# How to estimate the frequency of a feature in an image?

I have a background in the 1-D signal analysis but I am totally new to image analysis. I would appreciate any help.

Given the following original image and the downsampled image, as you could see, there are aliasing artifacts.

I want to understand this aliasing by comparing the downsampled image frequency and Nyquist frequency. I was told that the "fence" provides a good segment to estimate the frequency.

My question is how to estimate the frequency of the fence? How to find the Nyquist frequency of the image? (I know $$F_{nyquist} > 2 F_{max}$$, but how to determine $$F_{max}$$?)