|Building the scenes and characters to continue the Hogwarts legend.|
|The Moving Picture Company (MPC)|
London-based MPC handled two of the most stunning sequences in the film, the fireworks in the dining hall, and the final battle between Dumbledore (Michael Gambon) and Lord Voldemort (Ralph Fiennes). They also flattened Fiennes nose with digital makeup.
In the fireworks sequence, the Weasley twins fly over the students and teachers in the dining hall shooting off fireworks as they fly. Greg Butler, who won a VES award for his work on Gollum in “The Lord of the Rings: The Return of the King,” supervised the team at MPC that created the sequence.
“Basically, the fireworks were lots of little colored dots that were glowed in compositing,” he says. “But, we started off with the idea that the fireworks would look real and act real. So, we used only colors that real fireworks have.”
|To keep the colors real, the crew implemented a “black body radiation” algorithm in the shader that described the wavelength (color) based on the temperature of an element. Attributes on Maya particles specified when a particle was born and gave the RenderMan shader a starting point for determining color. |
Integrating the fireworks into the scene, however, meant that the actors and objects in the room needed to reflect the changing colors. “We knew one of the things about adding 3D self-emitting objects is that they would have to light up the room and we couldn’t do that in post in any decent way,” says Butler. Instead, they determined the color palette and locked down the animation in previz. Then, they used the previz as a guide for a turning on a bank of lights in the ceiling that they controlled with synchronized timers during the live action shoot.
“We could look at a plate and see that on this frame a blue light went off, so we put blue fireworks somewhere near that time and space,” Butler says. For the fireworks dragon that attacks the Ministry of Magic’s pernicious Dolores Umbridge, the visual effects artists emitted particles from the surface of an animated model.
MPC’s second group of effects happen in the Atrium, a set the visual effects studio extended digitally for shots early in the film when Harry is on trial as well as for the final battle. The battle begins when Harry runs into the room. The evil Bellatrix Lestrange (Helena Bonham Carter) taunts him. Voldemort appears and threatens Harry, and then Dumbledore shows up and the most powerful wizards in the world fight each other with visual effects. “It’s a big, fun sequence,” says Burke. “The battle is spectacular. MPC generated 60 foot CG snakes out of fire. We kept escalating the extremity of wand effects.”
|For the fire and water effects, MPC decided to use the FlowLine simulation tools from Munich-based Scanline, as the studio had for water and fire simulations in Poseidon. A Maya model that they rigged and animated in Maya became the emitting surface for the fire serpent. |
“FlowLine works with fuel and heat,” Burke explains. “We emitted fuel through cracks in the [dragon’s] surface, which created an interesting look, and from a pile of fuel beneath. Once the fuel ignites, it burns at different rates. The serpent’s movement changed how the fire reacted.”
For the watery prison, custom features that Scanline added to the software made it possible to control the liquid in unnatural ways. “Most of their simulations have water act as it’s meant to do,” says Butler. “We needed something more magical to create an antigravity effect.” Mental Ray rendered the fire and water. For the other shots, the studio used RenderMan.
Then, as the action escalates, MPC simulated a glass attack with particles that matched the speed and velocity of window fragments exploded by the special effects crew. “We used a flocking particle system with models of glass shards stuck onto the particles,” says Butler. PAPI, the studio’s dynamics system, which runs inside Maya, controlled the particles.
Lastly, to convert the glass into sand, particularly for close up shots, the crew had glass shards hit invisible 3D objects that started shrinking. As an invisible object shrank, it generated sand particles off the leading edge and the sand continued forward giving the impression that the glass literally turned to sand.
|Industrial Light & Magic (ILM)|
San Francisco-based ILM created the first big effects scene in the film, an attack on Harry Potter and his Muggle cousin Dudley Dursley (Harry Melling) by the soul-sucking Dementors. “They did our dark creatures,” says Burke.
To create the Dementors, ILM started with skeletons for the same creatures that they had covered with sheets in a previous Harry Potter film. “We had created Dementors for the third film,” says visual effects supervisor Tim Alexander.
“But, for this film, they wanted them to feel more physical.”Thus, ILM pulled back the sheets to reveal the creature’s head and arms. To create the “arms,” ILM cut the sheet – the cloth – into 12 strips and gave animators balls that attracted each strip so they could create a performance from the strips.
Then, to simulate the effect of a Dementor grabbing Harry by the neck, the crew filmed Daniel Radcliffe on greenscreen. At ILM, animators working from a matchimation of Radcliffe’s neck, used shape animation to bulge a digital replica and simulate what would have happened if the creature had grabbed Harry’s neck. But, rather than trying to render digital skin to match the actor’s neck, the crew rendered only an occlusion pass, the shadows, and used that as a matte in the comp to fake the motion.
ILM also created the Thestrals, horse-like creatures with 25-foot long dragon wings. Because the animals are emaciated, modelers had to build both the outer shell for the creature and the inner bones. The studio’s PhysBam engine developed with Stanford University simulated skin sliding over the Thestrals’ bones and sinking between their ribs. Subsurface scattering on the skin helped make the thin skin look fleshy.
The same basic PhysBam engine also simulated water crashing against the Askaban prison for an establishing shot during a prison escape. For areas that didn’t require interaction with the island prison, the studio used a deformed surface.
In addition, ILM built 11 digital doubles, which they used sparingly for a broomstick flight from London to Hogwarts. And, during the Dementor attack, created Harry Potter’s Patronus spell by bringing rendered strips of cloth into the studio’s Saber (Inferno) system.
“Harry’s Patronus is a stag and you can sense that, but it’s something between a stag and liquid light,” says Alexander. “We did the liquid light part with the cloth strips.” As the stag ran, the cloth flowed behind.
For its part, the London-based studio Framestore CFC created Centaurs, a house elf named Kreacher, Hawkes the Phoenix, and a Howler.
Two actors in blue suits played the roles of two of the Centaurs during live action filming. Meanwhile, at the studio, modelers sculpted the creatures in Maya from maquettes and then blended muscle groups to create one Maya rig that propelled the half-horse, half-human Centaurs. The main body of the Centaurs was human sized; the horse part, pony sized.
“We started with our skeleton rig and built a muscle rig on top of that, and then on top of that floated the skin,” says Craig Lyn, visual effects supervisor. “The rig and any dynamic movement drove the muscle simulation. So if the Centaur slaps a foot, the muscle jiggles and shakes, and on top of that the skin slides over the muscles.” A muscle system underlies the facial rig as well.
The muscles are geometry with soft body simulations; a volume preservation system handled only the extreme movements. “We often see more volume loss caused by rigging issues than with muscles,” says Lyn. “We find it’s easier and faster to use animated displacement maps in some areas, like movement across tendons, when it’s for a specific shot and we know the angles.”
Although the animators worked in Maya, Framestore CFC prototyped the muscle simulations in Houdini. “We developed in Houdini and then mimicked what we developed in Maya,” says Lyn. “It’s easier to do rapid prototyping for simulations in Houdini.”