Cleared Hot

Welcome back, intrepid pilots and nostalgia seekers. Today we have a deep dive into the world of NPC AI. But first allow me to tell you a story about the first AI behavior I ever wrote.

We have to go way back to a time so shrouded in nerdiness and embarassment that nobody in their right mind would ever revisit it. Computer camp.

Yes, 13 year old me learned to program at a week long camp surrounded by other uber nerds whose collective abilities quickly showed me I was not the smartest one in the room. Where I learned that you could actually be tired from just thinking all day. Where I learned that losing in Risk really sucks.

Our first task was to create a tic-tac-toe game in C++ (I really wish Python existed back then), and then create an AI player who would never lose. It was a monstrous if-else disaster of listing every possible case of what the AI should do. Eventually I got it working and it would never lose. It was a “perfect” AI player.

Soon I discovered that the game was no longer fun to play. Something about that stuck with me; a fun opponent is imperfect. If you’ve ever said “the AI is cheating” in a game, you know what I mean.

So the goal is to make AI that is human-like, instead of god-like. Well, let’s start with Behavior Trees.

Behavior Trees

Behavior Trees are basically a flow chart of decisions and actions. For example:

My first take on AI in Cleared Hot was pretty simple: If you see an enemy, shoot them. If you have a destination, walk towards it. If you were a friendly unit and the helo landed near you, get in. It was governed by a Behavior Tree.

Sounds pretty simple, right? Well here is what the behavior tree looked for this simple behavior.

The real culprit was this: Combining State and Actions in one tree. For the non-programmers, state is something you have to keep track of about the world, that will determine what you do. For example: Am I currently attacking, am in the helo or not, etc. So every time the tree was executed, you would have to dive into the right sub-tree for that state.

Behavior Trees inside State Machines

After experiencing enough pain with the behavior tree approach, I stumbled upon a GDC video. In this talk, Bobby Anguelov mentions an approach that uses State Machines for… state, and Behavior Trees for actions. This is based off his experience creating AI for the Hitman series! Pretty cool.

This approach made so much sense to me that I figured it was worth implementing before I added anything new to the NPCs. So I build a simple version. The NPCs have a small set of states:

• Idle
• Attack
• Flee
• Hit
• Dead

Each state contains it’s own behavior tree, that is set up when entering that state, and cleaned up when leaving. These behavior trees are just actions. They aren’t making decisions and they aren’t running any sensors. They are just executing actions. This cleans things up a TON. Look at the attacking behavior tree, which is currently the most complex one:

The really cool thing about this is that you can update your knowledge separate from running the behavior. Previously the NPCs were doing raycasts everytime they executed the behavior tree, but now they only need to update their knowledge base at the speed of a human reaction, and yet they can drive their behavior trees much faster.

Perception and Knowledge

In order to make decisions, the state machine needs some knowledge of the outside world. This is done by implementing “sensor” scripts that allow the NPC to see or hear things in the world. Each NPC updates their knowledge base about every 200-250ms (typical human reaction time) and will change state if needed.

These sensors react to “Stims” or AI Stimulations, which are just things you can drop in the world for an action that you want the NPCs to know about. So far I’m using these for: shots being fired, shots impacting a surface, explosions, all NPC and player positions, etc.

A trail of minigun bullet impacts create “Stims” in the world that NPCs can see and hear.

The game also adds some randomness into their update timing, so it tends to spread the processing time across frames, making the game run smoother. I’ve yet to fully stress test this, but I’ve put about 50 NPCs in view at once with no problems.

My favorite example of new behavior is the “RPG Warning”. When an enemy is about to fire an RPG (or other high damage weapon), they create a “RPG about to fire” Stim. If another NPC can see that, they will play an RPG warning voice line (RPG!!!) which also creates an audio stim. If another NPC sees the RPG enemy but already heard someone else yell the warning, they won’t repeat it.

All of this makes it possible for the NPCs to react to things in the world. Like being shot at.

Thank You

If you read this far, you need to be subscribed to my email list! https://clearedhotgame.com/email-list/

This is where I will release the first play test builds! I’ve already been interacting w some of you over email. Thanks for your thoughts and feedback, your emails are super motivating.

I am back to working on the hard to define task of “game feel”. This involves making the things you do over and over feel… good.

