[SEJeff]

This is sort of the inverse of gimp's "foreground select tool". It was a PHD thesis on the algorithm to select foregrounds that was contributed to the gimp photo editor as open source.

https://docs.gimp.org/en/gimp-tool-foreground-select.html

And the paper with an overview of the algorithm is here: "Image segementation by uniform color clustering"

https://www.inf.fu-berlin.de/inst/pubs/tr-b-05-07.pdf