Extracted from Harry Potter Film Wizardry.

Quidditch is essentially created on the computer. All the actors are filmed separately on their brooms, which are held and moved by a rig. They act and react while a motion-control camera films the performance in front of a blue or green screen. This coloured background is later eliminated when the actors are incorporated into the complete CGI environment by the visual effects department.

With little to interact with but the blue and green screens, filming Quidditch scenes was ‘one of the more gruelling experiences for the actors,’ says Chris Columbus, director of the first two Harry Potter films. ‘Riding those brooms was a tremendously difficult thing, but to create a sense of movement, a sense of urgency, and, at the same time, to feel as if there were real athletes playing a game, was the biggest challenge.’

To make sure the story of each Quidditch scene is clear, and that the necessary excitement will be built when all the pieces are assembled, every move in a Quidditch match is carefully plotted in a series of detailed storyboard sketches well before filming begins.

The movements between these key shots are then worked out on computers using simple animation known as previsualisation or ‘pre-viz’. This process gives the filmmakers a good sense of how many different elements they will have to combine in the final shot, and creates a preliminary shooting list for each individual player’s moves, which are subsequently filmed one by one.

It’s an incredibly time-consuming process. A shot that features 10 players, for instance, might involve a week’s filming before the visual effects team can even think about starting work. On screen, that shot might last just two seconds.

Technological advances during the making of the series have made it possible for Quidditch to be filmed in an increasingly exciting way.

‘Inside a computer we do just about anything,’ says visual effects supervisor Tim Burke. ‘Players’ cloaks can be made to flap and whip around, and moves in the game that would be impossible for an actor to perform can now be made by a digitally animate version of the actor.’