基于颜色复杂度和结构张量的恰可察觉失真模型

图像的恰可察觉失真(just noticeable distortion, JND)阈值是指人眼能够察觉的最小失真, 通常被用于去除图像/视频压缩中的视觉冗余. 针对 JND 模型对颜色和结构特征利用不充分的问题, 提出了一种基于颜色复杂度和结构张量的 JND 模型. 首先, 计算图像的颜色复杂度, 将其转换为与视觉敏感度相关的权值, 和对比掩蔽模型结合以提升模型的准确性; 然后, 利用结构张量对局部特征进行表示, 建立基于局部结构特征的调制因子, 估计结构不规则区域的视觉冗余程度; 最后, 将基于颜色复杂度的 JND 模型和基于结构张量的调制因子结合, 建立基于颜色复杂度和结构张量的 JND(complexity structure tensor based JND, CSJND)模型. 实验结果表明, 相比于已有的模型, 该模型在主观感知质量相同的前提下, 能使 PSNR 值明显降低; 该模型更加符合人眼的视觉特性, 能更准确地估计出 JND 阈值.

Spatial and seasonal variability of particulate organic carbon in the Wujiang River

Surface water samples were collected from seven cascade reservoirs across the mainstream Wujiang River and Hongfeng Reservoir on the tributary of Maotiao River, to measure the concentration of total suspended matter (TSM). Samples were analysed for elemental and stable carbon isotopic ($\delta ^{13}$C) composition and lignin phenols to investigate the variability in the spatial and seasonal distribution of particulate organic carbon (POC). The TSM in surface water varied greatly across the seasons. It was higher in the spring and summer than in autumn and winter. POC and lignin content ($\Sigma $8 and $\Lambda $8, respectively) in surface water showed high variability that were greatly influenced by season. POC in upstream waters was higher than in downstream waters in the autumn and winter, reflecting dam interception. $\Sigma $8 and $\Lambda $8 in the TSM samples exhibited a significant linear relationship, stronger in autumn and winter than in spring and summer. The $\delta ^{13}$C of TSM was significantly negative. This was potentially due to the more depleted $\delta ^{13}$C of terrestrial plant debris and freshwater plankton, that was likely to exist in surface water. Angiosperms were the dominant vegetation species reflected by lignin. There was a high diversity in the plant sources of terrestrial organic matter in surface particulate matter samples. The extent of degradation of TSM samples was relatively low, and aggravated degradation was observed in July and October, likely related to vigorous microbial activity.