For Killzone 2's theater of war, the design brief goes one step further: the environment shouldn't just react to player actions, it should do so dramatically. To some extent, this form of 'Hollywood realism' can be achieved by adding small-but-impressive details, such as bullet holes glowing red-hot from the impact of depleted uranium shells, or bits of plaster and concrete chipping off a wall when subjected to gunfire.

But sometimes, bigger really is better, and more really means more. A spectacular setpiece event, when initiated through player actions, can help strengthen the player's sense of control over the theater of war. It also serves as a reminder that the game environment is not a static, unchanging backdrop to the action, but very much a part of the action itself.



The Corinth River mission provides an excellent example. During one of its signature moments, the player (as Sev) engages a platoon of Helghast troopers holed up in a partially constructed building across a storm drain. The odds are stacked heavily against our protagonist, until he trains his fire on the exposed fuel tank buried within the building's structure.

The resulting explosion is at once expected and overwhelming. The tank ignites with a blinding flash, the impact sending billowing dust clouds in every direction. As chunks of debris rain down into the concrete canyon below, realistically colliding with each other along the way, the front of the building slowly cascades to oblivion. The collapse plays out in real-time, with a level of detail that wouldn't look out of place in a Hollywood blockbuster.



To achieve this stunning effect, the developers used a tool named - appropriately enough - after one of history's greatest magicians: Houdini. "The software was designed specifically for Hollywood's visual effects industry," Guerrilla Senior Artist and resident Houdini expert Ben Schrijvers says. "Previously it had little use in the games industry, but as games and movies begin to converge in terms of complexity and graphical quality, it's starting to make the jump from one creative arena into the other."

First, Ben took a traditionally modeled building, created in Maya, and used Houdini to procedurally break it up into hundreds of pieces. "With the building now made of discrete parts, we were able to run a full physics simulation of the explosion and the ensuing collapse, allowing a network of workstations to work out the movements of each piece in painstaking detail," he explains.



After that, it was just a matter of placing the resulting animations back in the game and augmenting them with real-time physics. "That's the magic of Houdini: because the physics have been pre-calculated and baked into the animation, they don't require a whole lot of processing power," Ben says. "This allows us to model much more variety in them. We can even increase the overall complexity of the scene by adding real-time physics on top."

Visual effects artist Iki Ikram further enhanced the collapse with particle effects to simulate dust clouds and smaller pieces of falling debris. Using Guerrilla's in-house destructible tool, he and artist Edouard Peregrine synced up the procedurally generated animations with in-game physics and particle effects to produce one smooth, seamless-looking collapse.



"Corinth River marked the first time we combined procedural animation with advanced physics modeling and special effects to tear down a large structure," Edouard recalls. "It was sort of a test case for us, and it turned out spectacularly well. Usually, setpiece events like these are presented as pre-rendered cutscenes, which means wresting control away from the player. But in Killzone 2, you remain in full control throughout the sequence, so the sense of immersion is never broken. You could continue shooting at the building, or back away from the dust cloud, or--"

"Or even look the other way," Iki interjects.

"That too," Edouard laughs. "But I'm confident that once the collapse commences, you won't want to. And besides, even if you accidentally miss it, there'll be plenty of spectacular setpieces like this in the game."