Google’s recaptcha v3 (or was it v4… whichever was the one that uses an excess amount of JS to try to scan everything about you) is trying for that, I may not like all the tracking and JS stuff it does, but it works quite well at figuring out browser ‘usage’ and tracking and such to determine who’s a bot, all without showing a captcha to the actual person (unless certain criteria are met).
OvermindDL1
cenotaph
yes it does work quite well but unfortunately in the hands of an evil corp that snoops on everything you do.
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Exadra37
You have others doing the same has Google reCAPTCHA V3, like:
Also a good alternative or addition is to use:
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eksperimental
cenotaph
olivermt
They are not caching the image url or anything, they are probably just caching a simple fourier transform of it which will technically be a cache of that unique combination.
Those transforms are immune to resizing etc, and the only way you can break similarity scores is by warping the image, hence why a lot of the old captchas had words that looked like they were viewed through a lens.
You need to randomly (even inside what is technically the same permutation like 1-3-2-1) place the images and skew them in ratio and placement for such a caching to not work.
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cenotaph
the famous FT hits us again 
Luckily very achievable with the way we stitch with ImageMagick. I am the only user so far, don’t think need to go to those levels just yet
I am finding out people just enter the sums on the first try rather than reading the instructions.
changing to sums would drastically increase the possibility of a right guess from min 1/44 to 1/8 but that seems like human nature or conditioned internet-based behavior so far.
ps: given stats are for a set of n=2 images as I use on my website. the number gets smaller as you increase to max n=9
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