Grand Theft Auto: San Andreas Q&A #2 - Physics, AI, Modding

There were no bigger games last year than Grand Theft Auto: San Andreas. Rockstar's blockbuster sequel sold more copies than any other video or computer game in 2004. And now, the hit PlayStation 2 game is coming to the PC. And despite the fact that we're getting the game months after the PS2 crowd did, we can look forward to a better San Andreas, courtesy of the PC's superior graphical power and its fan community, which has a very pleasant habit of using the game to create all-new, custom-made content. However, there's far more to San Andreas than just looks. You'll get the chance to guide your character through a heady life of crime that involves jacking cars, breaking and entering homes, and even engaging in aerial dogfights. To get some details on what makes San Andreas tick, we got a chance to throw some questions to some of the developers at Rockstar North.

CJ's in a hurry to make a name for himself.

GameSpot: What types of objects in San Andreas have physical properties? Is it mostly vehicles, or does every object work with the physics engine?

Obbe Vermeij: Pretty much every object has proper physics applied to it. Boxes can be pushed around or destroyed. Traffic and streetlights can be pushed over and destroyed. Objects now also split up into smaller objects when damaged. This looks particularly cool on benches. There is also a pool game with proper physics on the balls.

GS: How many different handling characteristics does each vehicle possess?

Sandy Roger: Automobiles have around 40 separate handling variables, which can be modified to set up the car's handling, while bikes and flying vehicles have an additional 15 to 20 variables tailored to their specific requirements. Each variable is influenced by other variables though, so you can't, for example, just increase wheel traction without having to "re-set up" the suspension to compensate.

GS: Are models given mass in the game? If so, how is it assigned? Is there a center of gravity, or is mass assigned to the entire model?

OV: Models do have a center-of-gravity set. The value is tweaked very carefully to make sure the object behaves as realistically as possible. In the case of vehicles, these values are tweaked for hours and hours to make driving as enjoyable an experience as possible.

GS: How are the collision dynamics for vehicles worked out? How is damage assigned in terms of how much damage a car can inflict?

SR: The damage a car can inflict is primarily determined by its momentum, or its mass and speed. Heavy and fast vehicles, such as the fire truck, can do more damage to other vehicles. We can also control how much damage a vehicle receives, or tweak how tough a vehicle is, through a variable in the handling.

You can jack pretty much any kind of vehicle in San Andreas.

GS: How are the floating-in-water mechanics worked out? How does this change between CJ and other character models and vehicles?

SR: The buoyancy forces that are applied through the physics engine are calculated based on the percent volume of an entity that is underwater. In the case of CJ or characters, this is simply based on the height of the water level--up the character's body--as a percentage of his or her height. For a vehicle, we must sample the water level at various points across the volume of the vehicle, and [we] use this to calculate the overall volume, as well at the center of buoyancy, which is the average position at which the buoyancy forces should be applied.