I have been reading the paper PALMPRINT IDENTIFICATION BY FOURIER TRANSFORM by WENXIN LI, DAVID ZHANG and ZHUOQUN XU about identifying persons based on an image of their palm, one version of the paper can be found here.
The first step in their identification algorithm consists of aligning and preprocessing the image - since they spent a few pages on that front, this gives me the impression, that this step is kinda important.
What I don't understand however: Why do we do this? The essential part in my opinion is the Fourier Transform afterwards, that is used to extract/generate the feature. But from my understanding, this should even work without this aligning/preprocessing step.
I have found a property of the DFT that states that rotating the spatial domain contents rotates the frequency domain contents, can be read here. In order to extract features, some rotation might be necessary, at some point. In my opinion however, this does not justify to think up such an elaborate method of aligning/preprocessing the image, there has to be more behind that.
Therefore, I am most likely overlooking/missing something. Thanks in advance.