# How to choose between taking the real part or the absolute value of an inverse discrete Fourier transform?

I kind of expected this to of been asked before, so if this is duplicate please point me in the right direction, however I couldn't find anything.

I am currently experimenting with low pass filtering. I take an image and apply a 2-dimensional fast Fourier transform to the image, next I apply a shift such that the zero frequency component is moved to the center (Using Matlab this is fftshift). Then I zero Fourier co-efficients which are not within some distance $r$ of the center pixel. I apply the inverse of the shift and then an inverse 2-dimensional fast Fourier transform.

Now I end up with a matrix of complex values, but to display an image I want a matrix of real values. With the matrix obtained after applying the IDFT I mapped the pixel values to their real part, imaginary part, magnitude and phase so that I could view the matrix as an image. I found that the real part and the magnitude looked like the original image and taking the imaginary part and phase rendered the image unrecognizeable compared to the original image. I also found that their wasn't a recognizable difference in quality between taking the real value of the transformed image or the absolute value.

My question is, is there advantages or disadvantages between taking the real part of absolute value after filtering (low pass, or in general)?

This is my first question on DSP.SE, so please leave comments on how to improve my question if it's not up to standard