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Reversible digital watermarking method based on histogram modification

2022-07-28 06:16:00 【zzuls】

0. I think the histogram of the image , It reflects the distribution of pixel values of the image , It tells you that the pixel value of this image is 0-255 How many pixels are there . The abscissa represents the pixel value , The ordinate represents the number of pixels corresponding to the pixel value in the image .ps： The pixel value of gray-scale image can also be said to be gray-scale value .

（ Image source reference watermark ）

1. Early foundation ： You need to find the zero point of the histogram ( The corresponding pixel value is z） And peak point ( The corresponding pixel value is p). This zero point can be simply understood as the histogram ordinate 0 The point of , If there is no zero , Choose the minimum value instead of zero , Also record 0 The value corresponding to the point . The peak point is the point with the most pixels in the histogram , The reason for finding the peak value is that the secret message needs to be embedded into the pixel value p Of pixels , It can maximize the watermark capacity .

Embedding watermark steps （ Suppose there is ）：

（1） First, calculate the histogram of the image , And find the zero point , That is, pixels without any gray value in the image are recorded as z; Then find the peak value of the gray value with the most pixels in the histogram , Write it down as p. For the convenience of narration , Hypothetical hypothesis p<z.

ps: The peak point may also be on the right of zero , But the actual steps of embedding watermark are similar

（2） From top to bottom 、 Scan each pixel in the image from left to right , The gray value of each pixel is $v_{ij}$ Express , When $v_{ij}$ >z or $v_{ij}$ <p when , The value of the pixel remains unchanged ; otherwise , The gray value of the pixel plus 1: $v_{ij}=v_{ij}+1$

（3） Pixels in the image whose gray value is equal to the peak point , Is the point where secret information can be embedded , Convert secret information into binary streams , use $s_{k}$ Express . Embed information sequentially have to $v_{ij}'=v_{ij}+s_{k}$ .

ps: How do you understand this , If p=128, There are... In the image 3 The gray value of points is 128 Respectively p1,p2,p3. If I want to embed watermark information as 101, Then I can make p1 = p1+1=129、p2 = p2+0 = 128、p3 = p3+1 = 129;

（4） The obtained image composed of gray value is the image after embedding secret information . meanwhile p、z Save in the form of a key .

Steps of extracting watermark information ：

（1） Read key , obtain p、z Value ;

（2） Progressive image , When $v_{ij}=p$ when , It indicates that this point is a hidden information point , Extract information 0 And keep the gray value of this point unchanged ; When $v_{ij}=p+1$ when , This point is also a hidden information point , Extract information 1 And reduce the pixel value 1.

ps: Why? $v_{ij}=p$ and $v_{ij}=p+1$ Time is the point of hiding information ？ Because in the original image $v_{ij}=p+1$ All points plus one $v_{ij}=v_{ij}+1$ , therefore $v_{ij}=p+1$ It must be a secret message 1 Only some

（3） When $v_{ij}>z$ or $v_{ij}<p$ when , The value of the pixel remains unchanged ; When $p+1<v_{ij}<=z$ when , The gray value of the pixel is reduced 1.