negative or positive

i can’t decide if this is something everyone will see, or whether its just something that jumped out to me, but i looked at this porcupine quill for a long time and then looked away to answer a question, and when i looked back, it was no longer a porcupine quill but a zipper rip in the middle of white fabric. what’s the negative space? what’s the positive space? apparently it depends on whether you’ve just tried to answer a question about factoring trinomials from your 8th grader.

african porcupine quill