If you have played Liquid War, you must have noticed that your army always takes the shortest way to reach the cursor. So the fundamental stuff in Liquid War is path-finding. Once you've done that the game is quite easy to code. Not harder than any other 2D game. Still the path finding algorithm is an interesting one, for it's not a common method that we used.
Basically, at each round (by round I mean a game logical update, this occurs 10 or 100 times/sec depending on the level and/or your machine), the distance from all the points of the level to your cursor is calculated. Now the point is to calculate this fast, real fast. In fact, a "gradient" is calculated for all the points of the level, and the value of this gradient is the distance required for a little pixel/fighter to reach your cursor, assuming that he takes the shortest way. Liquid War does this with a 10% error tolerance, and it's enough for keeping the game interesting.
Once you have this gradient calculated, it's not hard to move your fighters. Basically, you just have to move them toward the adjacent point that has the lowest gradient value, ie is the closest to your cursor.
The Liquid War algorithm has been invented by my friend Thomas Colcombet In fact the Liquid War algorithm has been invented before the game itself. The game came as a consequence of the algorithm, he just thought "mmm, cool, we could make a game with that!".
Later, I enhanced the algorithm, as I coded it. The consequences were a performance increase, especially on simple but big levels. I mean levels with wide areas for teams to move. Still the basis of the algorithm remained the same.
The Liquid War algorithm for path-finding is very efficient:
- When you have to move lots of different points toward one single point. Good thing that's the rule of Liquid War!
- When you have no clue about how your map will look like, ie if the walls are randomly placed. The complexity of the level doesn't influence much the speed of the algorithm. The size does, but the complexity, ie the number of walls, is not so important.
The Liquid War algorithm is very poor compared to other algorithms when:
- You have several target destinations, that's to say Liquid War would be really slow if there were 100 teams with 10 players only.
- You want to move one single point only.
- > You want the exact (100% sure) path. In fact, this algorithm finds solutions which approach the best one but you can never figure out if the solution you found is the best, and the algorithm never ends. In the long term, the algo will always find the best solution or something really close but I don't know any easy way to figure out when you have reached this state.
The first Liquid War algorithm used to calculate the gradient (the distance from a point to your cursor) for every single point of the map.
With Liquid War 5, I used a mesh system. This mesh system is a structure of squares connected together. Squares may be 1,2,4,8 or 16 units large or any nice value like that, and the gradient is only calculated once for each square. Squares have connections between them, and each connection is associated to a direction.
There are 12 directions:
- North-North-West (NNW)
- North-West (NW)
- West-North-West (WNW)
- West-South-West (WSW)
- South-West (SW)
- South-South-West (SSW)
- South-South-East (SSE)
- South-East (SE)
- East-South-East (ESE)
- East-North-East (ENE)
- North-East (NE)
- North-North-East (NNE)
Well, let me give you an example, supposing that you level structure is:
********** * * * * * ** * * **********
The * represent walls, that's to say squares where fighters can not go.
Then the mesh structure would be:
********** *11112233* *11112233* *1111445** *i1114467* **********
In this mesh, there are 7 zones:
- zone 1 has a size of 4. It's linked with zones 2 (ENE) and 4 (ESE).
- zone 2 has a size of 2. It's linked with zones 3 (ENE,ESE), 5 (SE), 4 (SSE,SSW) and 1 (SW,WSW,WNW).
- zone 3 has a size of 2. It's linked with zones 5 (SSW), 4 (SW) and 2 (WSW,WNW).
- zone 4 has a size of 2. It's linked with zones 2 (NNW,NNE), 4 (NE), 5 (ENE), 6 (ESE) and 1 (WSW,WNW,NW).
- zone 5 has a size of 1. It's linked with zones 3 (NNW,NNE,NE), 7 (SE), 6 (SSE,SSW), 4 (SW,WSW,WNW) and 2 (NW).
- zone 6 has a size of 1. It's linked with zones 5 (NNW,NNE), 7 (ENE,ESE) and 4 (WSW,WNW,NW).
- zone 7 has a size of 1. It's linked with zones 5 (NW) and 6 (WSW,WNW).
Why such a complicated structure?
Because it allows the module which calculates the gradient to work much faster. With this system, the number of zones is reduced a lot, and calculus on the mesh can go very fast. At the same time, this mesh structure is complicated to understand by us humans but it's very easy for the computer.
For each zone defined in the mesh, LW calculates an estimation of the distance between the cursor and this zone.
The algorihm is based on the fact that to cross a zone which size is n, n movements are required. Easy, eh?
Here's the way the algorithm works:
for each turn of the game, do:
- pick up a direction between the 12 defined directions. They have to be chosen is a peculiar order to avoid weird behaviors from fighters, but let's suppose we just pick up the "next" direction, ie if WSW was chosen the last time, we pick up WNW.
and then for each zone in the mesh, do:
- Compare the potential of the current zone with that of its neighbor zone. The neighbor zone to be chosen is the one which corresponds to the direction which has been previously picked up, and by potential I mean "the distance to the cursor, estimated by the algorithm's last pass".
- If potential_of_the_neighbor_zone > (potential_of_the_current_zone + size_of_the_current_zone) then potentiel_of_the_neighbor_zone = potential_of_the_current_zone + size_of_the_current_zone
How can this work?
Well, just ask my friend thom-Thom, he's the one who had the idea of this algorithm!
The basic idea is that by applying this simple rule to all the zones, after a certain amount of time, it's impossible to find any place in the mesh where the rule is not respected. And at this time, one can consider the potiential is right in any point.
Of course when the cursor moves the potential has to be recalculated, but you see, cursors move really slowly in Liquid War, so the algorithm has plenty of time to find a new stable solution...
It's possible to see this algorithm working by typing:
while playing, where [n] is the number of the team the gradient of which you want to view. The game is still running but you view a team's gradient being calculated in real time instead of seeing the fighters.
If you type ufootgrad0 the display comes back to normal mode.
Once the gradient is calculated for any zone on the battlefield, it's quite easy to move the fighters, hey?
The following method is used to move the players:
- A "main direction" is chosen for the fighter, this direction is chosen using the gradient calculated on the mesh.
- Knowing which direction is the main one, a "level of interest" is applied to the 12 defined directions.
There are 4 "level of interest" for directions:
- Main directions: the direction calculated.
- Good directions: these directions should lead the fighter to the cursor.
- Acceptable directions: ok, one can use this direction, since the fighter shouldn't loose any time using it.
- Unpossible directions: wether there's a wall or using this direction means the fighter will be farer from his cursor than before, it always means that this direction will not be used, never.
The fighters will try to find any matching situation in this list, and chose the first one.
- The main direction is available, no one on it, OK, let's follow it.
- There's a good direction with no one on it, OK, let's follow it.
- There's an acceptable direction with no one on it, OK, let's follow it.
- The main direction is available, but there's an opponent on it, I attack! By attacking, one means that energy is drawned from the attacked fighter and transmitted to the attacker. When the attacked fighter dies, he belongs to the team which killed him.
- A good direction is available, but there's an opponent on it, I attack!
- The main direction is available, but there's a mate on it, I cure him. That's to say that energy is given to the mate. This way, when there's a big pool of fighters from the same team, they re-generate each other.
- None of the previous situations found, do nothing.
Tips and tricks
The behavior of the armies is quite tricky to set up. I had myself to try many algorithms before I came to something nice. In fact, I had to introduce some "random" behaviors. They are not really random for I wanted the game to behave the same when given the same keyboard input, but for instance, fighters will prefer NNW to NNE sometimes, and NNE to NNW some other times. By the way, I think Liquid War could stand as a nice example of the thoery of chaos.