Cinesite, a visual effects company that worked on Harry Potter and the Prisoner of Azkaban, has divulged details of various projects from the film, including how they accomplished time travel, the moving portraits, invisible thestrals and more:
Cinesite's team of digital effects artists back in Soho completed many effects sequences for the film. These included filling the ceiling of the Great Hall with computer generated Candles, creating the Enchanted Ceiling, the Monster Book of Monsters, Marauders' Map ink effects, various ghost and ghost horsemen effects and the stunning panoramic shot where the camera follows Hedwig flying through time and season towards Harry in the Hogwarts clocktower.
The company said they started working in April 2003 and completed a year later. The film was released to theaters in early June and comes out on DVD in November.