1. CIS 488/588 Bruce R. Maxim UM-Dearborn
Movement
CIS 488/588
Bruce R. Maxim
UM-Dearborn
1/3/2019

2. Environment Structure Detail
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)
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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
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4. Defining Space - 1 Human players AI characters
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
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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
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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
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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
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8. Space Discrete Continuous
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
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9. Time Discrete Continuous
“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)
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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)
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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
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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)
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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
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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
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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
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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
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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*
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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
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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)
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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
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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
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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
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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
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