No. It's not a magic invisibility field inside. It's a magic object that works only on living "intelligent" creatures. It also doesn't work the same on different races. Only humans (hobbits are a subtype of human in Tolkiens lore) turn invisible. And because it's magic it also turns their clothing etc invisible. So either Frodo and his poop is invisible or nothing is.
I don't know the lore, but what initially happens with the ring? Anything that passes through it vanishes?
No. It's not a magic invisibility field inside. It's a magic object that works only on living "intelligent" creatures. It also doesn't work the same on different races. Only humans (hobbits are a subtype of human in Tolkiens lore) turn invisible. And because it's magic it also turns their clothing etc invisible. So either Frodo and his poop is invisible or nothing is.
Of note, it also straight up doesn't work on Tom Bombadil. He is immune to its temptation, and it doesn't make him invisible.
This guy Tolkiens!