Director James Cameron’s academy award winning masterpiece Avatar has set new standards by which all future VFX films will be measured against. The movie was first conceived in 1994 and then put on the back burner until James Cameron felt that CG technology was ready to bring his vision to life cinematically. The resulting movie uses CG throughout and required the combined effort of many top VFX studios to make Cameron’s dream a reality.
One of these studios was Framestore London who developed a wide range of visual effects for the film. The Houdini team included Gunnar Radeloff, Adrien Toupet, Ben Frost, Tom Bolt and was led by Guillaume Fradin. This small team of artists and technical directors were able to make a significant contribution to the overall shot count by using Houdini's deep toolset to help them meet the high standards set out by James Cameron.
Fire and Smoke
In background shots of the human camp on Pandora, Framestore needed to generate fire and smoke pouring from chimneys and chose to use Houdini’s PyroFX tools. To find the right look for this effect, the team decided to take different approaches for the fire and the smoke. This made it easier to create long elongated smoke trails on the one hand and more compact flames on the other.
For the flames, the team was able to easily create numerous fire variations using the built-in Pyro FX shader. “The shader provided by Side Effects met our needs exactly” says Guillaume. ”The smoke effect was achieved using custom VEX code. Depending on the situation, our artists could either create volume geometry or define it at render time. We soon realized that we had more control when we used volume geometry.”
“Shadow generations and render times are very fast and the geometry is visible in the viewport, meaning we could easily fine tune it. We could also modify it using either, transforms or by applying additional vex expressions if we needed it to fade or get denser in certain areas” Guillaume explains. “But sometimes the smoke was filling a very large area of the shot and volume geometry would have been too big to manage. In those situations, it was very handy that the smoke vex library could be called at render time from a shader. And we could still preview the effect at a low resolution in the viewport.”
As the action in the film intensifies, bullets strike metal and tarmac surfaces which generate sparks and smoke. No realism was spared by Framestore’s VFX team. In one particular shot, a helicopter was pelted with gunfire as it floated just off the ground. The smoke created as the bullets impact the metal had to quickly disperse due to the strong wind generated by the helicopter blades.
“In shots where the forces are quite high simulations tend to become unstable making it tricky to find the right amount of force needed. It has to be fast enough, otherwise it is not realistic, but it also has to be slow enough so that it stays pleasing to the eye” Guillaume says. “Some shots used particles with our in-house volumetric generator while more detailed shots used fluids. Houdini's tool set made it easy to find a general technique for all the impacts, while still allowing us to tweak each result separately”.
God is in the Details
In a film full of jaw-dropping visual effects, there are so many subtle details that help add polish and bring to life the film’s virtual shots. For instance, when the protagonist Jake Sully glides his wheelchair through a puddle. This interaction between the wheelchair’s wheels and the water puddle is given a sense of realism using a combination of particles for the water droplets and textures to create ripples.
The sultry climate of Pandora is very evident in all of the beautiful bird’s eye view shots of the environment, but what about when the Pandora is observed from the ground? Well, this too has been taken into consideration. When a helicopter lands in this film, the downward force of air from the blades creates a vortex-like light fog. This has been achieved by generating fluid from the helicopter moving downward. This fluid then bounces on the ground and creates the vortex. Particles are then advected through this fluid so the team can fine tune where it should be visible then begin adding layers of noise.
The human settlement in Avatar is full of high-tech machinery, much of which emits great amounts of heat. In a shot where Jake Sully is conversing with Colonel Quartich who is climbing into a robotic AMP suit, the team was required to animate heat haze projecting from the suit. This was especially challenging due to the stereoscopic nature of the film. Heat haze is typically created as a compositing trick where particle passes would be handed to the compositors so they can distort the background. This distortion works effectively in a non stereo movie, but in this case it would lose the 3D effect. “Houdini was a great help here since it allowed us to prototype many types of shaders, interact easily with Nuke and try out versions in particles, fluid or a combination of both” says Guillaume.
One of the subtler yet necessary effects created involved the mixing two different types of air. The air on Pandora is not the same as that found on earth. This is evident when doors from within the human settlement open to the outdoors and mix with the humid air of Pandora. This “combination of air sources” was achieved using normal fluid simulation and was rendered in Mantra using normal passes and RGB lighting passes so that compositors could use them in order to distort the background.
The human base was surrounded by an immense forest. Here, Houdini and Xfrog were utilized to prototype layouts for the forest. To easily test many layouts, many points were scattered where they desired trees and were used as instance points. With this they were able to vary the sizes and species of trees, make fine-tuned adjustments and then quickly render with Mantra. Once a layout was approved it would be sent to the lighters for final rendering.
Simplifying the Pipeline
Before tackling the actual visual effects, the Framestore team first needed to nail down their pipeline. The first issue they addressed was the importing of footage shot with a stereo camera. Using Houdini, they wrapped two instances of their in-house camera importer into a Houdini Digital Asset. This became their new default camera importer and guaranteed that any Technical Director would have a correct set of two cameras in a scene.
Working with both Houdini and Maya simultaneously in production, the team had to ensure that they could easily import precise geometries and animations into Houdini to allow the two packages to have perfectly synchronized renders. Framestore is no stranger when it comes to working with multiple software packages during a production however in this instance each shot they were working with contained a huge amount of assets. With this being the case, some tools had to be re-designed and Houdini made it easy to adapt quickly to changes and plug itself back into a constantly changing pipeline.
“Houdini’s flexibility was a big help and most of the solutions we ended up were solved with out-of-box Houdini nodes without too much scripting,” says Framestore VFX Artist Guillaume Fradin. “We only scripted a python node that would pull an XML file that was built from the layout department containing all asset position and geometry cache file. This let us recreate a very light set of geometry containing only one point per asset.”
This set of points had all the necessary attributes to load the asset geometry later if desired using a copy SOP, or for each with a simple file SOP. This way they could quickly visualize where all the assets were in space, which allowed the artist to have a clear idea as to where each point corresponds. Since it was impossible for a single computer to load all assets together, this was a huge time saver and they did not experience any slow-downs by while loading assets that weren’t needed.
Framestore’s approach to creating effects for Avatar show how highly polished details can make a big difference. By focusing on many small details designed to add up to a new level of realism, the Houdini team at Framestore was able to play a big part in bringing James Cameron’s vision to life.