可逆信息隐藏(Reversible data hiding,RDH)是一种能将信息隐藏在数字媒体中,并且能够保证提取出信息之后能无损恢复数字媒体的技术。
数字媒体包括:
1.音频
2.图片
3.视频
可逆信息隐藏具体可分为几种类型:
1.Lossless compression[2]
2.Difference expansion(DE)[3]
3.Histogram shifting(HS)[4]
4.Prediction-error expansion(PEE)[5]-[9]
由于本文创新算法属于PEE算法,下面介绍一些PEE算法的研究情况。
PEE算法可以分两个步骤:
对图像作出准确的预测,改进目标是让预测更准确。
修改获得的预测误差直方图(prediction error histogram),以此嵌入秘密信息。
两个改进方向以下会介绍。
本文的创新在Prediction stage中。
多使用临近像素来进行预测
方法有:
Difference predictor(DP)[3]
Median edge direction predictor(MEDP)[5]
Rhombus predictor(RP)[6]
Gradient adaptive predictor(GAP)[7]
Partial differential equation predictor(PDEP)[8]
Multi-predictor[9]
传统方法,由于locality和linearity的限制(limitations),减小预测误差(prediction-error)的能力有瓶颈。
预测误差:像素的预测值与实际值之间的误差(原文:The difference between the predicted value and the to-be-predicted value)。
于是,研究者在深度学习崛起,图像分类、目标识别等领域取得重大突破的背景下,改用CNN,力图实现预测性能的提升。
研究近况:
Luo等人[10]:
提出了一个基于CNN的立体图像可逆信息隐藏方法。(Proposed a stereo image RDH method based on the CNN.)
Hu等人[11]:
提出了一个应用于灰度图像的CNN预测器。(Proposed a new CNN-based predictor(CNNP) for the grayscale images.)
上作的作者们还提出,由于CNN有多个感受域[12],还能够进行全局优化,它在信息隐藏中还有更大的施展空间。(The authors claimed that since the CNN has the abilities of multi receptive fields[12] and global optimization, it can make more room for data hiding.)
Hu等人[13]:
改善了[11]中的图像划分策略,提出了一个性能更好的CNN预测器(CNN-based predictor,CNNP)。
重复一下这个阶段的工作,那就是研究新方法来修改获得的预测误差直方图。(The researchers focus on designing some new rules to modify the obtained prediction error histogram.)
1.生成有效的直方图映射关系(Construct efficient reversible histogram mappping)
一维映射方法[4]-[9](其中[6]的阶段方法被本文采用,以此结合成完整的RDH算法)
高维映射方法[14]-[16]
2.改善、发明新效的嵌入策略(embedding strategy)[17]
原理:
新的嵌入策略会选择嵌入效率更高(high embedding efficiency)的像素
1.采用更多临近像素来预测待测像素的预处理
在把图像放入CNN网络之前,先基于每个像素临近的8个像素生成预处理图片,这样的图片放入CNN预测器中会提升预测效果。
2.新的CNN预测器
设计了一个更好的CNN网络来预测前面预处理过的图片。经由这个网络,生成的图片预测准确率更高。
结合经典的PEE嵌入策略[6],本文的工作构成了一个完整的RDH算法。
实验取得了满意的结果,最终生成的载密图片的视觉质量,比起现有的RDH算法,得到了提升。
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