Jiazhong Chen

Distance-based weighted prediction for H.264 intra coding

Intra 4times4 is one of the two primary intra prediction types supported in H.264/AVC. For intra 4times4 prediction, eight modes are to capture orientations of images with DC mode to reflect flat regions. Therefore the smoothly-varying regions which are very common in ordinary images can not be well decorrelated. In this paper, to improve decorrelation performance of intra 4times4 prediction, we propose a distance-based weighted prediction (DWP) method to take the place of DC mode. In DWP, weighted prediction is performed with the weighting coefficients inversely relative to the distances between current pixel and its reference pixels. We also present an integral DWP (iDWP) method which is of much lower computational complexity than DWP. Experimental results demonstrate that DWP and iDWP yield average PSNR enhancements of 0.12 dB and 0.1 dB over current H.264 using 4times4 DC prediction respectively.

Image segmentation based on edge detection using K-means and an improved ant colony optimization

On the base of edge detection using the K-means algorithm and an improved ant colony optimization (ACO), a novel image segmentation algorithm is proposed. The proposed method can enhance advantages and avoid disadvantages of edge-based and clustering methods by embedding the clustering in edge detection, combining them in a novel way. Since the clustering centers are determined roughly using K-means in ACO, the proposed algorithm can address the problem of slow convergence of the traditional ant colony (AC) algorithm and reduces its complexity. Experiments are divided into two stages to test the results of edge detection and segmentation respectively. It is shown that the proposed algorithm based on superior edge detection achieves better performance compared to the typical image segmentation methods.

An improved hybrid fast mode decision method for H.264/AVC intra coding with local information

In this paper, an improved hybrid fast mode decision method for H.264/AVC intra coding is proposed, which is based on the analysis of edge filter and more efficient mode selection. In contrast to the conventional fast mode decision methods where multiple neighbor modes are determined by the same filter, a unique filter is applied to each directional mode to eliminate the correlation among neighbor modes. To reduce the complexity, the filters are only applied to the selected pixels that cover most of the block region. After that, the results along prediction residuals of these pixels are summed for fast mode decision. Moreover, local information is also taken into account through Most Probable Mode (MPM) from neighbor-coded blocks. Experimental results demonstrated that better coding performance is achieved as compared with well-known conventional fast mode decision algorithms for H.264/AVC intra coding.

The training of Karhunen---Loève transform matrix and its application for H.264 intra coding

In H.264/AVC, 4 × 4 discrete cosine transform (DCT) is performed on the residual signals after intra prediction for decorrelation. Actually, residual blocks with different prediction modes exhibit different frequency characteristics. Therefore, the fixed transform matrix cannot match the energetic distribution of residual signals very well, which degrades the decorrelation performance. Fortunately, the energetic distributions of residual blocks with the same mode are relatively coincident, which makes it possible to train a universally good Karhunen–Loève transform (KLT) matrix for each mode. In this paper, an optimal frequency matching (OFM) algorithm is proposed to train KLT matrices for residual blocks and nine KLT matrices corresponding to nine prediction modes of 4 × 4 intra blocks are trained. Experimental results show that KLT with trained matrices yields a persistent gain over H.264 using 4 × 4 DCT with an average peak signal-to-noise ratio (PSNR) enhancement of 0.22dB and a maximum enhancement of 0.33dB.

A new hybrid fast mode decision method for H.264/AVC intra coding

A new hybrid fast mode decision method for H.264/AVC intra coding is proposed in this paper. In the proposed algorithm, edge based filters are applied to some samples distributed in the block and each directional mode is related to a filtering result. After that, the absolute sum of Hadamard Transformed coefficients of the filtering results is utilized to help selecting the candidate prediction modes. In addition, local information is employed by using Most Probable Mode (MPM) from neighboring coded blocks which can significantly mitigate the increase of coding bits. Experimental results demonstrate that the proposed method is able to provide a better coding performance than existing algorithms.

