CV method for isolating high and low frequency elements of an image?

I've been playing a lot recently with limiting effects/filters to a specific luminance range or motion-detection range..but I'm trying to think of it's silly to ask if there's something similar to a high/low pass filter method for doing something like masking out sections of an image that appear relatively featureless like a wall or something out of focus in the background, and probably your face/skin/clothes to an extent

This seems like one of those questions that's staring me in the face, and I'll just go 'ohhhhh...duh' later on after I post this, but if anyone has ideas, let me know