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Murphy's Law of Combat #43
If you are short of everything but the enemy,
you are in the combat zone



The Pathfinding Files

In order to play a single player game in Battlefield 1942 an important part of the game, the AI engine, must be up and running. The AI is a mechanism, used by the game, to operate the robot soldiers.

The AI is basically divided into two parts. One part is devoted to determining a good strategy for the robot soldiers to follow; which point on the map the soldiers should try to take or hold. The other, called pathfinding, determines ways for the soldiers to get from point A to point B.

Piti's infantry pathfinding map

There are three different types of pathfinding files for each vehicle:

  • The search map files
  • The info file
  • The smallones file

Search maps

The pathfinding search maps are one bit images. The bots are allowed to move the given vehicle around in the areas of the pathfinding map where the bits are set to zero (DoGo), shown in black. Every place the bits are set high (NoGo) the bots are not allowed to go and if, by chance they end up in that area, that's where they stay. The picture to the right is the infantry pathfinding map from Piti. Since none of it extends into the water, if a bot ends up there vehicle-less, even feet from shore, it will sit there till it dies.

The names of these files have the following format:
   <vehicle name><map number>Level<level>Map.raw
An example: Tank0Level1Map.raw

The map numbers 0-4 correspond to vehicle types tank, infantry, boat, landingcraft, car respectively. The map numbers are used in the AIpathfinding.con to assign the proper pathfinding files to the given vehicle. The entry for infantry in the AIpathfinding.con looks like:
 

rem *** Infantry ***
ai.addSearchMap Infantry1 0 1.5 30 1.0 0.4 2.0 1
ai.addSearchType Infantry 1 0
ai.setMapSpawnPoints 1 1180/1020
ai.setSmoothing 1 10

The vehicle name is shown in blue, the vehicle number in purple and the number in red is the map index number. It doesn't refer to the vehicle number, but to the sequence it was loaded. In maps where there are no sea vessels, the car has a map number of 2. The number in orange is the minimum search level and refers to the lowest level (least compressed) map used by the vehicle. For boats and landing craft this is set to 2 and for land vehicles this is set to 0. This means that, even if there are level 0 & 1 pathfinding maps for boats and landing craft in the pathfinding directory, they never get used.

The level refers to how many times the map has been compressed. Each time the image gets compressed a level, every four bits (two across, two down) get merged into one, halving it's dimentions. If any of the original bits are set high (NoGo's), the resulting bit gets set high (NoGo). The opposite is true for the info maps.

SmallOnes mesh overlaying the info image for Iwo Jima

The way these images are stored is consistent with the way they are used. Notice the image to the right. Before a search map image is stored, it's broken down into 64x64 pixel tiles, displayed in the image as a numbered grid. Information for each of these tiles is processed and stored seperately from the other tiles. This information is stored as a chain of 64x64 pixel images. To save space in the file, if the tile is all DoGo's or all NoGo's, the image is replace with a 0 or 1 respectively (4 bytes long). if the information in the tile is a mixture of both DoGo's and NoGo's, a -1 marks the beginning of the record in the search map file, followed by 512 bytes of image information (64*64 bits equals 512 bytes).

The Info File

The search maps are used to create the smallOnes and info files. After the search maps have been broken down into tiles, each tile is examined and broken down into up to 4 contiguous (connected) areas in order of size. if more than 4 seperate areas exist in one tile, only the 4 larges are chosen and the others are ignored. Before being stored in the info file, the tile gets compressed 1 level, but unlike the search maps, if any of 4 pixels ( 2 across by 2 down) are DoGo's, the resulting pixel is a DoGo. This makes the DoGo regions slightly large than the regions in the search map.

A single info tile with and without grid imposed

The info files are 2 bit images. Each pixel can have a values from 0 (00b) to 3 (11b). The NoGo areas in the tile have a value of 3, which would be the same as the 4th area (if there is one). How the game handles that is only a guess, but if the bot is in a valid pathfinding point in a tile that has 4 areas and the info files has that point set to 3, it must be the 4th area.

The SmallOnes File

The smallOnes file is not an image. It's a mesh of sorts. It's used to connect all the pieces back together. A point is chosen for every area that exists in the info file. How the point is chosen is arbitratry. Some think it's a waypoint and it might be. More importantly, it's used by the AI to determine if the bot can move from one adjacent tile to the next. For every smallOne point in a tile there is a 4 flags that mark whether or not it is connected to any of the 4 possible points in the next tile in the adjacent column and row. So every smallOne point can be connected to, at most, 8 other smallOnes points, 4 each in the next row and col. Since any previous tiles will have these flags set if connected to the current tile, there is no need to mark connections to any tiles in previous columns or rows.

Moving Around

In order for a bot to get from any point on the map to another point. There must be a contiguous DoGo area connecting them. The game AI consults all three file types for a vehicle, looking for a combination of connected tiles that do just that.

In order to get from point A to point B, the bots need to first consult the lowest level search map for to see is both points are in valid pathfinding areas, then the info file to find out which smallOne points to use for both. After finding the starting and ending smallOnes, it needs to find a chain of connected smallones that get from point A to point B. With a chain of smallOnes chosen, the bot checks the info and search map files again to draw the path it needs to take.


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Since Dec 6, 2004


Last Updated: Jun  9, 2005