Upon completion of principal photography, the project enters by far the longest and one of the more critical aspects of film production: post-production. It is during this time that the film is edited, the visual effects and music are added and mixed, and re-shooting occurs if it is found to be necessary. This period culminates in the presentation of a final print of the film, from which all release prints can be made and distributed to the many theaters throughout the world. If a production is going to be released straight to DVD or VOD (video on demand) it will not need a release print unless required for film festival screenings. Release prints are also not necessary for theatres with digital projection systems.