Chan-Won Seo

Efficient Bit Allocation and Rate Control Algorithms for Hierarchical Video Coding

Hierarchical structure is a useful tool for providing the necessary scalability in adapting to the variety of channel environments. For schemes involving hierarchical picture structures, bit allocation, and rate control algorithms are vital components for improving video codec performance. Since conventional bit allocation schemes do not efficiently consider the hierarchical structure characteristics, it is difficult to optimize the video quality at an arbitrary bitrate. Similarly, conventional quantization parameter decision methods are not appropriate for controlling the bitrate generated by a codec using a hierarchical encoding structure. In this paper, we propose an effective bit allocation scheme that assigns the target number of bits to pictures or macroblocks (MBs) and improves the overall quality of images encoded by a hierarchical-based encoder. A rate control scheme is also proposed to ensure that the generated bitrate is equal to the assigned target bitrate. From the simulation results, the proposed schemes outperformed conventional methods from a rate-distortion perspective, by efficiently controlling the bitrate of the MB unit. The algorithms regulated the generated bits to achieve the target bits by using the proposed linear R-Q model.

Rate Control for Consistent Objective Quality in High Efficiency Video Coding

Since video quality fluctuation degrades the visual perception significantly in multimedia communication systems, it is important to maintain a consistent objective quality over the entire video sequence. We propose a rate control algorithm to keep the consistent objective quality in high efficiency video coding (HEVC), which is an upcoming standard video codec. In the proposed algorithm, the probability density function of transformed coefficients is modeled based on a Laplacian function that considers the quadtree coding unit structure, which is one of the characteristics of HEVC. In controlling the video quality, distortion-quantization and rate-quantization models are derived by using the Laplacian function. Based on those models, a quantization parameter is determined to control the quality of the encoded frames where the fluctuation of video quality is minimized and the overflow and underflow of buffer are prevented. From the simulation results, it is shown that the proposed rate control algorithm outperforms the other conventional schemes.

Enhanced Adaptive Loop Filter for Motion Compensated Frame

We propose an adaptive loop filter to remove the redundancy between current and motion compensated frames so that the residual signal is minimized, thus coding efficiency increases. The loop filter coefficients and offset are optimized for each frame or a set of blocks to minimize the total energy of the residual signal resulting from motion estimation and compensation. The optimized loop filter with offset is applied for the set of blocks where the filtering process gives coding gain based upon rate-distortion cost. The proposed loop filter is used for the motion compensated frame whereas the conventional adaptive interpolation filter (AIF) is applied to the reference frames to interpolate the subpixel values. Another conventional scheme adaptive loop filter (ALF), is used after deblocking filtering to enhance quality of reconstructed frames, not to minimize energy of residual signal. The proposed loop filter can be used in combination with the AIF and ALF. Experimental results show that proposed algorithm provides the averaged bit reduction of 8% compared to conventional H.264/AVC scheme. When the proposed scheme is combined with AIF and ALF, the coding gain increases even further.

Rate Control Scheme for Consistent Video Quality in Scalable Video Codec

Multimedia data delivered to mobile devices over wireless channels or the Internet are complicated by bandwidth fluctuation and the variety of mobile devices. Scalable video coding has been developed as an extension of H.264/AVC to solve this problem. Since scalable video codec provides various scalabilities to adapt the bitstream for the channel conditions and terminal types, scalable codec is one of the useful codecs for wired or wireless multimedia communication systems, such as IPTV and streaming services. In such scalable multimedia communication systems, video quality fluctuation degrades the visual perception significantly. It is important to efficiently use the target bits in order to maintain a consistent video quality or achieve a small distortion variation throughout the whole video sequence. The scheme proposed in this paper provides a useful function to control video quality in applications supporting scalability, whereas conventional schemes have been proposed to control video quality in the H.264 and MPEG-4 systems. The proposed algorithm decides the quantization parameter of the enhancement layer to maintain a consistent video quality throughout the entire sequence. The video quality of the enhancement layer is controlled based on a closed-form formula which utilizes the residual data and quantization error of the base layer. The simulation results show that the proposed algorithm controls the frame quality of the enhancement layer in a simple operation, where the parameter decision algorithm is applied to each frame.

Motion Vector Coding Using Adaptive Motion Resolution

In most conventional video codecs, such as MPEG-2 and MPEG-4, inter coding is performed with the fixed motion vector resolution. When KTA software was developed, resolution for MVs can be selected in each slice. Although KTA codec uses a variety of resolutions for ME, the selected resolution is applied over the entire pixels in the slice and the statistical property of the local area is not considered. In this paper, we propose an adaptive decision scheme for motion vector resolution which depends on region, where MV search area is divided to multiple regions according to the distance from PMV. In each region, the assigned resolution is used to estimate MV. Each region supports different resolution for ME from other regions. The efficiency of the proposed scheme is affected from threshold values to divide the search area and the entropy coding method to encode the estimated MV. Simulation results with HM3.0 which is the reference software of HEVC show that the proposed scheme provides bit rate gains of 0.9%, 0.6%, and 2.9% in Random Access, Low Delay with B picture, and Low Delay with P picture structures, respectively.

Optimal spatial-temporal weight prediction for inter-frame coding of H.264/AVC video sequences

The paper proposes a novel method for optimal weight prediction in coding inter-frame of H.264/AVC compressed video sequences. The weights are optimized so that the motion compensated frames is closest to the current frame. This minimizes the residual frame and thus helps reducing the number of bits required to code the residual frame. We first consider the general optimal spatial-temporal weight prediction and then specify the special solution in case of temporal weight prediction using two reference frames (bi-predictive coding). Simulations on a wide range of video sequences are performed to verify the effectiveness of the algorithm on both visual quality and rate-distortion relation.