By Team SimRacingWorld
8 March 2004
Dynamic and fully interactive track environment in Xpand Rally has been achieved by global physics simulation of all game's objects. Physics in Xpand Rally is based on rigid body simulator named Open Dynamics Engine - a complex solution for dynamics simulation created by Russell Smith - the author of the MathEngine system that has been used for physics in many games. During the development of physics simulation for Xpand Rally, ODE has been largely modified and underwent heavy optimizations. It enabled the simulation of physics of bodies of all shapes and connected with one another and with the environment by any ways. All objects joints are fully configurable which allows for independent setting of parameters such as: number of degrees of freedom, friction coefficients, hardness, maximum speed and more.
For Xpand Rally the system has also been extended with a set of new parameters of joints that allow for the proper simulation of car springs and absorbers. Besides of standard gravitation forces also air friction, rolling friction, wind force and liquid interactions have been handled.
Thanks to the above it has been possible to achieve an unbelievable level of interaction of the car with the environment. Each object on the track is dynamic and it's behavior (such as reaction to collision) is determined by the parameters of it's physical simulation. The car hitting fences, walls, adverts, lamps, signs or barrels can tip them, break, turn over, pull out from the ground. To make the realism high thousands of such objects have been on each track. Sounds have big influence upon the realism of the game and that's why the collision system in Xpand Rally is fully integrated with the sound system. At every collision of 2 objects the system takes into account the material of which they are made, and a fallen wooden post for instance will sound differently when hitting tarmac surface and differently when falling down on a green meadow.
As it was mentioned before the ODE system in Xpand Rally has been extended by the simulation of liquid uplift pressure forces. That allowed for making objects float on the surface of water. After the car hits a board, it may fall into nearby lake and float moved by the waves before it drowns. Of course the same refers to the car which will be especially important in some game modes. This phenomenon has yet another meaning. Let's imagine the car driving through a deep puddle. But this is something to be discussed in further materials about the game regarding car physics. It is worth mentioning that wind plays an important role in Xpand Rally's physics engine. It makes the antennas on the car's roof swing and the grass on the fields wave. But the most spectacular is probably the effect of the tapes surrounding the tracks that can be tore and wave in the wind. The air forces simulation has been also integrated with the sound system - the objects that quickly pass by near the camera create characteristic zips. Full simulation of air forces allowed for simulation of the reaction of plants to the car driving nearby when the air wave of the running car bends the grass and other vegetation.
The development of the new physics took more than a year. It's been worth it to make the system comply current standards and, above all, give the player maximum feeling of realism in the game.