Rihanna to Chris Brown -- How Could You Leave My Ass?

Rihanna just gave ex-boyfriend Chris Brown a little taste of what he's missing ... her perfectly sculpted ass.

RiRi just posted an extreme close-up shot of her nearly bare derriere in shorts that are so tiny, her THONG is hanging out of 'em.

Rihanna added a caption saying the pic was from the set of her new music video ... but we think it's pretty obvious the photo was for an audience of 1 -- Chris ... who was recently spotted out with his other sometimes-GF Karrueche Tran (who definitely doesn't have an ass like Ri).

Think he's regretting the breakup yet?