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I reckon filters were initially intended to remove specific frequencies/noise and the term grew from there. Which is how ffmpeg and other software ended up with video filters that don't actually remove anything.

Someone else mentioned lens filters as a possible source for the term but I'm pretty sure the signal processing term is at least partially responsible for the current usage.

In image processing "filtering" has for over 30 years meant any algorithmic manipulation of an image as a whole (exhibit A: Photoshop's "Filters" menu). Simplest filters are also filters in the signal processing sense, but the meaning in this context is considerably broader than that.

I ran into something similar with maven builds today. Filtering resources with maven means replacing text in the resource with text from maven properties.

So we should be calling these "face mappers" then >:D