IEEE Trans Image Process. 2013 Dec;22(12):4613-26. doi: 10.1109/TIP.2013.2274386. Epub 2013 Jul 23.
Block transform coded images usually suffer from annoying artifacts at low bit rates, caused by the coarse quantization of transform coefficients. In this paper, we propose a new method to reduce compression artifacts by the overlapped-block transform coefficient estimation from non-local blocks. In the proposed method, the discrete cosine transform coefficients of each block are estimated by adaptively fusing two prediction values based on their reliabilities. One prediction is the quantized values of coefficients decoded from the compressed bitstream, whose reliability is determined by quantization steps. The other prediction is the weighted average of the coefficients in nonlocal blocks, whose reliability depends on the variance of the coefficients in these blocks. The weights are used to distinguish the effectiveness of the coefficients in nonlocal blocks to predict original coefficients and are determined by block similarity in transform domain. To solve the optimization problem, the overlapped blocks are divided into several subsets. Each subset contains nonoverlapped blocks covering the whole image and is optimized independently. Therefore, the overall optimization is reduced to a set of sub-optimization problems, which can be easily solved. Finally, we provide a strategy for parameter selection based on the compression levels. Experimental results show that the proposed method can remarkably reduce compression artifacts and significantly improve both the subjective and objective qualities of block transform coded images.
块变换编码图像通常在低比特率下会受到令人讨厌的伪像的影响，这是由于变换系数的粗量化造成的。在本文中，我们提出了一种新的方法，通过从非局部块中估计重叠块变换系数来减少压缩伪像。在提出的方法中，通过自适应融合基于可靠性的两个预测值来估计每个块的离散余弦变换系数。一个预测值是从压缩比特流中解码的系数的量化值，其可靠性由量化步长决定。另一个预测值是来自非局部块的系数的加权平均值，其可靠性取决于这些块中系数的方差。权重用于区分非局部块中系数预测原始系数的有效性，并由变换域中的块相似性决定。为了解决优化问题，将重叠块分为几个子集。每个子集包含覆盖整个图像的非重叠块，并独立进行优化。因此，整体优化可以简化为一组子优化问题，这些问题可以很容易地解决。最后，我们提供了一种基于压缩级别选择参数的策略。实验结果表明，所提出的方法可以显著减少压缩伪像，并显著提高块变换编码图像的主观和客观质量。
