Every time I watch those videos of people in the Motion Capture suits (whether they're ready to film a scene for a movie or fight someone for a video game or whatever), I always am amazed at the direction technology is headed in!
For those who don't know, Motion Capture is defined as the process of recording the movements or actions of human actors for use in animating digital characters. Motion Capture is widely used in the fields of entertainment (video games, movies, animations), sports, health, and even research!
The reason game developers sometimes use Motion Capture is because some animation sequences are extremely difficult to create; motion capture helps with complex character skeletal movement. Without Motion Capture, think about all the bones on the skeleton that the animators would have to move and time accordingly, depending on the type of animation used. It can be very overwhelming, especially since game development is challenging enough with the amount of programming, artwork, asset creation, music, and even testing! Lots of animators in video games use an animation process called "Keyframing", which is the process of filling in the movement of a character animation over time. Essentially, Keyframing is almost like taking a picture at one point in the animation, and then another picture at a later point in the animation, and automatically filling in the animation transition between these two animation poses in time.
Motion Capture involves a lot of acting. Physical actors have to put on suits that either optical or non-optical. Optical suits have tracking cameras that may or may not have markers on the suit, whereas Non-Optical suits have inertial or mechanical sensors (meaning detecting changes in motion). Optical systems with markers track the position markers on the actor's body (those ping-pong balls attached to the suits) and use the data that was tracked to try and approximate the actor's actual movements digitally on a virtual character. Markerless optical systems use algorithms to find certain types of features on the actor's body, such as his/her nose, mouth, or clothing. Once this data is captured, it is then mapped onto the virtual character's skeleton using software. Usually developers will use "Virtual Cinematography" on set, which allows them to see the animation footage previewed on their computer monitors before assuming it is good to use in the game; otherwise, you wouldn't know if the animation data transferred to the computer correctly until you replayed the footage hours later!
Since Motion Capture typically requires having a studio, motion capture suits, props, and software, it is very expensive to use. However, some companies have their own Motion Capture area, or they rent a place. Pricing has come way down within the last few years though, and you can buy markerless motion capture devices for around $300!
Here is the link of the article I used for reference:
https://www.engadget.com/2014/07/14/motion-capture-explainer/
Also, Here is a great video of the Motion Capture footage from Friday The 13th: The Game