The “split down the middle” Not-Bug
Well good thing, that's no longer my job!
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While it’s not exactly a 1-to-1 relationship1 it’s close. All you need is a sorted list of base64 characters (A..z, 0..9, etc) weighted by their luminance, which, well, apparently that doesn’t exist anywhere, but it’s easy to generate a few (64) images with a single character on each, and count dark/light pixels using numpy.
Bonus: Scan with TruffleHog.