One of my main playtesters is a friend I’ve known since 1st grade. I’ve played every generation of video game with him. We’ve been comrades in arms in n64 goldeneye, being repeatedly gunned down by his older brother, I think we actually got into a fight one time over Tekken3 or maybe some Dreamcast boxing game, and we’ve shut down entire battlefield servers as helicopter pilot & copilot.

So when I send him a build, I can easily tell if it “feels good” or not. The last time he played, it was clear something was missing from the minigun. He sent me back a gif from Blackhawk Down and said “this is how I want to feel”. Alright. Mission accepted.

Game feel is more of an art than a science… The best way to improve it is just to experiment with a bunch of things.

So let’s get our paint out and Bob Ross this minigun, shall we?

Making the minigun feel more powerful

The Before

Screen shake + Barrel Flash Effects

Screen shake is a common game feel tweak- you make the camera move slightly in reaction to powerful events in the game scene.

So much better already. even without audio it’s clear when you’re shooting.

Thankfully the asset we’re using for camera work (Cinemachine) has a nice system for adding screenshake impulses to events. All we have to do is define the rotation + position waves that shake the camera.

As you can see, the camera shake involves 3 different frequencies rotating on the 3 separate axes. This gives a good shake effect that feels natural without having a weird oscillation in only one direction.

I decided not to use any position shake, since that tends to be more jarring than rotation.

Splash Damage + Spread

Part of the issue I wanted to fix was how lame it felt when you shot right next to an enemy, and nothing happened. The easy improvement here was to add splash damage to the rounds. So now when a shot impacts a surface, it tries to add damage directly to what it hit, and then uses the existing splash damage system to add damage to anything in a 2 meter radius around it.

The other part of this was that the rounds would go EXACTLY where you were aiming. This sounds like a good thing, but it looked and felt strange. So I added an angle of spread that can be adjusted.

Better Tracers

Ok I have to admit something. Before now the tracers didn’t actually go where the bullets were hitting. Forgive me. It was good enough at the time but not good enough anymore. Even worse, they would go through surfaces that stopped the bullets. This irritated me for awhile and I was so happy to fix it.

Now the tracers actually follow the path of the bullets, and stop where the bullets stopped. This means the spread of the minigun is directly visible in your tracers.

Aiming Parallel

Previously both miniguns (left and right) would aim towards a single point.

By keeping them parallel, you increase the actual area covered by their shots, and I think it just looks better too.

I also improved the look of the lasers which designate their aim, and added small laser dots to the end. This makes it clear to the player where the shots will land, if they’re blocked by an object, and it responds slightly to changes in the helicopters angle.

NPCs are now scared of your bullets

Another way to make something feel powerful, is to make the NPCs afraid of it!

This took an entire rewrite of the NPC AI. Which was on my list for a long time and this was reason enough to do it. More on this next time, it will probably be the topic of the next devlog.

Each NPC has sensors that it can use to make decisions about what state to be in. If they see or hear a shot near them, and don’t see a current target, they will run away. This uses a common method of adding AI Stimulations or “Stims” to the world that each NPC can choose to react to.

A fun bug that exists currently: The NPCs don’t differentiate between their own shots and their enemies shots. So they can essentially shoot and then startle themselves, and run away. I’m working on a fix to give them a bit more courage.

More Bass

The minigun sound effect doesn’t have much in the low frequencies, and it’s hard to really distinctly feel when the individual rounds are being fired. I experimented with adding a small bass pop when the gun actually fires each bullet. It’s subtle, but it feels a lot better.

I don’t have a gif for this. 😉

Recoil

This one is subtle… I wonder if you noticed it in the above gifs? The minigun has a bit of recoil that it exerts onto the helicopter when firing. Notice how in the before gif, the helicopter is totally stationary, but in the other gifs it’s moving slightly backwards.

I think I will use this more for the minigun upgrades in the game. If you unlock a more powerful gun, it might have higher recoil.

In Summary

Tell your friends!

Know someone that would like this game? Ask them to wishlist it + follow here: https://store.steampowered.com/app/1710820/Cleared_Hot/

Wishlists help me a ton. And thank you all for your continued support. I’ve heard from a bunch of you over email and twitter and it helps a lot to know you’re along for the ride!