Bot design
Contents
Generalities
The main situations we want bots to work in are:
- In a game where a single user plays in a team against a team of bots (let's call this 1v0)
- In a game where a few users play against each others, with a considerable amount of bots on both teams (let's call this 1v1, 2v2, etc.)
- In a game where a considerable number of players team up to play against a horde of bots (let's call this PvE)
- In a game where a single user (or few users) play against a considerably bigger number of other users (let's call this 1v3, etc.)
In games where bots are a minority in both teams, bots are not that important. When playing PvE, the variations of bot skill are also not very important, as players can't feel like they got an unfairly weak ally.
Bots are built from roughly 3 different "modules":
- navigation, which is done with a combination of A* or Dijkstra and navmeshes;
- decision, which is done with the paradigm of behavior trees;
- computer vision, to complete what navmeshes can not provide;
Not a direct part of gameplay
Bots are not a gameplay feature, they are only around to help making interesting games when there are not enough players around. This means that it is ok for them to exploit gameplay's weaknesses, by doing things such as camping, like real players would do. Bots' objective is to win the game, too, so they should avoid feeding at least at average and above skill levels.
While they are not a direct part of the gameplay, they still need to "understand" game's rules as much as possible and adapt to gameplay changes. For this reason, generic changes should be preferred, in opposition to hard-coded trigger values (like 0.51's healScore or baseRushScore, and like current 0.54's GetEnemyPriority).
Non-cheating
Bots should not perform any action that could not be performed by a non-bot client. Bots currently cheat in few cases, because of technical and historical reasons. However, this is un-desired status and considered as bugs (see details in next §). This property does take some care to maintain, as bot functions often act directly on sgame state rather than using client commands.
It would be nice if additionally, bots only used information that could have been learned by a normal player. This is far from true in the current state: for example, bots have omniscience of the closest (in ghost distance) buildable of each type in the level, which also applies to friends and foes..
When playing against bots, it should not feel like a bot is cheating, i.e. doing something that a human player could not have done. The bot should not have impossibly fast reaction times or seem like it's using an aimbot, triggerbot, etc.
All bots should be able to reach any part of the map that does not require special training.
Computer vision
Bots should be able to detect any enemy in their FoV. It would be nice to have bots not being too good at finding hidden buildables on the roof. This makes hiding eggs unfun and next to impossible in the current state. Unless they are placed outside of a navmesh, in which case bots are totally unable to discover them.
To improve this and make bots feel more like human players, we should make bots:
- not automatically know the egg locations when rushing: https://github.com/Unvanquished/Unvanquished/pull/2683
- attack enemies that are visible but outside the navmesh
- for human bots, not be as great at spotting things vertically as they are with thing horizontally (like regular humans)
- maybe not discovering things always with the same reliability — but this would bring randomness.
Decision
Bot levels
For historical reasons, bots have a range of skills numbered from 1 to 9, all integers.
Skill 1 is considered to be training dummies.
Skill 5 should never be so strong that an average player can not kill it in duel, nor so weak than an average player considers the bot negligible.
Skill 9 should be made as hard as possible.
In some cases, bots currently cheat (they "smell" enemy buildings depending on some conditions) . This is because of technical limitations of the code infrastructure, but is not desired.
Bot personalities
In 2022 and early 2023, we did some experiments about giving each bot a personality. The idea was to have a number of skills that are valuable in playing. Each bot would receive its unique set of skills, by some random selection. That would make them different from each other.
Some examples of "bot skills" we used are: various alien trick moves, the ability to aim better than other bots, smartly buying human equipment/weapons and using the human medkit.
The pros of this approach:
- Bots do not behave all the same. That makes them more difficult to predict.
- Bots simulate real users more accurately.
The cons:
- It is harder to balance bots against each other. We would have to test a significant subset of all possible combinations. This matters a lot in 1v1 or 2v2. It matters even more in 1v3.
- Users will discover what skills are more valuable. They can tweak the skills the bots on their team have, by voting to remove them and add them again until they hit a sweet spot.
Those two cons should be mitigated by having sufficiently many skills and skills that have well adapted costs: that is by having costs reflect the skill's usefulness.
Distributing Skills over Bot Levels
Let All denote the set of all bot skills. Let S1, S2, ..., S9 denote the sets of skills given to each level from 1 to 9. We should aim for S1 ⊆ S2 ⊆ ... ⊆ S9 = All.
As of May 2023, we have:
All = {
aim-barbs,
aim-head,
buy-modern-armor,
fast-aim,
feels-pain,
flee-run,
goon-attack-jump,
goon-flee-jump,
mantis-attack-jump,
mantis-flee-jump,
mara-attack-jump,
mara-flee-jump,
medkit,
predict-aim,
prefer-armor,
safe-barbs,
small-attack-jump,
strafe-attack,
tyrant-attack-run,
tyrant-flee-run
}
There is also movement, fighting, which seem to be a little mysterious.
This table summarizes the proposed skills for each level:
| Level | Skills |
|---|---|
| S1 | {prefer-armor, medkit} |
| S2 | S1 |
| S3 | S2 ∪ {strafe-attack} |
| S4 | S3 |
| S5 | S4 ∪ {aim-barbs, feels-pain, flee-run, *-attack-jump, *-flee-jump, buy-modern-armor} |
| S6 | S5 |
| S7 | S6 ∪ {aim-head, predict-aim, safe-barbs} |
| S8 | S7 |
| S9 | S8 |
Current Questions
- Should we keep bot personalities, or make all bots the same?
- How much randomness should be involved in a bot's behavior?
- How many details of bot configuration should be hard-coded, and how many should be exposed by cvars?
