Digital Domain: The Lightning Thief

For Percy Jackson & the Olympians: The Lightning Thief, director Chris Columbus and visual effects supervisor Kevin Mack looked to Digital Domain to create some of the key creatures and effects in the film. We talk to DD’s visual effects supervisor Kelly Port and animation supervisor Erik Gamache about their work for the Hydra beast, the six-winged Fury character and the film’s final New York face-off.

Hydra

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After discovering he is the son of Poseidon, the Greek God of the sea and earthquakes, Percy Jackson soon comes face to face with many beings of Greek mythology. A fire-breathing creature called Hydra, disguised originally as five night janitors, attacks Percy and his friends inside the Parthenon replica in Nashville. Percy takes to the air via winged shoes and attempts to cut off the Hydra’s five heads, only to find that new ones grow. Pursued by a now ten-headed beast, Percy is able to magically summon a wall of water to stop the Hydra’s fire before using Medusa’s head to turn the beast to stone.


10Mar/thief/HY0050_v112Digital Domain completed about 80 shots for the Hydra sequence, which was filmed on the Fox studios in Vancouver. A shoot early in production benefited the visual effects team so that they could concentrate on the look of the Hydra. “The biggest challenge of the creature,” noted visual effects supervisor Kelly Port, “was how to make this character go from five to ten heads. Where do you put the base of the neck, for instance? It was very important to us that it was anatomically feasible, which helps all the way through lighting and animation.”


10Mar/thief/HY0380_v103 The Hydra was modelled in Maya, with the initial five heads based on a different janitor. “Each head is a slightly different colour and shape,” said animation supervisor Erik Gamache. “In one shot they even had us put a pair of glasses onto one of the heads to match it up, although it’s pretty subtle and happens really quickly.”

Artists consulted dinosaur-type movie reference for the general look of the Hydra, although because of the number of heads and the frenetic nature of the character’s movement, the design changed to something more snake-like. “They kept calling it a junkyard dog, like it was salivating and on its leash,” added Gamache. “With the five necks, we bowed the area so it was more like a hull, with the centre neck leaning more forward. It was actually originally more like a hand with the other necks coming more from the shoulders.”


When a head was cut off the Hydra, animators created the movement of a new growth on a separate neck, pushing through the skin and scaling up at the same time. “We had morph targets for all the different parts of the head,” said Gamache. “You could grow the snout and the jaw differently which gives you that morpheous kind of thing. It then went to the effects department and they put a gel-filled sack around it so as it comes through it actually pierces that sack and flaps away and then they put all the goo between the necks.”

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For the skin, Digital Domain employed sub-surface scattering techniques and point-based rendering out of RenderMan. Skin and muscle jiggle of the Hydra was all simulated. “We had a small dynamics team just for the Hydra,” noted Gamache. “We had a scaled down collison for the skin. Then using a bunch of weighting techniques, wire deformers and lag, you’d get an overall jiggle. We wanted to control it so that in the fat or muscle areas you got more jiggle. It was all completely art-directedable.”


10Mar/thief/079_HY0530_v096Coming up with an appropriate animation style for the Hydra was made slightly more difficult by its threatening nature. “It was such a big creature with long necks which could easily have gone and used its different heads to snap these guys up,” said Port. “So one of the challenges was getting the audience to believe that it wouldn’t just eat them straight away.

We did this by making the heads compete with each other. So if they came to a column, one head would go one way and the other try and go the other way. They’re not exactly working in concert together. All the heads are after the same meat, but going after it in a different way.” Facial animation was handled using blend shapes or wire deformers, with animators given a series of sliders in Maya.


10Mar/thief/HY0190_v165To help sell movement of the Hydra’s neck when it breathes fire, artists relied on animated displacement maps to show detail and ripples in the muscle areas. Explains Gamache: “In animation we could change some sliders where the muscles should move, and that information was transferred to Nuke where we had a whole bunch of different displacement maps – ‘neck on, neck off’. Inside Nuke the curves we would specify in animation would drive the fading of them on and off.

You’d send that out to the farm and it would be rendered and come back and the lighters would just apply that one displacement map.” To place the Hydra in the environment, Digital Domain undertook an entire survey of the Vancouver set and a LIDAR scan. “When Percy’s flying around there was quite a bit of rigging that had to be painted out,” said Port. “Plus there was no ceiling so we had to put that in there. The LIDAR helped us out with the fire and water interaction against the walls.”

10Mar/thief/079_HY0550_v155For the fire emnating from the Hydra’s jaws, theoretically a methane gas igniting at the back of the creature’s throat, Digital Domain used a mix of Houdini and 2D elements in Nuke. “We shot a bunch of locked-off fire elements over black using propane flamethrowers,” said Port. “Even though they were impressive, it didn’t work so well with a moving creature’s head.

