1. Ch 7.1: Using a Spatial Database for Runtime Spatial Analysis

2. Purpose
Dynamic environment – player’s actions are unpredictable, player can change environment (i.e. game has a physics engine)
High levels of interactivity, coupled with dynamic environments, require sophisticated AI
AI needs way to process information in environment

3. Example

4. Possible Representations
Static
Place “hints” in the game world
Analyze world and build a data structure
Dynamic
Influence maps (establishes implicit boundaries between opposing forces)
Want a solution that represents all relevant data

5. Spatial Database 2D grid overlaid on top of the game world
Multiple independent layers superimposed on the same grid
Most useful layers:
Openness
Area occupancy
Area search
Line-of-fire
Light level

6. Use of Spatial Database
Combine multiple layers to compute composite desirability values
AI examines nearby cells, moves accordingly
Implicitly coordinates behavior of multiple AIs

7. Openness Layer
Needed for tactics which require certain amount of open space (ex – “Circle of Death”)
Cells contain values that indicate how close it is to a wall/object
If blocked, value = 0; else, value = 1
Propagate influence of 0-value cells to neighbors

8. Example Propagation 1
0.2 1
0.2
0.4 1
0.2
0.4
0.6

9. Uses of Openness Layer
Can determine the direction of increasing/decreasing openness
Can use this gradient to find the middle of the room (high openness) and the walls (low openness)

10. Area Occupancy Layer More for RTS games, “fog of war”
Each unit propagates an area of influence around itself
Cells with nonzero values are visible
Units update grid values when moving (or created/destroyed)
Used in other genres for AIs to know how populated an area of the world is

11. Uses of Area of Occupancy Layer
Higher values if cells are closer to more agents
Avoid moving into occupied areas when maneuvering
Difference between “fog of war” and occupancy – values depend on distance between AI units for occupancy, not “fog of war”

12. Area Search Layer
Games with stealth components, AI needs to search for player
Scouts in RTS games, uncovering the map
Initialize the layer to zero values
Increment cell values as they enter AI agent’s view arc

13. Implementation for Stealth Games
Guard considers surrounding area a “hidden map”
Has clues to aid search; knows where to begin and where to look
Store database of evidence, bias search towards it
Decay database over time (randomly?)
Automatically coordinates multiple NPCs

14. Thief 3

15. Line-of-Fire Layer For game genres with ranged weapons
Want to ensure that AIs don’t block each others’ shots
Each cell starts as zero
When AI aims, propagates values in its viewing arc
Each AI ignores its own contribution (temporarily subtract value before reading)

16. Light Level Layer Needed for games with dynamic lighting
Stealthy characters/vampires would avoid light, guards would prefer light
Initialize to 0s
Each light source propagates values based on current radius, lighting type, intensity, etc.

17. Handling 3D Environments
Single 2D grid can’t represent vertically overlapping areas
Solution: transform grid into one of sub-grids
Assign a sub-grid to each different height
Works well for human environments

18. Updating certain areas constantly is a waste of time/memory
Potential Issues
Spatial database system could be a performance nightmare
Large grid + high resolution + large # of AIs + frequent updates =
Updating certain areas constantly is a waste of time/memory

19. Optimizations
Pare size of the grid (make cells represent larger areas of environment)
Don’t recalculate static layers
Decompress sub-grids as needed
Combine cells if all values are the same
Use short-circuit evaluation
Update cells only when used

20. Sources
Tozour, Paul, “Using a Spatial Database for Runtime Spatial Analysis,” AI Game Programming Wisdom 2.
HL2 video -
Thief 3 video -
BSOD image -