Real shapes and real patterns are things you would observe in nature like the marks on the back of a cobra's hood or the markings on a fish or a lizard. Imaginary shapes are just that symbols that come to a person in dreams or reveries and are charged with meaning.
You get ideas from daydreaming. You get ideas from being bored. You get ideas all the time. The only difference between writers and other people is we notice when we're doing it.