基于DCT和小波的图像和视频编码性能的比较研究

在比较编码性能时,许多人分析了图像和视频编码的离散余弦变换(DCT)和小波变换性能差别,普遍认为图像的小波编码在压缩比和编码质量方面优于传统的DCT变换编码.说明了研究图像和视频压缩算法应当从全面的系统观点来考虑,即把重点放在各要素之间的量化、熵编码和编码系统中的复杂的相互影响的研究上.     

方块DCT硬件算法结构研究及其ISE实现

在阐述DCT（离散余弦变换）在图像／视频压缩中变换编码算法原理的基础上，重点研究了方决DCT（8×8点二维DCT）易于VHDL实现的适合的硬件算法结构，并在Xilinx公司的ISE7．1i平台上实现了算法设计与功能仿真。     

几种常见编码方法的比较

通过对DFT／DCT、子带、小波变换、分形编码的特征分析，得出如下一些结论：DFT与DCT理论成熟，前者因存在复数计算，使得硬件结构较后者复杂，但DCT变换也因分块数限制，在还原图像时可能存在块效应；具有多分辨率分析的小波变换编码，虽然是目前变换编码中较为高级的一种，但也存在选择小波基函数的困难，子带编码具有并行处理和分层传输之优点，但镜像滤波器组的设计难以达到理论要求；根据事物间普遍存在的相似性建立起的分形编码，以其高压缩比换取图像的极端相似性，失真表现为整个屏幕的随机现象。     

基于DCT域的任意分辨率视频可伸缩编码

空间分辨率可伸缩编码能够满足网络视频对不同码率的要求以及不同用户显示终端的需求,成为近几年研究热点.由此提出一种基于DCT域准卷积的任意分辨率下采样方法,该方法通过分析子块与宏块DCT系数之间的关系,实现了由子块DCT系数预测宏块DCT系数,通过低通滤波器截取预测的宏块低频DCT系数,实现任意分辨率下采样.实验证明:该方法适合诸如H.264、AVS等基于DCT变换的视频编码标准,且提供灵活的空间可伸缩能力.     

一种基于小波的多光谱图像压缩方法

文章提出一种基于Karhunen2Loeve变换(KLT)和小波量化编码的多光谱图像压缩方法。该法首先使用KL变换步骤来去除谱间冗余,而后对各变换波段图像进行小波变换,并利用均匀阈值网格编码量化方法来量化小波子带图像,最后使用算术编码对量化结果进行熵编码。为使编码器能为所有谱段各子带获取率- 失真意义上最优的量化阈值,本文提出基于子带图像统计特性和网格编码量化器率- 失真特性的比特分配算法。实验表明,本方法能高效地压缩多光谱图像,表现出优异的压缩性能。     

变换编码中直流偏移的快速处理

根据变换编辑码的持点,对变换编码中直流偏移处理提出一种快速算法。该算法通过交流常规的直流偏移和变换的顺序,可大大减少直流偏移的运算次数,余弦变换编码器的每个编码块的直流偏移运算次数降低至一次,小波变换变换块后的真流运算降低至分析增益为1的子带大小。例如对8×的DCT块直流偏移次数只需1次,对采用MALLAT方式的L层分解的二维图像,直流偏移次数只有原来的1/4L.     

基于子带谱间变换的多光谱图像压缩

针对星载多光谱图像压缩,提出了基于子带谱间变换的压缩算法。该算法首先对多光谱图像序列的每个波段分别进行空间二维小波变换,以此去除多光谱图像的空间相关性;为了去除多光谱图像的谱间相关性,将小波分解后的每一层子带作为整体,采用串行成对变换的方式对两个波段进行子带谱间KLT变换;最后,利用最优截断的嵌入式块编码算法对变换后的所有主成分同时进行最优率失真压缩。实验结果表明,该算法能够获得较好的压缩性能,同时具有较低的编码复杂度,适用于星载多光谱图像的压缩。     

Gabor小波变换与图像分块的人脸识别算法研究

以二维Gabor小波变换提取人脸图像特征作为全局特征,对图像进行不等分块,人脸图像区域所在块加大权重,并提取每个子块的特征作为局部特征,对全局特征结合局部特征采用DCT进行降维处理,用支持向量机分类模型进行人脸识别。实验表明:与较同等分块的识别算法相比,该算法可提高人脸识别率。     

