SSIM指标是一种测量两幅图像结构相似度的算法，是一种图像质量评价方法。SSIM指标可以看作图像质量的一种测量方法。这个版本是之前2004年6月发表在IEEE TRANSACTIONS on IMAGE PROCESSING上Image
Quality Assessment: From Error Visibility to Structural Similarity,文中方法的改进，效果要好一些。
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