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An Efficient Mode Decision Algorithm for

H.264/AVC Encoding Optimization

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

Senior Member, IEEE

Outline

- Introduction
- Proposed Early Termination of Motion Estimation

and Mode Decision - Proposed Algorithm for H.264 Mode Decision
- Experimental Results
- Conclusion

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.

Proposed Early Termination of Motion Estimation

and Mode Decision

- In video coding, its common that all DCT

coefficients in a block are quantized to zeros. - ME can be early terminated if an all-zero block

is detected.

Proposed Early Termination of Motion Estimation

and Mode Decision

- 4x4 DCT
- C(n) , for n0, C(n)1, otherwise
- denotes to round x to the nearest integer
- Quantization
- f is

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

A. Sufficient Condition to Detect All-Zero Blocks

- The case of uv1 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

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

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

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.

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)
- gtwe can early stop checking EMET if one of

the 4x4 blocks does not satisfy the condition in

(13).

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

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) - gt skip 8x4, 4x8, 4x4, INTRA4 and INTRA16
- If( )
- gt skip 16x8 and 8x16
- Additional computation few comparison

operations

B. Thresholds Based Prediction

- We define two thresholds and
- If( )
- gt skip 8x4, 4x8, 4x4, INTRA4, and INTRA16
- If( )
- gt skip 16x8 and 8x16
- From our exhaustive experiments, we found good

results can be achieved when and are

set to 50 and 200 respectively.

C. Monotonic Error Surface Based Prediction

- RD cost of 4x4
- If( )
- gt skip 16x8 and 8x16
- If( )
- gt only check 16x8 and 8x16 and skip all the

other unchecked modes

D. Overall Algorithm

Set all modes active. Select M16 and record C16

16x8 inactive

TSC

Record C4 and EMDT C4

True

false

false

True

True

false

EMDT 16x8

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

Done

True

True

EMDT C16

C16gtC8 and C8gtC4

false

Done

Done

8x16 inactive

False, record C8

True

True

false

True

false

C8-C16gtTh16

EMDT C8

C16ltC8 and C8ltC4

false

Done

EMDT 8x16

True

false

True

false

True

false

Done

C16-C8gtTh8

false

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

TSC and C16ltC8

True

Done

8x4 inactive

Set 16x8 and 8x16 to inactive

True

Set 16x8 and 8x16 to inactive

false

True

false

D. Overall Algorithm

false

True

EMDT 8x4

True

Done

false

4x8 inactive

True

false

EMDT 4x8

True

Done

false

Check I4 and I16 if they are active. Select the

best mode

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

Experimental Results

Experimental Results

Experimental Results

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

Experimental Results

Experimental Results

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.