In 2005, Guerrilla Games leaked a teaser trailer for KILLZONE 2 to give gamers a taste of what was being planned for the Playstation 3. The game community was enthusiastic yet remained a bit skeptical that such a high degree of realism would be possible “in game.” Three years later, Guerrilla Games KILLZONE 2 has just been released and has clearly lived up to the teaser trailer’s promise. The game is a hit and has already garnered high scores from reviewers throughout the gaming community.

For KILLZONE 2 to achieve such a high degree of realism, Guerrilla Games needed a new pipeline that could handle the Playstation 3’s support for bigger, more detailed environments and real-time HD video output. Houdini was therefore brought in for a few different parts of the pipeline such as procedural modeling, dynamic simulations and object placement.

“The gaming experience needed to be cinematic in scope and Houdini’s reputation for high-quality feature-film VFX made it a great fit for the new pipeline,” says Guerrilla Senior Artist Ben Schrijvers. “Guerrilla Games was looking for a new approach to create next gen assets and Houdini offered a solution with its node-based procedural approach.”

One of the most critical parts of a gaming pipeline is the ability to get art into the game engine. Guerrilla Games had already set up Maya for converting data into the game engine format. To pipe in Houdini, the first task was to get a versatile conversion path between the two packages. Using Houdini’s native geometry format, a plug-in was created that imported the geometry and its vertex and primitive attributes into Maya. In addition, a Python script was made to import Houdini's native bclip animation format.

Waves of Destruction

To make KILLZONE 2’s wartime setting as immersive as possible, Guerrilla Games set out to make objects and environments destructible in both a realistic and dramatic manner. To smash up buildings, they had to first be pre-fractured in a way that created nice breaks and good output geometry. Initial tests were not satisfying as the broken up models contained too many polygons and, in some cases, unnecessary edges. A better approach was therefore chosen where the object was converted into a volume primitive using an ISO Surface operation then cut up using voxels defined by a VEX volume shader. The resulting voxel chunks where then converted back to polygons and reduced with a Poly Reduce.

The original object was divided by transferring chunk id attributes from the internal voxel chunks. Then a Foreach Sop was used to loop through all chunks and stitch them to the corresponding bits of original geometry. Breaking up models still required an artist’s attention but modeling time was cut from more than a week to less than a day. After wrapping the system up in a couple of Digital Assets, artists who had never used Houdini before were able to get to work right away.

In Game Cloth Simulations

Throughout the game the weather plays a big role. To create the sense of interaction between the weather and the environment, flags, banners and other cloth-like materials were given dynamic properties. Although the Playstation 3 is powerful enough to process a cloth simulation in real-time another approach was needed because there was already a lot of detail being calculated in a typical 1/30th of a second between frames.

Therefore a non real-time solution was chosen to give the option to tweak and maximize the dramatic character of each simulation. Houdini’s cloth dynamics was used to create separate simulations for slow wind, medium wind and fast wind for each cloth object. The game engine interpolates between the three simulations based on the wind speed at the particular position and time in the game.

But getting the cloth simulations into the game engine turned out to be a challenge since a sequence of deforming meshes or an animation channel per mesh point would take too much memory. On the other hand, the game engine is highly optimized for processing skinned meshes because of the vast amount of character animation in the game. Therefore the simulations were converted to skeletons with animating bones.

“The in game cloth effect was achieved by using Sticky Objects in Houdini to capture the motion at key positions on the cloth” says Schrijvers. “This resulted in an optimized data set which generated surprisingly good results especially since the Sticky Objects also captured rotation information.”

For cloth objects covering another object like a tank, a simple VEX operator was used to create fake collisions. The whole system was then wrapped up into a Digital Asset so that the effect could be easily repeated. The system was extended to also handle wire simulations and in the end more than fifty Houdini-made cloth objects can be found in the game ranging from flags and banners to spider webs and snapping electrical cables.

Terrain Generation

In addition to urban and industrial areas many game levels also take place in desert and mountain landscapes. Modeling terrain manually can be a lot of hard work because of the organic shapes and large dimensions and typically the art team can only start working on a level once the design team has finalized a low-resolution proxy version. This geometry is made up of simple meshes which only show the general shapes without any detail to provide a starting point for the art team to refine.