So sometimes it was an all digital gas sim and fire and sometimes it was combined with the live action elements.” Seeking to make the fire-breathing head of the Hydra, stand out, artists added stripes on its ears, a nod to one of the director’s previous films. “Chris Columbus wrote Gremlins,” said Gamache, “so we threw those stripes in and he noticed!”


Fury

Additional character work was required for an earlier sequence in which school teacher Mrs. Dodds transforms into Fury, a six-winged and particularly sharp-toothed being who attacks Percy. Digital Domain modelled Fury in Maya, with some minor design changes taking place once her animation had been worked out. “When the character was designed,” said Gamache, “she was configured to have her arms forward. But when she would jump off her feet would naturally lag behind. That meant when we put the arms in that position the hip would get messed up. So we went back and forth a bit with modelling to make it work.”

Animators developed a flap cycle for Fury’s six wings, ensuring each wing did not look like it was intersecting. To enable Fury to deliver some dialogue, footage of the actress speaking lines was referenced. “Also, she was so dynamic with her flapping all the time,” added Port, “that we had to exaggerate it so much to make it read. It’s way more pronounced than it would normally be if it was still.”

Using a survey and LIDAR of the set, Digital Domain’s environment team worked in Nuke to recreate the room, paint out the rigging and allowing for more dynamic camera moves. “It was actually a really interesting lighting environment,” said Port, “with a high contrast, low-light abandoned room and a big window source light.” Artists accommodated the look of Fury’s skin to match, adding translucency where necessary.

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New York

10Mar/thief/111_RT0725_v126The Empire State Building becomes the scene of a showdown between Percy and his adversary, Luke, the son of Hermes. Here, the characters fly from the top of the Empire State down to a rooftop where Percy summons water rooftop tanks and ultimately flushes Luke away. Digital Domain started the shots by constructing a nighttime New York environment for all the backgrounds. “We went on location for a week or two,” recalled Port, “and shot hundreds of thousands of still frames, HDRs and big panoramas from about 15 to 20 rooftops.

We would know the path that Luke and Percy were going to fly from the top of the Empire Stare Building down to the rooftop and then back up. Once we knew that path, we’d go to the different rooftop locations, take the pictures and then we’d take the urban dataset of Manhattan and re-project the images onto this. On top of that we would render reflection and ambient occlusion passes, so it was not just looking like a projection. And for the high shots looking down we would splice in some moving traffic as well.”

10Mar/thief/111_RT0708_v009Scenes of the characters flying through the New York locations were achieved with a combination of wire work and digital doubles. “As good as the wire work was,” said Port, “sometimes you could tell that Percy was swinging a little bit. We had to figure out how much of his motion to track versus the camera motion and then add additional motion on top of that to help unify them. It was always a challenge to get the ‘wire curse’ off of the shots.”

Digital doubles were created via stereo-paired photographs for some shots. “There was this whole construction site that they fly through and that was all CG,” said Port. “It was based on a site we found in Vancouver. We were able to use pretty low-res geometry and then project a lot of the added detail of the photography. Given the speed they’re going through it, a lot of it is pretty forgiving, especially with the electricity and interactive light.”

10Mar/thief/111_RT0700_v146As Percy and Luke battle it out, they shoot lightning arcs at each other. “The director and Fox wanted the lightning to look very photorealistic,” said Port. “We started off by blowing out the frame, but of course you want to see the characters. It ended up being a balance between how much you want to see and how much you play realistic photographically.”

The arcs were rendered in 3D with interactive lighting created using the HDR images and 3D sets. “We were able to use some of that higher end of the dynamic range as interactive light,” noted Port. “Had it been shot at night, it would just be black and you wouldn’t be able to fill in the detail.

What we would do was render almost like a reveal matte. So we’d render a light source from the lightning and then it would show you where that would be. In Nuke we could reveal that as lower-end detail.” Shots featuring parts of the Empire State Building or other buildings being blown up by the lightning were achieved using rigid body dynamics sims in Houdini, and the surrounding clouds were formed via volummetrics to give a simulated cloud tank look.

10Mar/thief/111_RT0670_v114Landing on a rooftop, the two characters face off with their lightning wands before Percy is able to summon water from a prototypical water tank. The water surrounds them and eventually caves in like a giant wave on Luke. “It was basically a lot of layers,” recalled Port. “You’ve got your main sim and then many, many layers of spray down to a fine mist. Again, it needed to look physically accurate but it’s doing something that is completely odd that you would never see. That was the big challenge actually for all of the visual effects shots – just working out how it had to work physically versus artistically.”