Why does the 2D DFT assume periodic boundary conditions on images?

[nylonman]

Confusion with 2D DFT

Hello everyone,

I'm trying to figure out why the third transform in the picture I attach here is different to the other ones, why it's 'distorted'. I know that it's related with the fact that if you copy the image in a grid the resulting image has discontinuous lines, unlike the other two transforms where the resulting lines are continuous, but I can't see the relation with the distortion.

Thank you for any help!

 

[MATLABdude]

Could it just be a bug in your code?

 

[nylonman]

I found this: http://www.cs.unm.edu/~brayer/vision/fourier.html

There is an explanation in "rotation and edge effects" section.