What is motion capture and how does it work?

Motion capture is a popular cutting-edge technique with a broad range of uses across film and TV, science and gaming. In this article, we’ll examine the history of motion capture technology and how it’s used. Let’s take a look at the different approaches, the advantages and disadvantages and how to do motion tracking.

What you’ll learn:

What is motion capture?

Motion capture or mo-cap, is the name for technology that records the movement of people or objects. The motion is captured by the tech and the data is transferred to a computer program to enable photorealism in a virtual environment.

Mo-cap has become increasingly popular in recent years, as a method of imbuing computer-generated 3D characters with the natural and often subtle movements/mannerisms of human actors.

Motion capture sessions involve the movements of actors many times per second. Only the movements are recorded - not their real visual appearance. The animation data is then mapped to a 3D model which is enabled to perform the actions that were captured.

What is motion capture used for?

You’ll find motion capture used in a variety of industries. These include:

  • Films and TV. These are the most well-known examples of mo-cap. It’s often used in previsualisation (previs) during pre-production. Motion capture brings storyboards to life - helping directors to block out motion within a scene and prepare for visual effects (VFX).
  • Video games. Motion capture animation is a quick and effective way for game developers to build up a large library of motions for their characters.
  • Health and sports. Mo-cap is used to diagnose injuries and to help with rehabilitation.
  • Military. Combined with virtual reality (VR), motion capture can enhance training.

Types of motion capture.

There are four different ways to use motion capture technology:

  • Optical passive. This is the most flexible and commonly used method. Optical passive uses infrared cameras to track retroreflective passive marker motion capture.
  • Optical active. Special cameras track LED motion capture markers as they emit light.
  • Video/markerless. Instead of markers, this technique relies on software.
  • Inertial. The subject wears inertial sensors (also known as IMUs). These sensors transmit data wirelessly to a computer or smart device. Cameras are only used as a localisation tool.

Advantages and disadvantages of motion capture.

Mo-cap is an excellent tool for many reasons, but there are disadvantages as well as advantages to this technology.

Advantages

Disadvantages

Low latency (almost real time) results, reducing the cost of keyframe-based animation.
Specific software and hardware needed to obtain and process the data.
The amount of work it takes isn’t dependant on complexity or performance length as traditional techniques. Different styles and deliveries can be tested, with personality only limited by the actor’s talent.
If making a small production, cost of equipment and software can be prohibitive.
Realistic, complex interactions and movements (e.g. weight and exchange of force) can be easily recreated in an accurate way.
Camera field of view or magnetic distortion may affect the capture system’s specific requirements.
Large amounts of animation data can be produced when compared to traditional animation, helping with meeting deadlines and cost effectiveness.
If there are problems during shooting, few mo-cap systems can allow real-time viewing to decide if a retake is needed, so it’s easier to reshoot.

History of motion capture animation.

1878: English photographer Eadweard Muybridge made a series of photos known as The Horse in Motion. These pictures of the movement of a horse in rapid succession were the first examples of chronophotography, which proved to be an important part of the development of motion pictures.

1915: American animator Max Fleischer, who went on to create the Betty Boop cartoons, invented rotoscoping. This animation technique was used to add realism to the movement of characters by using live-action film footage to paint over frames.

1939: Walt Disney Studios’ Snow White and the Seven Dwarves became the first film to use rotoscope.

1959: American animator Lee Harrison III created a bodysuit, lined with adjustable resistors called potentiometers, which enabled him to record and animate the movements of actors. The first example of real-time mo-cap.

1980s: Animators used large cameras to film actors in bodysuits lined with active markers.

1999: Star Wars: Episode 1 - The Phantom Menace was released, featuring the first fully-computer-generated supporting character in a feature film - Jar Jar Binks, played by Ahmed Best.

2002: Motion capture as we know it today was used in the film The Lord of the Rings: The Two Towers. Andy Serkis, as Gollum, wore a mocap suit on location, with special cameras recording his movements and facial expressions.

2004: American director Robert Zemeckis’ The Polar Express became one of the first major films to be shot entirely using motion capture technology.

2009: Canadian filmmaker James Camerons’s Avatar uses a new virtual camera system to take motion capture in film to a new level.

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How to do motion tracking.

Here’s a step-by-step guide to motion tracking using Adobe Premiere Pro:

  1. Import your footage into Premiere Pro.
  2. When it appears on the timeline, press File in the top menu.
  3. Select New and then Title. When the pop-up appears, press OK.
  4. A window will open. Choose the reference point you require and start typing. Close the window and drag the title on the timeline.
  5. Go to Effects. In the Effects Controls tab, select the stopwatch icon next to Position and set the keyframe.
  6. Move the footage forward by around 5-10 frames. Do this by holding down Shift and pressing the Right Arrow. The reference point will have moved.
  7. Drag the X and Y co-ordinates over the position keyframe panel. The respective Title will be moved and track to the new reference point.
  8. Repeat steps 6 and 7 until the process is done.
  9. Double-click the Title and type in the desired text to make it appear there.

Motion capture FAQs.

Is motion capture considered animation?

The Academy’s definition states that motion capture is not an animation technique by itself. A significant number of major characters must be animated and at least 75% of the running time should be animated. This helped the Academy to define animation at the same time as several films, including Avatar, District 9 and A Christmas Carol, blurred the boundaries of motion capture and animation.

What are some examples of motion capture?

In addition to the films already mentioned, motion capture also features in:

  • The Mummy (1999)
  • Hulk (2003)
  • Pirates of the Caribbean: At World’s End (2007)
  • The Curious Case of Benjamin Button (2008)
  • The Avengers (2012)
  • Grudge Match (2013)
  • Paddington (2014)
  • Fantastic Four (2015)
  • Guardians of the Galaxy (2017)
  • A Quiet Place (2018)
  • The Lion King (2019)
  • Ghostbusters: Afterlife (2021)

What are the three goals of a motion capture system?

The three goals of motion capture are:

  1. Sensing the motion
  2. Processing the sensor data
  3. Storing the processed data

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