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K.Dot

K.Dot

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The AI of Street Fighter 2

By today’s standards, the Artificial Intelligence in Street Fighter 2 World Warrior isn’t very sophisticated. These days, when most people talk about AI they’re talking about machine learning. There’s not any of that in SF2. Anyone looking for some insight into how to write an AI engine for a game today will be disappointed.

The AI engine has three main modes of operation:

Waiting for an attack. Simple scripts are chosen at random which consist mainly of walking backwards/forwards small amounts.
Actively attacking. Scripts such as the one above are selected.
Reacting to an attack. Scripts suitable for countering the attack are selected. Sometimes. Depending on the AI difficulty setting the computer lets plenty of attacks through unguarded, of course.
For the first two modes, there are 8 levels of scripts, which are chosen based on how much time is left in the round. When reacting to an attack the scripts are chosen based on something called a yoke.

Each frame of animation for both avatars and projectiles contains a value for the yoke in the metadata, which the AI peeks at to select a script suitable for responding to that attack. The computer sees the yoke of your move as soon as you have input it, before the first animation frame has even displayed. As such it gets one more frame of advantage on top of your reaction time.

A question addressed in an earlier post in this blog was whether the AI “cheats”, and it certainly does. Charge moves such as blade kicks are simply executed as instructions, so they cannot fail. Guile can do a bladekick from a standing position simply because that’s what’s in the script. It’s probably possible for the AI to command special moves in the air. One instruction is available to disable collisions against the computer player for a certain number of frames. Using it, you could write a script to simply walk through an approaching fireball
Click to expand...

The AI Engine
 
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