BP's have pretty good vision, it’s about the same as a cats. Some BP's have a habit a associating certain characteristics when it comes to food. Most BP's just smell the rat, see the shape of the rat, see the heat coming off the rat and that's all they need to mark it as food in their minds. Some need the rat to also move to associate it as food. Some need the rat to have dark or light fur to associate it as food. BP’s are notorious for imprinting on certain characteristics for their food.
I have a big female BP that will only eat male rats. Drop a female rat in with here and she doesn't identify the smell as food.