How Motion Capture Works in Game Development

Motion capture, or mocap for short, is a process of recording movement and translating it into digital animation. It’s been used by the game industry since the 90s when Mortal Kombat was the first video game to copy an actor’s movements onto a digital character. In the last decade, motion capture technology for video games has come a long way and can now realistically recreate biped (humanoid) movements in 3D. In this blog post, we'll take a closer look at how motion capture works and is used in game development. We'll also explore some benefits of using motion capture technology in your video game versus keyframe animation techniques. 

Why is motion capture used in video games? 

For indie developers and small game studios, motion capture is an affordable and fast way to create animation and speed up the production process. It’s much quicker to record animation than manually key-frame it in the traditional method. Learn more about the difference between animation techniques for games. 

A full body performance capture solution will include:

• Physical mocap suits record the full-body movement of all limbs.
• Optical technology captures facial movements and expressions. 

Optical mocap can be used to capture full-body motion as well, but motion capture suits allow you to skip the complicated setup required for high-end optical mocap and record any extreme pose without concern for overlapping limbs and obstructed body parts. With the right setup, you can record motion capture animation and retarget it to your character rig in real-time. This makes for a streamlined workflow that reduces editing time and the need for additional retakes. 

Understanding the types of motion in game development

In game development, there are two approaches to making an animated character move. 

1. Animation-driven movement refers to a motion that follows the root animation. It looks more fluid and cinematic, but the player can’t control actions as fast. 
2. Script-driven movement is root movement that’s driven by code. Game players can have millisecond reaction times, but movements will feel jerky and not as fluid. This style is often used in first-person fighter games. 

The use of motion capture is suitable for all digital character animation, but knowing what movement style you’re using is crucial to your recordings. For example, animation-driven movement can afford your mocap actor to be more flowy or extravagant in their activity. On the flip side, when recording movement for a script-driven system, minimize all unnecessary actions to keep animation clean and sharp. 

In some cases, you might want to use both types of animation, considering you might need game character animation for the following purposes: 

• Player-controlled animations where the player is driving movement to achieve tasks. These are typically looped, linked, and one-shot animations for main character actions like walking, running, swinging a weapon, etc. 
• Non-playable characters (NPC) animations for background game characters that players interact with. The animation might be looped with special scripted events that randomize movement and allow for interactions with the environment. 
• Cinematic scene animations where you provide a cut-scene with storytelling elements. These animations are typically higher quality and take more finesse to create. 

How mocap impacts player feel, and fluidity

While feel refers to how much control a player ‘feels’ like they have over a character, fluidity is more about how well your animations blend when transitioning between animations. You can significantly impact these elements in-program, but a motion capture actor’s movements also play a large part in how a player ‘feels’ about a particular motion. For example, when trying to improve how "free" a player feels to move around in your digital world (aka feel), you should ask yourself the following questions: 

• Does this action add to the player feeling in control? 
• Does this movement communicate the impact it should? 
• Does this action add to the player’s fantasy of being the character? 
• Does this animation add to the character development? 

Fluidity, on the other hand, is a more systematic process. In an ideal world, you want to ensure your animations blend seamlessly with #unwanted deformation. To do this, we must build a “State Machine” capable of switching and blending looped and one-shot animations. The State Machine should respond to a player’s input and quickly combine animations and cycle through actions. For example, a character can cycle from walk → run →  sprint → walk → jump without any limbs snapping or looking off.

In some cases, you will need to keyframe in what’s called a “link” animation.  Links are in-between animations that smooth motion between actions that otherwise do not line up — for example, going from mid-jump to a power move with no in-between states would require a link animation.

Free download: The Ultimate Guide to Motion Capture in Game Development →

How does motion capture improve a game character’s readability? 

Readability refers to the player’s ability to understand what’s happening in-game at any point. You should always check for readability in any game where a player can see the CG character from multiple angles. Because a player can see movement at any camera angle and position, actions should be readable from any angle. A mocap actor must perform actions on the X, Y, and Z axes. Take a look at the example below.

Notice the character’s movement from the back angle is difficult to read. The player might not understand the action. We can easily fix this by asking our actor to swing wider, covering a broader range of the motion for the arm on the X axis. 

So now the movement is more readable, does it still have the right ‘weight’ or ‘feel’ behind it? Now you’re thinking like a game developer! Considering the character’s bulk, we may continue to iterate by asking our actor to lean back more with their upper body pre-swing. By reviewing all your captures with readability in mind when you’re recording mocap, you will cut down on retakes and be able to better communicate with your actor. There are a few other tips you can give your mocap actor (or yourself if you plan on recording your own animation): 

• Your character’s rig limitations, weight, and bulk, strength, size/scale to the surrounding world
• Physical limitations/augmentations such as armor or a limp
• Learn to visualize movement in 3D space by playing back takes on the actual character model and not just an armature.  

What does a full motion capture setup for games look like? 

Motion capture comes in two different forms; optical and inertial. Optical motion capture is the kind of tracking that uses cameras and brightly colored and reflective markers to translate motion into data. Inertial tracking typically comes as a mocap suit and uses tiny gyroscopes placed at key movement points to record movement into animation data. Inertial motion capture does not require specialist cameras, a dedicated operator and actor, or studio space. In indie mocap setups, a developer can throw on a suit, act, and record the motion all by themselves.

A typical motion capture setup will use inertial motion tracking for the body and hands and optical motion capture for the human face. Facial mocap can be done using something as simple as an iPhone or captured in ultra high-definition using a complex multi-camera setup. Your method depends on the degree of realism you want to achieve and your budget. Assuming you're looking for an affordable full-body setup, Rokoko’s full performance capture bundle offers excellent value to a small studio and is a great place to start. It contains the following: 

• Rokoko Smartsuit Pro: Full body motion capture suit
• Rokoko Smartgloves: Motion capture for hands
• Rokoko Remote: Software to capture facial expressions (requires an iPhone X or higher)
• Rokoko Studio Plus: Software to record motion capture data 
• Optional: A free retargeting plugin for your game engine (Unreal Engine / Unity) to record animation data right onto your character in real-time

You can get up and running in less than an hour. Here’s a video tutorial that shows the entire process: 

Learn more about motion capture for games

Want to continue learning about how to record excellent motion capture for games? Download this ebook: The Ultimate Guide to Motion Capture in Game Development.