离散余弦变换编码的现状与发展研究

1974年由Ahmed和Rao提出的离散余弦变换,至今已有30年历史。此间,DCT编码已发展成为JPEG、MPEG、H.26x等图像/视频编码标准中的核心。尽管Shapiro的EZW以及Said等人的SPIHT小波编码的成功应用,对传统的DCT编码提出了挑战,但Xiong等人利用嵌入式DCT块变换之间的直流相关性,以及对DCT后的系数进行策略性重组或层式DCT同样具有小波多分辨率图像的分解特性。此外,基于层次嵌入式DCT、形状自适应DCT、截短DCT、感兴趣区域支撑DCT以及形态DCT等改进形式的编码,都是将基于DCT变换编码推向更高层次。就DCT改进的变换,以及DCT系数的应用,如利用DCT系数实现信息隐藏等,也使得基于常规的DCT变换编码有了更广阔的应用与发展空间,文章就此作了广泛的比较和深入的研究,并指出了发展方向。     

一种基才DWT-DCT变换强鲁棒性的数字水印算法

提出一种基于DWT-DCT的新的数字图像水印算法。该算法充分利用离散小波变换多分辨率特性和离散余弦变换能量压缩特性．综合两种变换的优点。在嵌入时,先对宿主图像进行DWT变换,得到4个子带。选择HL子带进行DCT变换．然后将加密后的水印嵌入DCT变换后得到子带中频系数。为获得较强的安全性,在嵌入前先对水印进行混沌加密。试验结果表明基于DWT-DCT联合的水印算法具有较强的不可见性,对常见的攻击具有较强的鲁棒性。     

Edge-based motion compensated classified DCT with quadtree for image sequence coding

The motion compensated discrete cosine transform coding (MCDCT) is an efficient image sequence coding technique. In order to further reduce the bit-rate for the quantizied DCT coefficients and keep the visual quality, we propose an adaptive edge-based quadtree motion compensated discrete cosine transform coding (EQDCT). In our proposed algorithm, the overhead moving information is encoded by a quadtree structure and the nonedge blocks will be encoded at lower bit-rate but the edge blocks will be encoded at higher bit-rate. The edge blocks will be further classified into four different classes according to the orientations and locations of the edges. Each class of edge blocks selects the different set of the DCT coefficients to be encoded. By this method, we can just preserve and encode a few DCT coefficients, but still maintain the visual quality of the images. In the proposed EQDCT image sequence coding scheme, the average bit-rate of each frame is reduced to 0.072 bit/pixel and the average PSNR value is 32.11 dB.     

Prioritized DCT for compression and progressive transmission ofimages

An approach is based on the block discrete cosine transform (DCT). The novelty of this approach is that the transform coefficients of all image blocks are coded and transmitted in absolute magnitude order. The resulting ordered-by-magnitude transmission is accomplished without sacrificing coding efficiency by using partition priority coding. Coding and transmission are adaptive to the characteristics of each individual image. and therefore, very efficient. Another advantage of this approach is its high progression effectiveness. Since the largest transform coefficients that capture the most important characteristics of images are coded and transmitted first, this method is well suited for progressive image transmission. Further compression of the image-data is achieved by multiple distribution entropy coding, a technique based on arithmetic coding. Experiments show that the approach compares favorably with previously reported DCT and subband image codecs.     

基于DCT变换的快速分形编码方法

本文提出了一种基于DCT变换加速相似块匹配的分形编码方法.通过图像块与相似块在DCT变换域少数低频系数的比较,来确定匹配的相似块.并根据DCT变换的性质,方便的确定对比度因子和亮度偏移量.并采用将图像块与相似块的DCT变换系数预制成查找表的方法减少重复运算.结果表明,编码时间大幅度减少,而PSNR只是略有下降.     

Video coding scheme using DCT-pyramid vector quantization

A new and effective video coding scheme for contribution quality is proposed. The CMTT/2, a joint committee of CCIR and CCITT, has proposed a video coding scheme (already approved at European level by ETS) working at 34-45 Mbit/s. Basically this proposal includes a DCT transform for spatial correlation removal and motion compensation for temporal correlation removal. The individual transform coefficients are then scalar quantized with a non uniform bit assignment. Starting from the CMTT/2 proposal, the study presents a new video coding scheme designed using a vector quantizer solution instead of the scalar one. Specifically, the pyramid vector quantization (PVQ) has been chosen as the vector quantization method as it is able to reduce the DCT coefficients Laplacian distribution. Simulation results show that the proposed video coding scheme gives the same contribution quality at 22 Mbit/s as the one obtained with the CMTT/2 proposal at 45 Mbit/s.     

