DEV DIARY - AI AWARENESS / Dev Diary / 5/21/21



I’m an AI Tech Designer on Back 4 Blood. I was asked by our wonderful Community team to write up a short article about the AI in Back 4 Blood. 

I hit up our DISCORD SERVER and asked the community what sort of things they wanted to know about. The overwhelming majority wanted to know about AI targeting: how the player gets targeted by AI and what the rules are for targeting the player. Before I get into that, there is a whole other system that you need to know about: Awareness. 


Here’s how it works, for the most part. When a Ridden spawns into a level it initiates its Wander behavior. While it’s Wandering it is completely unaware of the player and every once in a while, picks a random spot within a range and moves there. 

The next behavior it does is called Alert. Alert is usually triggered by 3 things: Sight, Sound, and Damage. Each Ridden (that has eyes/can see) has vision cone data. To oversimplify, if you enter this vision cone, the zombie sees you and targets you. Ridden also have a hearing range that differs per variant. Cleaner actions generate noise; shooting generates a lot of noise unless you have a suppressor, each step a player takes generates noise, opening doors, etc. 

If the noise radius overlaps the Hearing radius of the Ridden it will become Alerted and move toward the location of the noise. And of course, if you shoot a Ridden they will target you directly. There are other things that generate noise, but these are the primary sources. 

Here is a shot of a Common Ridden ambling along. The Yellow radius is her “Hearing” radius. The Green radius and cone displays her “Sight”. That Orange radius right next to her? If you enter that radius she will become aware of you no matter what. 

Some AI interact with the Awareness System differently. Common Ridden (the AI you kill a billion of in every map) just move to the Alert location and mill about if they don’t find an enemy. Other AI go into an Ambush behavior, like the Retch. This means they know the player’s location as soon as they become alert and try to “hide and wait” for the player. The Snitch will just ...scream. No matter where it is, if it gets alerted you’d better kill it quick. You might be saying, “Yeah that’s really cool, Adam, but I asked about targeting.” Well, the Alerted behavior has a direct impact on how an AI gets its target. 

Once the Ridden has a target, each one reacts a little differently. Common Ridden just move along the shortest path to get to their target and will constantly reevaluate if a Cleaner is closer, then retarget that Cleaner. Some, like the Retch and ... others you haven’t been introduced to yet, go into “Ambush” mode. They will try to find places to hide from their target and attack from there. Other Ridden types you haven’t seen yet can target a Cleaner and stick to that target until they deal damage. 

Here is a shot of a Reeker in its “Ambush” behavior. 

I guess that was just a really long way of saying, “they usually target the closest player” but keep in mind, not all AI behave the same and we haven’t revealed all the AI in Back 4 Blood yet. When we do, I’ll be back to talk about them!