FFT (DFT) of 2D Images

Question

Branching off from the question [Mapping the value of a sample in a 2-D DFT to cycles/pixel], I have a few more questions related to 2D FFT of images.

• I correct in assuming that the fundamental frequency is the image itself?
• And one copy of the image is equivalent to 1 wave cycle?
• The sampling frequency is the number of pixels in the image?
• And If this is correct, how do I compare two distinct images?
• Can I compare the sampling frequencies ( $\frac{1}{N_1}$ & $\frac{1}{N_2}$ ) and frequency peaks of one image with a second image ($\frac{1}{N_2}$)? ($\frac{1}{N_1}$ & $\frac{1}{N_2}$ are the dimensions of an image)

What I am trying to understand is - what the X & Y axis of a complex 2D FFT plot is? and how to compare information from 2 plots?

Answer 1

Consider a 1D wave of 10sec duration.

I correct in assuming that the fundamental frequency is the image itself? Does the 10 sec sample equal the fundamental freq? No. same for images.

And one copy of the image is equivalent to 1 wave cycle? Repeat above. Answer: No

The sampling frequency is the number of pixels in the image? One pixel can be thought of as the sampler. But, this could be changed. You could go for sub-pixel sampling also.

With this I think the last question has no relevance. Comparing two images, to the best of my knowledge is an open ended problem.