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.
That Moorish architecture is all over the place, of course. It affects me everywhere I see it, as it does so many people. But Brand Library was a special place to me, and I know I've paid homage to it many times in my drawings.