Movement

1. Movement CIS 488/588 Bruce R. Maxim UM-Dearborn

2. Environment • Structure • Parts of game environment that can physically affect movement (e.g. doors) • Player cannot simply push these asid • Detail • Cosmetic parts of the environment that players cannot collide with in a significant way (e.g. books)

3. Living Creatures • Affect movement during game • Often have been forced into either structure or scenery as needed by game designer • Other players and NPC’s can be considered obstacles in large crowds • It may be best to consider moving creatures as category of their own when considering movement issues

4. Defining Space - 1 • Human players • Have to assimilate the environment based on imperfect visual information • AI characters • May get a simplified or incomplete version of the world off line before the game starts (e.g. a list of way points) • They may be expected to perceive and interpret the world on-line like humans

5. Defining Space - 2 • Space is an abstract concept that cannot be fully understood • Each representation scheme will involve considerations of design tradeoffs • Once an entity has “figured out” space it is possible for the entity to determine which parts can be traversed and which cannot

6. Dimensions • Some game take place in 2D worlds (Warcraft II) and some in 3D (Half Life) • It is important to note the the game world dimensions do not always match its rendering • Most academic AI deals with 2D movement algorithms • In many cases it is permissible to generalize a 2D movement algorithm to 3D

7. Pitfalls 2D to 3D • Complex environments may contain widgets (e.g. jump pads) that are rare in a 2d world • Naively applying the 2D algorithm without custom tailoring it to the problem at hand • Gravity plays a role in realistic environments and should not be ignored

8. Space • Discrete • Game environment limited cells on a grid using integer coordinates • Continuous • Have unlimited (real) coordinate values • Usually can be done using single precision (32 bit) floating-point numbers

9. Time • Discrete • “Clock” advances at regular fixed length intervals • Continuous • Actions can be continuous and appear concurrent • A single CPU shares its clock cycles with each object and a portion of the game code is devoted to determining what happened since the last update (say every 0.1 seconds)

10. Conversions • For digital computers everything ends up being implemented as if it was discrete • It is possible for the game world to use one discrete conversion scheme and the AI different scheme • This allows the use of an grid-based A* path finding routine in a continuous game world • There may be tradeoffs to consider (e.g. compromising behavior quality)

11. Handling Movement • The allowed movement is imposed on the creatures by their environment • In both 2D and 3D worlds the patter is: • Game simulation loop • Integration • Collision detection

12. Role of Game Engine • Game engine takes into account all movement requests from players and NPC’s • Physics system resolves all collisions and updates the creature data structures with new velocities, positions, and orientations • Animation system handles low-level “limb” control as predefined animations (abstracting this low-level control as a “locomotion” layer greatly simplifies programming)

13. Dr. Spin • Only behavior is to move and turn • Often bumps into walls • Can get stuck in corners • Supposed to demonstrate the importance of world simulation and structure on movement

14. Navigation - 1 • Process of purposely steering course of some entity though a physical environment • Autonomous navigation means the software system can steer its course unassisted • Navigation produces proactive rather than reactive movement • Purposeful navigation is not possible by trial and error of actions

15. Navigation - 2 • Most living creatures require spatial information to interpret that provides them a sense of their world and helps them select their “next step” • Much of the complexity of navigation comes from the acquisition of world information • A rough description of space is encoded by animals to help them pick out empty and solid spaces

16. Game Bots - 1 • Very few computer games use sensing and environment interpretation to achieve navigation behavior • Virtual game worlds can be stored entirely in the computer’s memory • Algorithms process the world description to extract structural information form it

17. Game Bots - 2 • The bot is part of the game simulation world so they have perfect position information • By dropping waypoints or processing polygons a compact terrain description can be given to the bot before the game begins • Bots are particularly good at global movement and allow very effective use of standard algorithms like A*

18. Game Bots - 3 • Bots do not sense the world like animals • Bots perceive the world using preprocessed environment knowledge and this is not always good • The terrain model might be based on erroneous assumptions • The terrain model may not be up to date in dynamic environment • Bots can become stuck or ignore obstacles

19. Animats • Autonomous navigation is possible by giving animats the ability to sense the environment and selectively process the relevant information • This give animats fresh information about their local surroundings • This reactive approach has good local navigation, but has problems reaching points in space (similar to hill climbing)

20. Criteria for Effective Motion • Realistic • Movement must be similar to real world analogs • Efficient • Economical use of processing cycles • Reliable • Must guarantee NPC can complete standard situations with getting stuck • Purposeful • Must be able to achieve goals in space

21. Motion Tradeoffs • These criteria often comes in conflict with one another and compromise is needed • Efficiency has historically been the greatest concern of game developers • Realism has become more important in recent years • Different game situations require different tradeoff decisions

22. Obstacle Avoidance • With no obstacles near, animat is free to travel in any direction • With walls on one side, animat should be turned away slowly in a preventative manner • With obstacle in front, turn should be executed to avoid danger on one side • When stuck in a corner, animat should attempt a complete turnaround

23. Bouncer • Runs around the game monitoring its own movement • If its forward movement is not what was expected, a collision is assumed • Following a collision, animat takes off in a random direction until it can move again