How to keep fourier transform data when converting to uint8 image?

I'm making an image watermarking system based on dft.I add a small blqck and white logo in the fourier transform noise, then i reverse the fourier transform ifft2() to get spatial image and convert it to uint8() so that data can display image in imshow(). The problem is inserted data are disappered in fourier transform when i do fft2() to uint8 watermarked image.So how do i keep the watermark data in fourier transform as well as to display image in spatial domain.Thank you for your help.

Answer 1

[Walter Roberson]

1) you make larger changes to the fft so that you get larger changes when you ifft

2) do not just code the bits once. Use an error correcting code so that even though an individual position might be lost in the ifft/uint8 /fft cycle, the code helps you recover the bits

[hein thet]

I tried with smaller bigger watermark.The code looks similar to this, just looping throught fft matrix and add the watermark image bits.But I trying to export the watermarked image with imwrite(). https://www.mathworks.com/matlabcentral/fileexchange/48393-watermarking-based-on-fourier-transform

[Walter Roberson]

I know what you are trying to do, and in order to do it with uint8 you need to make bigger changes to the fft.

If you just want an approximate graphic to be retrieved then you might not need error correcting codes. If you want to store and retrieve data or you want the extracted image to look reasonable then you will start needing error correcting codes.

[hein thet]

Walter Roberson, Yes you are right! I made more changes to the fft and watermark appears in the noise.Here's my stupid logic is more value = more change so I multiply Logo image with a large number and image finally appear in the fft. Thank you very much.

fftimage(10:(10+LogoRows -1),5:5+LogoColumns-1)= Logo*2000;

[kinda magableh]

could you help me please,

how wecanl extract a watermwark that impeded in the image using FFT techbique?