Harry Potter and friends magically travel to the Quidditch World Cup courtesy of Double Negative. All images courtesy of Warner Bros. Pictures. © 2005 Warner Bros. Ent. Inc. — Harry Potter publishing rights © J.K.R.

Since his very first year at Hogwarts School of Witchcraft and Wizardry, Harry Potter has vanquished a giant troll, arch nemesis Voldemort, Aragog the spider, the mighty Basilisc, a deadly werewolf and the terrifying Dementors. And he also dealt with that pesky little elf, Dobby. Yet, in Warner Bros.’ Harry Potter and the Goblet of Fire, Harry faces his most dangerous challenge to date: represent his school in the Triwizard Tournament, a competition in which the players participate in a series of hazardous contests…

Since Mike Newell (Four Weddings and a Funeral) is a director with little experience in vfx, the task of guiding him through the arcane world of plug-ins and shaders fell to overall visual effects supervisor Jim Mitchell, a seasoned veteran of the saga. The effects load was spread among nine vendors in four different countries, which could have created serious organizational problems. In order to review the work, Mitchell and visual effects producer Theresa Corrao used a variety remote review tools provided by vendors. The London-based production visual effects unit could thus keep track of the hundreds of shots that were being completed in locations as distant as Australia or California.

Concepts for the frenetic portkey effect included wormholes and inverted tornadoes. Double Negative decided the environment would vortex into a giant tornado and leave the characters hanging in mid-air, as their new destination forms around them.

Vortex Transportation
Early in the movie, Harry Potter magically travels with the Weasleys to the Quidditch World Cup by using a “portkey.” Lead vendor Double Negative, which produced more than 500 shots, tackled the sequence, lead by visual effects supervisor Mark Michaels and CG supervisor Richard Clarke. Early concepts for the frenetic portkey effect included wormholes and inverted tornadoes until the artists arrived at the idea that the environment would vortex into a giant tornado, leaving the characters hanging in mid-air as their new destination takes form around them.

“We first set about isolating one ‘hero’ shot on which the surrounding shots would be based,” explains Double Negative’s visual effects producer Dom Sidoli. “In this master shot, the camera rotated 360º around the characters while the environment transformed around them. A giant panoramic matte painting was created using our proprietary stitching software, Stig. This gave us a 360º cylindrical texture for the distant fields and sky detail. Field and grass textures were painted for the close-up hill with CG grass rendered to cover the hill. The hill and surrounding landscape were then converted to particle systems, or softbodies, which were simulated to create the vortex effect. This was then turned into a geometry cache, so we could add finer detail and deformations to achieve the twisting tornado-like effect. Further layers of debris, leaves and clouds were rendered as particles to add volume to the environment. Finally, the composited shot was re-timed using Kronos, which created a more visually abstract result.” Depending on the shot, the characters were either the real actors shot on wires in front of a greenscreen, or digital doubles built from cyberscans with clothes animated via a cloth simulator.

The Quidditch World Cup
From the moment the characters arrive at the World Cup venue, the sequence features visual effects provided by Industrial Light & Magic (ILM), which created 250 shots for the movie. Since each of the three previous installments of the series had featured a Quidditch match, it was decided early on that a fourth one was not necessary. This time around, the sequence would focus on the spectators before the start of a game. ILM’s brief for the sequence was to create a giant stadium filled with 80,000 digital characters. ILM first built a CG stadium, based on artwork provided by the production. Then, the CG department, led by Doug Smythe and Robert Weaver, set out to populate it with a convincing crowd. The spectators were created from one single model with 20 different built-in outfits. For each digital character, the crowd system randomly chose a selection of the various parts, and at render time, the pieces of outfit that were not selected for the shot were turned off. For textures, ILM used still photographs of extras in costume that were mapped onto the models.