如何在频域中添加水印？

Question

我是 Matlab 的新手，我有一个作业要求使用 DCT 变换为图像加水印：

Read Lin.jpg color image and apply DCT.

Threshold the logo.jpg (watermark) into binary and ten times of its strength, and then add it to the coefficient of transformed Lin image.

这是两张图片：

我有三个问题：

我应该把 Lin.jpg 分成 8x8 块和 logo.jpg 分成 2x2 块还是没必要？
“强度的十倍”是什么意思？这只是乘以 10 吗？
如何得到变换后的Lin.jpg图像的系数？

这是我尝试过的：

img = imread('Lin.jpg');
wImg = imread('njit_logo.jpg');

wImgBinary = imbinarize(wImg) * 10;
[rows, cols] = size(img(:,:,1));
[Wrows, Wcols] = size(wImgBinary);

% make the watermark image as large as the original
watermark = zeros(size(img), 'uint8');
for column = 1:cols
    for row = 1:rows
        watermark(row, column) = wImgBinary(mod(row,Wrows)+1, mod(column,Wcols)+1);
    end
end
watermark = watermark(1:rows, 1:cols);

% apply dct and add with watermark at each channel
for i = 1:3
    imgDct = dct2(img(:,:,i));
    C = imgDct + double(watermark);
    Iw(:,:,i) = round(real(idct2(C)));
end

IIw = uint8(Iw);
figure, imshow(IIw), title('watermarked image');

Answer 1

通过组合离散余弦变换分量对图像加水印

在这种情况下，我发现在此示例中使用 30 的 Watermark_Strength（强度因子）会显示更突出的结果。添加水印的流水线如下：

• 使用 padarray() 函数对水印图像进行零填充以匹配要添加水印的图像的大小，或者放大图像。
• 将图像和水印图像拆分为它们的RGB channels/components。
• 使用dct2() 函数对所有颜色通道进行离散余弦变换(DCT)。
• 将带水印图像的离散余弦变换(DCT) 分量乘以强度因子。
• 基于颜色通道添加相应的离散余弦变换(DCT) 组件。
• 使用idct2() 函数对3 个合成的离散余弦变换(DCT) 分量取反。
• 组合逆向分量以在空间域中创建带水印的图像。

Image = imread('Lin.jpg');
Watermark = imread('njit_logo.jpg');

%Grabbing the image and watermark dimensions%
[Image_Height,Image_Width,~] = size(Image);
[Watermark_Height,Watermark_Width,~] = size(Watermark);

%Padding the watermark to match the size of the image to be watermarked%
Side_Padding = (Image_Width - Watermark_Width)/2;
Top_And_Bottom_Padding = (Image_Height - Watermark_Height)/2;
Watermark_Padded = padarray(Watermark,[Top_And_Bottom_Padding Side_Padding],0,'both');

%Binary image of watermark%
Watermark_Binary = imbinarize(Watermark_Padded);

%Converting the watermark image to frequency domain using DCT%
Watermark_Strength = 30;
Red_Channel_Watermark_DCT = Watermark_Strength*dct2(Watermark_Binary(:,:,1));
Blue_Channel_Watermark_DCT = Watermark_Strength*dct2(Watermark_Binary(:,:,2));
Green_Channel_Watermark_DCT = Watermark_Strength*dct2(Watermark_Binary(:,:,3));

%Converting the image to frequency domain using DCT%
Red_Channel_Image_DCT = dct2(Image(:,:,1));
Blue_Channel_Image_DCT = dct2(Image(:,:,2));
Green_Channel_Image_DCT = dct2(Image(:,:,3));

%Adding the frequency components together%
Combined_Red_Channel = Red_Channel_Watermark_DCT + Red_Channel_Image_DCT;
Combined_Blue_Channel = Blue_Channel_Watermark_DCT + Blue_Channel_Image_DCT;
Combined_Green_Channel = Green_Channel_Watermark_DCT + Green_Channel_Image_DCT;

%Inversing the combined frequency domain image%
Combined_Image(:,:,1) = idct2(real(Combined_Red_Channel));
Combined_Image(:,:,2) = idct2(real(Combined_Blue_Channel));
Combined_Image(:,:,3) = idct2(real(Combined_Green_Channel));

%Displaying combined image%
Combined_Image = uint8(Combined_Image);
imshow(Combined_Image);

运行使用 MATLAB R2019b

如何在频域中添加水印？

How to do watermarking in the Frequency Domain?

matlab

watermark

dct

通过组合离散余弦变换分量对图像加水印