   FFT and frequency domain
  Hello everyone,I have implemented a FFT, I have high, low, band pass filters, and I have to implement a filter, which will denoise the picture (if it has some noise, for example like horizontal, vertical or diagonal stripes or spots). I have tried to use a threshold and "erase" wrong artifacts but it doesn't work quite good :(. Do you some ideas? Thnx a lot.   
  You get "ringing" artifacts, right?  This happens whenever you aggressively threshold in the frequency domain.  If you make the transition smoother (not just a sharp cutoff) you'll get better results.If you have a good noise model, then there is a least-squares-optimal filter called the Wiener filter that you can use.One issue, though, is that mean-squared-error isn't actually a very good metric for how good something looks.  So often more heuristic things (like median filters) end up looking better (these are good for "salt and pepper" type noise).   