8x16 to inactivefalse

True

Record C4 and EMDT C4

false

C16>C8 and C8>C4

falseDone

True

C16<C8 and C8<C4

false

16x8 inactive

Set 8x4,4x8,I4, and I16 to inactive

True

false

EMDT 16x8

false

8x16 inactive

falseDone

True

EMDT 8x16

false

8x4 inactive

falseDone

True

Set 16x8 and 8x16 to inactive

True

True

True

True false

Set all modes active. Select M16 and record C16

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D. Overall Algorithm

EMDT 8x4

false

Done

4x8 inactive

false

True

EMDT 4x8

false

Done

True

Check I4 and I16 if they are active. Select the best mode

false

True

True

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Experimental Results

H.264 reference software : JM9.5 ME with ¼ pixel resolution Number of reference frames : 1 Motion search range : 16 Without B-frames The period of I-frames : 10 Five values : 24, 28, 32, 36, and 40

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Experimental Results

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Experimental Results

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Experimental Results

With B-frames The period of I-frames : 4 GOP structure is IBBP The other encoding parameters are the

same.

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Experimental Results

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Experimental Results

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Conclusion

A theoretical analysis is carried out to study the sufficient condition to detect all-zero blocks in H.264.

The proposed algorithm also introduces TSC, TBP, and MESBP methods to further skip examining redundant modes.

In the future, we will consider other fast mode decision techniques such as [3],[4],[7] to further optimize the proposed algorithm.