These meshes were the ideal starting point for the art team to generate a more detailed game-ready terrain model using Houdini’s procedural techniques. This made it possible for the design team to modify the base mesh without forcing the art team to start from scratch. The art team quickly picked up on the benefits of the new procedural workflow and was able to react more quickly to changes in the general design of the landscape. Multiple iterations became part of the process instead of forcing hard work to be thrown away and new work started from scratch.

The terrain creation procedure began when the input mesh was given more resolution using a progressive subdivision system driven in Houdini by a Foreach operator. Next, the terrain was split up in sections which would form natural seams to both hide UV discontinuities and make large models easier to work with. Each section would have vertex UV's assigned using a combination of pelting and a custom VEX-based un-wrapping operator. By measuring the area of each section, this custom operator was able to easily achieve uniformly scaled UV's over the complete terrain which can be hard to keep consistent when applying UV’s manually.

Another custom VEX operator was used to add noise to the terrain using the vertex UV's to make sure the rock geometry matches the orientation of the rock texture. Another noise pattern was added to horizontal areas to create little hills and sand dunes. In order to make sure that A.I. characters and vehicles could move uninterrupted across the terrain, an attribute was painted onto the control mesh to limit the amount of deformation.

In all cases, the number of polygons had to be kept to a minimum to maintain fast game play. The biggest terrain was a complete level containing around 110,000 polygons. All UVs and noise patterns were first applied on the subdivided mesh and then reduced using a Poly Reduce. By taking into account where the player would walk and what would be most visible even more reduction was done.

The KILLZONE 2 game engine allowed for the use of complex shaders that could make use of vertex colors. A final custom VEX operator was therefore used to apply vertex colors to differentiate between sand and rock types. The results from this procedure were then further refined by environment artists who could make adjustments where needed using a paint-based workflow.

It’s the Little Details that Count

Apart from the breakup tools, cloth simulations and terrain generation, Houdini was also used to do add a variety of other details to the game. All lightning effects were created using a Digital Asset that could turn a simple curve into a finished lightning model including textures and vertex colors. The asset had various sliders that not only controlled the look but also the optimization of the finished model.

Bullet wounds on all characters were also made with Houdini. The wounds themselves were pre-modeled and a Digital Asset was used to position and control the direction of the bullet impact. This job had to be completed in a really late phase of the project but the Digital Asset tools made it a simple task that enabled an artist to produce wounds for about five characters within a day. Other things Houdini was used for were RBD simulations of grand scale destructions and even a nuclear explosion.

For Guerrilla Games, Houdini offered an opportunity to bring feature-film quality effects into the gaming world. At the same time, its procedural nature provided many workflow enhancements that complemented their existing pipeline. The results are stunning and KILLZONE 2 is destined to be a shining example of how far the new gaming systems can be pushed.

About Guerrilla Games

Founded in 2000 as the result of a merger between three small game development studios, Guerrilla Games now employs 140 developers. Outstanding technology and a strong artistic vision have elevated the company to a prominent rank among Europe's game developers. After the successful release of KILLZONE for PlayStation 2, Guerrilla was acquired by Sony Computer Entertainment in 2005. It went on to create the critically acclaimed KILLZONE: Liberation for PlayStation Portable, and has just released KILLZONE 2 for PlayStation 3.

Guerrilla Games is committed to recruiting, developing and retaining the best talent in the industry. Under the banner of the ‘United Nations of Guerrilla,’ employees from 20 different nationalities work to achieve incredibly high production standards. It’s not all work and no play, however, as the company has an in-house massage therapist, a gourmet chef, and weekly Friday Frag Fests. And while Guerrilla benefits from the SCE Worldwide Studios network, it has ensured to retain the culture of an independent developer.

In contrast to its informal atmosphere, the company is based in a very regal 17th century canal mansion in the heart of Amsterdam. As one might expect, the garden house, the oldest in Amsterdam, has been converted into a movie-grade screening room. For more information, visit www.guerrilla-games.com.