When I try visualizing a high-pass filtered image, all I see is gray, similar to the middle subfigure in the attached figure from a related paper. The authors claim they normalize the image to have unit variance, so I tried plotting $(x - \mu) / \sigma$, which still resulted in a gray image. Here is my process for high-pass filtering:

1. Shifted DFT
2. Apply radial mask, zero out low frequencies
3. Inverse shifted DFT
4. Normalize image to $[0, 1]$

[![enter image description here](https://i.stack.imgur.com/VF7Rb.png)](https://i.stack.imgur.com/VF7Rb.png)