If we have a 1-dimensional wave in time domain, it can be represented in frequency domain with x axis indicating the frequency of the wave and y axis indicating amplitude/magnitude of the wave. But what about a 2-dimensional wave in case of images in Frequency Domain? Because in 2-dimensional wave we have a cordinate (x,y) to denote the magnitude. So what is the frequency in this case?
If you're working with a 2 dimensional data set, you need to do a 2D FFT on it (which can be dome using many 1D FFTs: doing all the rows and then all the columns, or if you prefer, the columns and then the rows).
The 2D FFT gives you a 2D array of complex numbers. Horizontal waves will give peaks on the horizontal axis. Vertical ones will give peaks on the vertical axis, and diagonal waves will give peaks away from both axes.