studios was the difference in adult-themed material to make way in Japan. Tezuka's thought that animation should not be restricted to kids alone has brought about many studios that are employed in the production of adult-themed adaptations of classic stories such as Heidi (Heidi, Girl of the Alps), One Thousand and One Nights and The Diary of a Young Girl and many more. In the 1980s, animation studios were led back to their theatrical roots due to the success of Hayao Miyazaki's film Nausicaä of the Valley of the Wind, which led publishing house Tokuma Shoten to finance a new animation studio, Studio Ghibli, which would be used for the personal works of Miyazaki and his close friend, Isao Takahata. Many of Ghibli's works have become Japan's top-grossing theatrical films, whether in live-action or animated form. OAV/OVA market The market for 'OAV's or 'Original Anime Video' later the acronym would be better known as 'OVA' meaning 'Original video animation' as the term 'OAV' could often be misunderstood for 'Original Adult Video', began in 1984. These are often tended towards the home video market, while not tending to the television or theatrical audience as such. They refer to those movies that are launched as direct-to-video releases and not meant to be released in theatres. Video productions can run from half an hour productions to well over two hours. They require that premise or story be original in order to be counted as an OVA, though sometimes, the story can be derived from a longer running manga or animated series. As the OAV market is not adapted to the rigors that are faced by television shows or feature films, they have been known to show gratuitous amounts of violence and/or pornography. Some OAV's have registered such strong acclaim that they have been remade as anime television series as well as theatrical releases. Since most new OVA's are derived from other animated media, many animation studios that have previously worked on animated series or movies, and adaptations of Japanese manga, have now entered the OVA market, looking to capitalize on the popularity of their flagship shows. Studios participating in such circumstances include Production I.G and Studio Deen. Animator's contracts Although there are permanent/full-time positions in studios, most animators work on a contract basis. There are some animators that are considered to be in the core group of the studio, which can either be