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一种保持细节的人脸照片瑕疵去除算法

Pores-Preserving Face Cleaning Based on Improved Empirical Mode Decomposition

  • 摘要: 人脸照片在日常生活中使用频率高,对人们的生活具有重要影响。由于先天或者后天原因,人们脸部经常出现一些瑕疵如斑点,凹坑等,对肖像照片的视觉效果造成不利的影响。
    在传统的图像处理中大多采用去噪算法去除脸部瑕疵。由于去噪算法未对脸部细节如毛孔和瑕疵进行区分,脸部毛孔和细节都被认为是图像噪声而被去除。这类方法获得的结果往往使人脸过于光滑,产生橡皮或者塑料般的感觉,结果不够真实。另一类去除瑕疵的方法是使用Adobe Photoshop交互地选择源区域和目标区域,这种方法强调两种区域的无缝融合,并不保证将原有区域的细节保持下来。而且当瑕疵较多时需要大量的交互工作。此外,这种方法需要用户具有一定的技巧和经验。
    本文提出一种基于经验模态分解的脸部瑕疵去除方法。首先,在对图像进行经验模态分解后,我们对每一层的每个像素抽取其归一化的局部能量。我们发现,脸部细节如毛孔和瑕疵对应像素的归一化局部能量具有明显不同的特点即随着分解层数的增加,对应毛孔的像素其归一化能量越来越小,而对应瑕疵的像素的归一化能量则越来越大。基于此,我们对脸部图像上每一点建立定量描述即不理想程度。通过实验发现我们定义的不理想程度反映了人们对该像素的直观感受,不理想程度高的区域往往对应此瑕疵部分。反之,不理想程度低的区域则大多对应一般的脸部区域。进一步地,我们根据不理想程度对像素周围的领域进行不同程度的滤波,瑕疵区域滤波程度较大,而毛孔区域滤波尽量小,从而达到去除瑕疵保留细节的目的。
    本文的创新点在于:
    (1)我们对瑕疵区域和细节区域提出了一个统一的表示框架。我们的算法不需要明确的检测瑕疵的区域。相反,我们根据归一化的局部能量所定义的不理想程度能定量的刻画人们对这个区域的不满意程度。利用此,我们将不理想程度同滤波程度关联起来。本文的方法不仅能有效地去除各种瑕疵,而且能保持脸部的细节不被模糊。此外,我们还引入了几个高层次的,直观的参数便于用户控制增强的程度。
    (2)针对传统经验模态分解容易出现的灰度斑现象,提出了一种基于自适应均值和限邻域的经验模态分解算法。该算法从两个方面即局部均值的选取和分解临域的选取两个方面改进了传统的算法。通过自适应的获取局部均值,可以有效地避免传统插值算法的过冲和欠冲。通过限制分解临域,可以使算法避免为了达到局部最优而影响临近区域。改进后的算法能有效地去除传统经验模态分解中极易出现的灰度斑现象。

     

    Abstract: In this paper, we propose a novel method of cleaning up facial imperfections such as bumps and blemishes that may detract from a pleasing digital portrait. Contrasting with traditional methods which tend to blur facial details, our method fully retains fine scale skin textures (pores etc.) of the subject. Our key idea is to find a quantity, namely normalized local energy, to capture different characteristics of fine scale details and distractions, based on empirical mode decomposition, and then build a quantitative measurement of facial skin appearance which characterizes both imperfections and facial details in a unified framework. Finally, we use the quantitative measurement as a guide to enhance facial skin. We also introduce a few high-level, intuitive parameters for controlling the amount of enhancement. In addition, an adaptive local mean and neighborhood limited empirical mode decomposition algorithm is also developed to improve in two respects the performance of empirical mode decomposition. It can effectively avoid the gray spots effect commonly associated with traditional empirical mode decomposition when dealing with high-nonstationary images.

     

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