I’m trying to wrap my head around this filter. Like some other filters, I find that I don’t understand how it works, both visually and in terms of the algorithm. I’m hoping that by learning the latter, the former would also somehow click in.
High pass is the opposite of gaussian blur
If you take an image and blur it you only keep the “low frequencies”. High pass makes the opposite, it only lets the “high frequencies” pass, or what most people call “the details”. Any image can be deconstructed into these two components.
Have you used unsharp mask to sharpen an image? That filter is actually equivalent to taking an image and adding a high pass filtered copy to it. You can confirm this in Photoshop by applying the high pass filter on a duplicated layer and change the blend mode to overlay.