Image Dimming Perceptual Model Based Pixel Compensation and Backlight Adjustment

In this paper, an image dimming perceptual model is proposed as the foundation of image compensation and quality evaluation. Specifically, under this model an ideal compensation function is derived out and objective quality metrics SSIM and PSNR are adopted to evaluate the compensation performance. A practical compensation function is obtained by modifying the ideal compensation function under the consideration both of visual perception and objective quality. By using the SSIM and PSNR metrics, the relationship between image mean values, backlight levels, and image quality is further investigated. Base on this a novel scheme of backlight level adaptive adjustment is presented. Numerous experimental results verify the effectiveness of the model and demonstrate the advantages of the compensation and backlight level adaptive adjustment techniques.

A Lifting Scheme of Symmetric-antisymmetric Multiwavelet Transform for Image Coding

A symmetric-antisymmetric (SA) multiwavelet lifting factorization of matrix filter banks is presented as an extension of Sweldens’ traditional scalar wavelet lifting scheme. A prefilter absorbed (PA) multiwavelet lifting factorization is also presented to reduce the redundant computations in prefiltering. Then an algorithm of multiwavelet lifting is designed and used for signal decomposition. The experimental results show this lifting scheme can achieve lower complexity while preserve high quality for image coding.

A Very Low Bit Rate Video Coding Combined with Fast Adaptive Block Size Motion Estimation and Nonuniform Scalar Quantization Multiwavelet Transform

We describe a very low bit rate video coding framework in which motion correlation between successive video frames is exploited in the multiwavelet transform domain. Some complicated techniques, such as spatial prediction in intra coding, adaptive block size motion estimation, more than one previous frames for prediction in inter frames, and content adaptive binary arithmetic coding (CABAC) are used in H.26L standard. The testing results show that H.26L can greatly outperform MPEG-4 ASP in both PSNR and visual quality. However, the encoding of H.26L costs too much time for it is complex to use fast motion search in adaptive block size motion estimation, and CABAC needs much time to generate the code list for entropy coding. Whereas, only four types of symbol are generated after zero tree wavelet coding so that the entropy coding can cost less time than CABAC. Moreover, if we select 8× 8 sized block as a basic mode, which can be united into the large size mode if neighbored 8× 8 sized blocks have same reference frame and motion vector, then the fast motion estimation can be feasible. Accordingly, a fast motion search algorithm, multiwavelet transform, and a novel adaptive quantization schemer are applied to the proposed coding frame. Experimental results reveal 0.2–0.5 dB increase in coded PSNR at low bit rates over the state-of-the-art H.26L recommendation, and similar improvements over MPEG-4 at high bit rates, with a considerable improvement in subjective reconstruction quality, while simultaneously supporting a scalable representation.

A hybrid M-channel filter bank and DCT framework for H.264/AVC intra coding

In H.264/AVC, discrete cosine transform (DCT) is performed on the residual blocks after prediction. However, the mismatch between variable block sizes and the fixed transform matrix not only degrades decorrelation performance but also causes severe blocky artifacts inside the blocks. In previous work, M-channel filter bank system (MCFBS) was proposed to overcome these defects. However, the increased percentage of encoding time by using MCFBS is very high, especially for intra coding. More seriously, the constructed M-channel filter bank with floating-point coefficients is an obstacle to hardware implementation. In this work, a hybrid M-channel Filter bank and DCT (HMD) framework is proposed for intra coding. Besides, the integer transform of a newly constructed M-channel filter bank is also implemented for HMD. Experimental results demonstrate that HMD can reduce 64–69% of the complexity of MCFBS with negligible quality degradation.

Integer M-channel Filter Bank Design for H.264/AVC Intra Coding

In the previous work, M-channel filter bank systems(MCFBS) were proposed to overcome the mismatch between the fixed transform matrix and variable transform block sizes in H.264/AVC intra coding. However, the constructed filter banks for MCFBS are with irrational coefficients, which bring great obstacles to implement integer transform. In this work, anew class of M-channel filter banks is firstly constructed and then several integer M-channel filter banks (IMFB) derived from this class are proposed for MCFBS. Experimental results show that IMFB can save about 61% of encoding time per frame, with nearly negligible loss of peak signal-to-noise(PSNR) value.