Half life 2/ Counter Strike: Source bot

Charlie Cross

CIS 488

2-14-05

Background

• Counter Strike is a multiplayer modification for Half life, released in June of 1999.

• Valve software hired the counterstrike team and ported Counter Strike to their new Source engine.

• Outsourced a spin off single player product- Counter Strike: Condition Zero.

• Ended up being finished at Turtle Rock Studios.

Background (2)

• Turtle Rock developed artificial intelligence for condition zero.

• AI was ported to CS: Source last month.

AI features:

• Navigates map using automated systems (waypoints) combined with human teaching (navigation meshes).

• Communicates with human players, telling them where enemies are located.

• Follows human instructions (e.g. “follow me”, or “team fall back”)

• Selects best weapon for tactical situation. Avoids tactically disadvantageous situations.

• Conveys range of emotion-driven behaviors. Rushes aggressively when winning or retreats when outnumbered.

Navigation Meshes

Navigation

• Use of nav meshes allows bots to be less predictable and to use less structured behavior the way humans do.

• A* pathfinding.• Uses reactive obstacle avoidance (like the quake

bots) To avoid crashing into walls and other objects.

• Monitor’s average velocity to detect when the bot is “stuck”, then uses a “random wiggle” to try to jerk free. (I have never seen this happen)

Using the tools to build paths. (Source: Mike Booth’s presentation at GDC)

Using and viewing the environment

• Nav meshes include “hiding spots” that are labeled by the programmer.

• When retreating or “camping” the bot will find a hiding spot to ambush the opponent.

• Looks around smoothly and avoid jerky movements.

• Looks around corners and checks openings.

Tactics

Tactics and Bot emotions

• Nav Areas hold a danger value. • Increased danger means increased cost (A*)• This forces bots to change routes.• Danger values are increased when bad things

happen (such as team mates dying)• Morale determines level of aggression.• High morale: Bot is on winning team, bot is

killing enemies.• Low morale: Bot is on losing team, bot is killed,

hurt.

Combat and Weapon selection

• Since it is a rule based system, there are an elaborate collection of rules to determine combat actions.

• Examples:– Using the appropriate weapon for the range: (Sniper

rifles if enemy is far, pistol if close.)

– Aiming for the head.

– How to use grenades effectively (flush out enemies if attacking, slow them down if defending)

Team awareness

• Follows the leader (you)

• Communicates enemy locations

• Uses stealth (sneaking slowly to reduce noise) when heavily outnumbered

• Avoids stupid problems like shooting teammates, (will yell at you if you do).

Effects on Gameplay

• Why make a bot for a multi-player game?– Some servers don’t have a lot of players on

them. Bots can fill in the gaps to make a full team.

– Lousy internet connection wont prevent players from enjoying game.

– Bots don’t accuse you of cheating when you kick their ass.

Strengths and weaknesses

• Strengths:– Bot is almost convincingly human.– Bot difficulty is scalable to match different skill levels.– Bots don’t change flow of game, they are just like other

players.

• Weaknesses– Bots can see perfectly in dark areas.– Bots often prioritize killing enemies over completing

mission objectives. (Then again, so do most human players.)