A Motion Picture Coding Algorithm Using Adaptive DCT Encoding Based on Coefficient Power Distribution Classification

A motion picture coding algorithm using motion-compensated interframe prediction and the adaptive discrete cosine transform (DCT) encoding technique is proposed. High coding efficiency is obtained by the adaptive DCT encoding technique in which encoding parameters are fitted to widely varying characteristics of the interframe differential signal. Segmented DCT subblocks of interframe prediction error are classified into categories based on their coefficient power distribution characteristics. The adaptation gain results from using a suitable variable word length code set designated by the above classification for encoding each quantization index of DCT coefficients. In addition, a new coding parameter control method is introduced based on the information rate estimation of the current frame. This classification promotes high stability because good estimation accuracy of bits consumption for each DCT subblock is obtained by utilizing the category indexes. Simulation results show that the proposed algorithm has enough coding efficiency to transmit videoconferencing motion pictures through a 384 kbit/s channel.     

WHT-based composite motion compensated NTSC interframe directcoding

The motion compensated interframe differential pulse code modulation (DPCM) and discrete cosine transform (DCT) hybrid (MC DCT) coding was nominated as a standard scheme for component TV signals by ISO and ITU-R. However, in cases where an NTSC composite TV signal is used such as the United States and Japan, applying the MC DCT scheme with its luminance/chrominance separating and composing process causes unavoidable quality degradation. The reason for this additional process required for MC DCT is that a composite TV signal presents a “color subcarrier phase shift problem” in which the color subcarrier phase varies between a coding block and reference block according to the motion vector. In this paper, we propose a Walsh Hadamard transform (WHT)-based composite motion compensated NTSC interframe direct coding scheme. In this scheme, phase shifts of a color subcarrier and modulated chrominance components between a coding block and reference block can be effectively compensated by a simple process of coefficient permutation and polarity changes of several pairs of WHT coefficients to which 100% of the subcarrier energy and most of the modulated chrominance component's energy are packed. In the motion compensated DCT scheme, however, the energy of the color subcarrier and modulated chrominance components are spread over too many coefficients and a pair-based coefficient handling rule is not given to solve this problem. This paper demonstrates that the proposed scheme provides higher coding performance for a composite NTSC signal than does the motion compensated DCT scheme with its luminance/chrominance separating and composing process     

一种基于联合信源信道自适应的不等错误保护方法

针对误码信道和信源编码的统计特性,提出了一种能有效提高基于DCT变换的视频编码码流的抗误码性能的自适应不等错误保护方法.它基于信道误码的反馈信息,选择DCT的量化参数q和信道编码码率,并进行DCT重排和VLC编码,再对数据流进行不等差错保护.通过实验证明,它在高误码信道中性能优于EEP等.     

Inverse pyramidal decomposition with multiple DCT

This paper presents a method for pyramidal image decomposition called “inverse” because of the order followed to obtain the pyramid levels: from top to bottom, in correspondence with the requirement for “progressive” image transmission. The pyramid top (level zero) consists in selecting the low-frequency coefficients of the discrete cosine image transform. The following pyramid levels are made up of low-frequency discrete cosine transform (DCT) coefficients of the subimages obtained from quadtree division at each level. The quadtree root coincides with the pyramid top. The first level is the difference between the image and its approximation obtained by inverse DCT. The following (second) level is a difference too, between the previous (first) level and its approximation obtained with inverse DCT for every subimage in the first level, etc. The paper describes the principle of image decomposition, the possibilities for recursive calculation, its basic characteristics and modifications. The block diagram and the generalised scheme of the decomposition are given and some results of its modelling show the application capacities in image coding systems.     

Improvements of Transform Coding Algorithm for Motion-Compensated Interframe Prediction Errors-DCT/SQ Coding

A new hybrid coding method for transmitting videoconferencing images at a bit rate 384 kbits/s is proposed. Considering the characteristics of motion-compensated interframe prediction errors for typical videoconferencing scenes, a filter is introduced to separate pulsive components on which conventional discrete cosine transform (DCT) coding method does not work well. These separated pulsive components are coded by using scalar quantization (SQ). The remainder are DCT coded. For DCT coefficients, an adaptive coding method based on the classification of DCT coefficients is applied in order to improve the coding performance. Since the proposed method employs both DCT coding and SQ of prediction errors, it is named "DCT/SQ coding method." Experimental results show that the DCT/SQ coding method is effective to reduce so-called mosquito effects, and thus it can improve the quality of decoded images.