Meaning of lines on x- and y-axis of a centered FFT-spectrum of an image
For an image containing an oriented 1D sine pattern looking like a corrugated sheet like the ones displayed, the 2D Fourier transform magnitude spectrum generally contains information along a line (aligned dots). One indeed may expect (at least) three dots: one in the center for the average or DC component (which could be zero if the mean is subtracted from the image), and two symmetric ones whose distance is in inverse proportionality with the frequency of the sine.
Since the image is limited in size (hence windowed) and the spectrum is computed using an FFT (which hypothetizes periodicity of the input), one may observe &quot;ripples&quot; (corresponding with a cardinal sine kernel in the frequency domain, dual to the 2D rectangular window) and border effects (with a centered cross).
You can find additional descriptions in Fourier Transform or
slides 8 to 14 of 2D Fourier transforms.