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此篇论文是基于 JM 的 H264 的亚像素运动估计的快速算法。 该论文级别较高,属于 SCI级别,值得反复研究分析; 该论文里的算法思想相对比较新颖,虽然是基于 JM,但完全可以应用到 x264、openh264 等开源编码器中来加速亚像素的运动估计模块; 从结论上可以看到,差不多无质量损失情况下,减少接近 50%的亚像素搜索点,也就相当于减少一般的亚像素编码耗时,进一步减少整体的编码时间。
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1
A Fast Sub-pixel Motion Estimation Algorithm for
H.264/AVC Video Coding
Weiyao Lin
1
, Krit Panusopone
2
, David M. Baylon
2
, Ming-Ting Sun
3
, Zhenzhong Chen
4
and Hongxiang Li
5
1
Institute of Image Communication and Information Processing, Dept. of Electronic Engineering,
Shanghai Jiao Tong University, Shanghai 200240, China
2
Advanced Technology Department, Mobile Devices & Home, Motorola, Inc.,
San Diego, CA 92121, USA
3
Dept. of Electrical Engineering, University of Washington, Seattle, WA 98195, USA
4
School of Electrical and Electronic Engineering, Nanyang Technological University, Singapore
5
Dept. of Electrical and Computer Engineering, North Dakota State University, Fargo, ND USA
Abstract
Motion Estimation (ME) is one of the most time-consuming parts in video coding. The use of
multiple partition sizes in H.264/AVC makes it even more complicated when compared to ME in
conventional video coding standards. It is important to develop fast and effective sub-pixel ME
algorithms since (a) The computation overhead by sub-pixel ME has become relatively significant
while the complexity of integer-pixel search has been greatly reduced by fast algorithms, and (b)
Reducing sub-pixel search points can greatly save the computation for sub-pixel interpolation. In this
paper, a novel fast sub-pixel ME algorithm is proposed which performs a ‘rough’ sub-pixel search
before the partition selection, and performs a ‘precise’ sub-pixel search for the best partition. By
reducing the searching load for the large number of non-best partitions, the computation complexity
for sub-pixel search can be greatly decreased. Experimental results show that our method can reduce
the sub-pixel search points by more than 50% compared to existing fast sub-pixel ME methods with
negligible quality degradation.
I. Introduction
H.264/AVC is the state-of-the-art video coding standard established by ITU-T and ISO/IEC.
2
H.264/AVC uses many new techniques and is able to save more than 50% in bitrate while having
similar video quality compared to the MPEG-2 video coding standard [1].
Motion Estimation (ME) is one of the most time-consuming parts in video coding. Developing
fast algorithms for ME to reduce computational complexity in video coding has been an important and
challenging problem. In the H.264/AVC Joint Model (JM) [5], the ME process contains two stages:
integer pixel search over a large area and sub-pixel search around the best selected integer pixel. Since
H.264/AVC uses 7 partition sizes for inter-frame prediction (16×16, 16×8, 8×16, 8×8, 8×4, 4×8 and
4×4), the complexity of multi-partition ME is high [2]. It is becoming more critical to develop fast and
effective sub-pixel ME algorithms for H.264/AVC. Firstly, the computation overhead by sub-pixel
ME has become relatively significant while the complexity of integer-pixel search has been greatly
reduced by fast algorithms. For example, there have been integer-pixel ME algorithms [4
,
10, 16] that
only need between 3 and 5 integer search points to calculate the final integer Motion Vector (MV).
The computation in the 16-point sub-pixel search method used in the JM thus becomes comparatively
large. Secondly, typical sub-pixel searches require interpolating sub-pixel values for computing the
Sum of Absolute Difference (SAD). Reducing sub-pixel search points can also reduce the
interpolation computation time.
In this paper, a novel sub-pixel ME algorithm is proposed for H.264/AVC, which performs a
‘rough’ sub-pixel search before the partition selection, and performs a ‘precise’ sub-pixel search for
the best partition. By reducing the searching load for the large number of non-best partitions, the
computation complexity for sub-pixel search can be greatly decreased. Experimental results show that
the proposed algorithm can significantly reduce the number of sub-pixel search points compared to
other fast sub-pixel ME algorithms [6-9], with negligible quality degradation.
The rest of this paper is organized as follows. Section II reviews existing research on sub-pixel
ME. Section III provides in-depth analysis on how to further reduce the search points for sub-pixel
ME for multiple partitions. The proposed algorithm is described in Section IV. Section V shows the
experimental results and Section VI concludes the paper.
3
II. Related Work
Chen et al. [6] analyzed the difference between the integer-pixel matching error surface and the
sub-pixel matching error surface. According to Chen’s analysis, the integer-pixel matching error
surface is far from a unimodal surface inside the searching window due to the complexity of the video
content. The assumption of unimodal will easily result in trapping in a local minimum. However, for
the sub-pixel matching error surface, the unimodal surface assumption holds in most cases because of
the smaller search range of sub-pixel ME as well as the high correlation between sub-pixels due to the
sub-pixel interpolation.
There has been much research on fast sub-pixel ME [6-9, 17]. Most of these methods are based
on the unimodal surface assumption and perform the sub-pixel search in two steps:
1) Predict a sub-pixel MV (SPMV), and
2) Perform a small area search around the SPMV to obtain the final sub-pixel MV.
The method to get the sub-pixel predicted MV can be summarized in two ways: using
spatiotemporal information and modeling the Sum of Absolute Difference (SAD) surface.
Chen et al. [6] and Yang et al. [8] used spatiotemporal information to get the SPMVs. In [6], a
Center Biased Fractional Pixel Search (CBFPS) fast sub-pixel ME method is studied, where the MVs
of neighboring MBs were used to get the SPMV as in Eqn (1),
β
)%_( MVmvpredSPMV −=
(1)
where pred_mv is the MV prediction of the current partition (in sub-pixel resolution), MV is the best
integer-pixel MV of the current partition (β=4 in the 1/4-pixel case and β=8 in the 1/8-pixel case) and
% represents the modulo operation. In [8], a larger partition MV (e.g., 16x8 inter-mode MV takes a
16x16 MV as a reference) or previous frame MV was used to get the SPMV. If combined with the
SPMV from CBFPS, the accuracy of the SPMV can be greatly increased.
A more popular way to get the SPMV is to use a function (in most cases a second-order function)
to model the SAD surface [7, 9]. If the matching errors of the best integer-pixel MV and its
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