What is a mathematical (described with a function) equivalent of mirroring bits?

Question

So imagine we have a function F(x), where x can be any 8bit number, and our functions returns mirrored number. How could this function look in decimal format or maybe do you even would think of binary representation?

Examples:

• F(0b1110.0000) = 0b0000.0111
• F(0b1001.0000) = 0b0000.1001