It cost, IIRC, a tenth of a cent per image URL. Rather than being based on skin tone, it was created based on algos to specifically identify labia, anuses, penises, etc. REST API: send a URL, get back a yes/no/maybe. You decided what to do with the maybes.
My experience:
- Before launch, I tested it with 4chan b as a feed, and was able to produce a mostly clean version of b with the exception of cartoon imagery.
- It could catch most of the stuff people tried to post to the site. Small breasted women (being that breasts are considered 'adult' in the US) was the only thing that would get through and wasn't a huge concern. Completely unmaintained public hair (as revealing as a black bikini) would also get through.
- Since people didn't know what I was testing with they didn't work around it (so nobody tried posting drawings or cartoons), but I imagine eg a photo of a prolapse might not trigger the anus detection as the shape would be too different.
- pifilter erred on the side of false negative, but one notable false positive: a pastrami sandwich.