Yes, the image functions are from Compass, but the underlying mechanics of mapping Sass functions to Ruby code is purely Sass's domain. So, perhaps I misspoke

Yes, the image functions are from Compass, but the underlying mechanics of mapping Sass functions to Ruby code is purely Sass's domain. So, perhaps I misspoke: Sass's power is in its ability to tie it's functions to arbitrary Ruby code, which can then access and manipulate files.