Ukiyo-e, or "pictures of the floating world", was a popular art genre in Japan during the Edo Period (1603-1868). By using woodblock printing, depictions of folk tales, landscapes, kabuki theatre scenes and erotica, were widely spread throughout Japan.
While, in the movie, O-Ei and her younger sister, O-Nao, are the only children (or at least the only children revealed) of Hokusai, history says the famous painter fathered two sons and three daughters with two wives - O-Ei actually being the youngest daughter, although some sources say Hokusai fathered four daughters, the youngest possibly being O-Nao.