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Studies On The Postprocessing Algorithm For Coding Effects Reduction In Image And Video Coding

Posted on:2008-12-10Degree:DoctorType:Dissertation
Country:ChinaCandidate:Y H ZhaoFull Text:PDF
GTID:1118360215476786Subject:Information and Communication Engineering
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Image and video compression technology could greatly reduce the image/video storage volume and the transmission bandwidth, thus boost the widely application of image and video. However, the compression technology also introduces coding artifacts, such as the blocking artifacts, the ringing artifacts and the mosquito artifacts etc. These artifacts to some extent destroy the spatial smoothness of the original content, and degrade both the objective and the subjective quality of the reconstructed image/video. In order to alleviate the coding artifacts and improve the quality of the reconstructed image/video, it is essential to combine post-processing with the decoding process of image/video. This dissertation focuses on several algorithms reducing the coding artifacts caused by the conventional"DCT + Quantization"in the image/video compression procedure. Firstly, the deblocking algorithm in the DCT domain; secondly the deblocking algorithm in the spatial domain; then the deringring algorithm and lastly the postprocessing to reduce the coding artifacts in video. The originality innovations of this dissertation includes:Progressive DCT domain blocking artifacts reduction (PDCT-BAR). This algorithm reduces the blocking artifacts by constraining coefficients in the shifted DCT blocks according to frequency characteristic parameters of the decoded DCT blocks. The undesired high frequency components, mainly caused by the blocking artifacts, are constrained by progressively constructing the shifted DCT blocks and adaptively shrinking the high frequency coefficients in the shifted blocks. Then the DCT coefficients are clipped by the quantization constraint set. Comparing with other deblocking algorithms related with shifted blocks, the algorithms in this dissertation constructed shifted blocks with different shifting position in each shifting. Experiments show the PDCT-BAR algorithm could both effectively reduce the blocking artifacts and highly preserve the details in the original image by take the advantage of different shifted blocks. The post-processed images give satisfying PSNR and visual quality.DCT domain adaptive filtering of blockiness artifacts reduction (DCT-AFBAR). First, four groups of filter coefficients matrix for DCT domain deblocking are deduced. Then frequency characteristic parameters are extracted to instruct the selection of the filter coefficients matrix. The selected filter coefficients matrix multiplied with the DCT coefficients and the blocking artifacts is alleviated in the filtered DCT blocks. One advantage of this algorithm is the reduction of heavy computation burden of calculating the shifted blocks. Beside, in order to further reduce the computation complexity, partial process to the DCT block is also tested in this dissertation, and the result is also acceptable. Experiments show: the DCT-AFBAR algorithm outperforms the similar algorithms both in objective/subjective quality and computation complexity.Spatial adaptive filtering of blockiness artifacts reduction (SAFBAR). SAFBAR is the deduction and improvement of DCT-AFBAR, the DCT domain filtering is transformed into spatial filtering. SAFBAR extracts frequency characteristic parameters to instruct the selection of filter coefficients. To avoid blurring of the real edge, this algorithm also combined the gradient information of several block boundaries to distinguish the blockiness and the real edge. MSAFBAR is a modified SAFBAF algorithm, the filter coefficient is simplified. So that the floating point calculation is converted into integer calculation. Furthermore, to avoid the DCT domain calculation of frequency characteristic parameters, approximate transform or gradient variation information are used to approximate the frequency characteristic parameters. Experiments show: MSAFBAR outperforms other algorithms in PSNR under a wide range of bitrates. A ringing artifacts reduction algorithm is proposed, which combines gray level transform and low-pass filtering. This algorithm selects candidate blocks by the frequency characteristic parameters. The parameters for block level, includs one weighting value and two trageted gray level values. The gray level transform approximates the grey level values of pixels to the targeted value; and the low pass filtering gives another smoothed output. Then weighting the output of gray level transform and the low-pass filtering to get the result without ring artifacts. To avoid excessive processing, the result is limited to give the untimate result. Experiments show: the proposed algorithm is especially effective in PNSR and visual quality of the decoded images.This dissertation also propose a scheme of post-processing with decoder to alleviate the coding artifact in the decoded video bitstream. This scheme utilize some informations of decoding process to calculate the frequency characteristic parameters for different frame types, then the parameters are utilized to instruct the deblocking and deringring. The proposed algorithm is satisfying for I, B and P frame types as compared to like algorithms.
Keywords/Search Tags:image coding, video coding, post-processing, DCT transform, quantization blocking artifacts reduction, ringing artifact reduction, MPEG, JPEG
PDF Full Text Request
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