Unreal Engine Experiments: Last Known Position Visualization

The blog has been dark for a while now. But the past few months have been a quite fun experience as I got to experiment with a whole host of interesting gameplay systems in Unreal Engine. And I have to admit that the prospect of writing about them is not nearly as exciting as working on them. But I have finally summoned the willpower to get one article published over this weekend. So I figured that I'll go ahead and write about the most exciting project that I've worked on (since the recreation of Blink ability from Dishonored): the Last Known Position mechanic from Splinter Cell Conviction.

As the title suggests, we're going to cover the process of visualizing the player character's last known position (as perceived by the AI). The mechanic itself should be quite familiar to those who have played either of the last two entries in the Splinter Cell franchise. But in case you're not, here is a short animated preview of what exactly the end product is going to look like:

 
Alright, so with that out of the way, let's get into the nitty-gritty of the experiment. Basically, there are three main steps required for implementing the visualization system: 

• Create a translucent silhouette material
• Setup an animation pose capture & mirror system
• Implement a basic AI perception system for tracking purposes

Now let's go over each of them in order, starting with the material creation process.

Silhouette Material

I started out with this because I had absolutely no clue how to get this working. So if anything was going to be a showstopper, it was probably going to be this one. I mean you can't just throw in a basic translucent material and call it a day. The material script also needs to be able to cull the inner triangles of the mesh. Being a complete noob at materials, I turned to the internet for help. Thankfully, Tom Looman had already posted a custom depth based solution in his blog and it involves the use of two similar overlapping meshes: a translucent mesh rendered in the main pass, and an opaque one rendered in custom depth. Here is a preview of what the final result:

Well, with that taken care of, let's head over to the next step in the process.

Visualization Pose Capture

I'm not very familiar with the animation side of UE4, but this part of the process actually had a relatively more straightforward solution. While the first idea that came to my mind was to copy the player character's animation poses over to a new skeletal mesh component, I wasn't particularly keen on going down that route. The reason being that there was no real need for a full-fledged animation system for our visualization mesh. We just need to set a pose once and then forget about it. Fortunately, after doing some research, I stumbled upon this neat little thing called Poseable Mesh component.

The Poseable Mesh component was exactly what was required for this scenario. It was intended to be used for one and only one thing. To mirror a single pose from another skeletal mesh. No unnecessary features involved. And it comes with an inbuilt function that lets you do that by passing in a reference to the target skeletal mesh component. Just copy the target's transform coordinates as well and we're done.

And now on to the final part of the experiment.

AI Perception

I went ahead with Unreal's inbuilt AI Perception system for this one. I'm not going over the details here as there are quite a few good resources available within the community already. But the basic gist is that I'm using it to keep track of AI agents gaining/losing track of the player character.

With this information, we just plop down our visualization actor every time the player evades the AI. 

And there you have it: a recreation of the Last Known Position mechanic from Splinter Cell Conviction. Here is a video preview of the system in action:

With that, we have come to the end of another experiment. I've shared the project files on GitHub. So feel free to use it in your work. Also head over to my YouTube channel if you're interested in checking out more cool experiments in Unreal Engine.

Alright, so that's it. I hope to publish the next post sometime during the next weekend. Until then, goodbye.