Going into the shoot, the brothers endeavored to make both principal photography and the visual effects work as efficient as possible, tapping into their expert knowledge to do as much as possible in camera and resort to CGI, only when and where it was necessary. "Other than the exterior spaceship shots, there are no pure CG shots," says Colin. "The lighters and the modelers had something real in every shot that they could actually reference. It was kind of a big thing for keeping the whole movie grounded."
That meant using practical costumes for both Alien and Predator -- though some adjustments were made. For example, the Aliens' tails as done in previous movies required puppeteers and wire removal, so those plus the Aliens' mouth strikers were done with CG this time out.
An unexpected side effect was the extent to which rabid fans of the Alien and Predator films would go to get information on the film, Colin says. "These are the guys who will go see the movie six times in the theaters and it's pretty wild about how aggressive they are about getting information," Colin says. "We even had a couple kids break into one of our trailers and steal some of our stunt Aliens and actually were trying to sell them on eBay while we were filming."
Shot on a tight 52-day schedule in Vancouver, the film posed several vfx challenges, including digital characters, matching the look of the original films and set pieces that include a nuclear explosion and the crash of the predators' ship.
One example was the cloaking effect used by the Predators. "We wanted to make sure it didn't look too digital," recalls Colin. The original had plenty of imperfections such as gate weave that they tried to preserve. "All these little inaccuracies, it makes it more of an analog effect."
A nuclear explosion sequence required fluid simulations for the explosion and the shockwave, as well as dynamic structures were destroyed in the blast, all of which was done using Maya fluids and BA Volume Shader.
The Predator ship and its crash were two of the largest sequences in the film. First was the interior of the ship, unseen in previous films, which had to impress and look functional at the same time. "We decided because of how crazy big it was going to be it wasn't going to be cost effective to build a set, so we did full digital interiors for all the shots," Colin adds.
The crash of the ship was another huge sequence, showing the vessel crash into the ledge on a mountain and setting off a huge shockwave of its own. Colin compares it to the crash shots in Transformers, but with a much larger ship.
Most unusual was the use of the Maya hair simulator to build a forest of trees that devastated by the crash. A procedural tree building system added branches that could break off and start bouncing around, while the hair sim created the flow of falling trees. "That was a really tricky shot because there are so many trees getting plowed over by the big ship, so quickly, that we probably spent a good four five months getting that whole system worked out," Colin says.