Adding high frequency noise to images in matlab

Question

The function imnoise adds noise to the image. How can I add only high frequency noise?

Answer 1

High frequency noise in appears mainly as "salt and pepper" noise. So to introduce this kind of noise, pick N pixels at random and change their intensities randomly. The randomness implicitly does the job of a high pass filter, because it is unlikely that many neighbouring pixels will be picked among the N.

Answer 2

If you generate noise samples with a flat spectrum (white noise), it will contain all frequencies. Then you can filter that noise to shape it any way you want. For high frequency noise, you would want a high pass filter.