Physics also makes more work for the CPU outside of the physics in itself

• The use of physics in one level makes the pathfinding far more complex. In fact, it must adapt on-the-fly to a changing environment. If a large block falls down in the middle of a road, the artificial intelligence of the game must take it into account, so that the NPCs controlled by the AI move around the object or use it for cover.

• The lights and the shadows must be dynamic. If objects move or walls are destroyed, pre-calculated shadows or lights cannot be used. Thus, the use of dynamic lighting becomes essential. This can be avoided by choosing environments with little contrast, but the graphical quality of the map suffers.

  
  

The challenge posed in multiplayer gaming

The use of physics in a multiplayer game offers very interesting perspectives from the gameplay point of view, as the gamer can then modify the topology to his or her advantage. However, when compared to a single player game, the multiplayer game offers additional constraints as well: the available bandwidth for sessions played on the internet, and the latency. The challenge is to synchronize the events that have a direct impact of the gameplay on all machines.

Until now, in most games, only the position of the gamers and their projectiles had to be synchronized. If physics is to have an impact on gameplay, all physics events should be synchronized as well: physical objects, clouds of particles that are likely to block the view, damage zones etc.

Unfortunately, this constraint is largely independent of the computing power of the game machine, but several solutions enable the use of physics in a multiplayer session. The strategy to follow is to have a physics implementation whose impact is focused on the client machines and which requires little synchronization. Thus, the number of large objects that have an impact on the gameplay should be limited, as they have to be synchronized on all machines.

On the other hand, a cloud made up of particles or small objects does not require perfect synchronization on all machines. Synchronizing the position of the cloud and its possible damage volume would suffice. Thus, if synchronizing the effects of the fall of a watchtower is desired, what should be synchronized on all machines is the damage volume associated with the fall of debris and not the debris itself, of which there is far too much. Physics locally manages the collisions between debris.

The level design should take this constraint into account from the very beginning by allowing that a maximum number of physical effects should be managed locally, and by providing a "budget" of objects to synchronize.

The incompatibility between the scripted dimension of certain games and the chaotic nature of physics

All games that offer a single experience aim to provide the gamer with the best sensations. For game development cost reasons, such games offer a linear level design, thus ensuring that the gamer takes advantage of the projected special effects, cutscenes and scripted combat situations. The linear level design also allows controlling the rhythm of the game. If the gamer starts to roam in a place where nothing interesting happens, they might get bored.

Of course there are many exceptions that offer open game environments such as most of the RPGs or certain action games that take place outdoors, such as Far Cry, but the majority of action/adventure games provide linear or semi-linear architecture.

In such a context, the use of physics may lead to situations that are unpredicted by the level designer. This is called emergent gameplay.

This type of gameplay is usually welcome, but the use of physics provides for the possibility to change the topology of a level and therefore create situations like blocked movements, disturbance of the characters' AI or disappearance of key items for the script. These kinds of problems may also occur in the multiplayer maps.

Thus, in CTF-Tornado we quickly became aware that the movement of large objects by the tornado or gamers could block the access to the bases and therefore prevent them from getting back or laying down the captured flags. This option had to be offered, as it opens new tactical possibilities, but care had to be taken not to block play on the map.

Consequently, various solutions were developed: the number of access paths was increased and we offered gamers the opportunity to get rid of the possible obstacles themselves by using the Impact Hammer, one of the Unreal Tournament 3 weapons that provide a repelling effect.