Understanding Motion Capture for Computer Animation

Alberto Menache

• Editore: Morgan Kaufmann
• Anno: 2011
• ISBN:
  9780123814975
• Edizione: 2
• Argomento: Computer Science
• Categoria: Computer Graphics

Understanding Motion Capture for Computer Animation discusses the latest technology developments in digital design, film, games, medicine, sports, and security engineering. Motion capture records a live-motion event and translates it into a digital context. It is the technology that converts a live performance into a digital performance. In contrast, performance animation is the actual performance that brings life to the character, even without using technology. If motion capture is the collection of data that represents motion, performance animation is the character that a performer represents.

The book offers extensive information about motion capture. It includes state-of-the-art technology, methodology, and developments in the current motion-capture industry. In particular, the different ways to capture motions are discussed, including using cameras or electromagnetic fields in tracking a group of sensors. This book will be useful for students taking a course about digital filming, as well as for anyone who is interested in this topic.

• Completely revised to include almost 40% new content with emphasis on RF and Facial Motion Capture Systems
• Describes all the mathematical principles associated with motion capture and 3D character mechanics
• Helps you budget by explaining the costs associated with individualized motion capture projects

1. Motion Capture Primer

Chapter Outline

Motion Capture and Performance Animation2

History of Performance Animation in the Entertainment Field3

The Rotoscope3

Brilliance4

Pacific Data Images7

The Jim Henson Hour7

Exoskeleton10

deGraf/Wahrman11

Kleiser-Walczak Construction Company12

Homer and Associates13

Party Hardy13

Steam14

Motion Capture Service Bureaus16

Types of Motion Capture16

Optical Motion Capture Systems17

Advantages of Optical Systems20

Disadvantages of Optical Systems21

Radio Frequency Positioning Systems22

Global Positioning System (GPS)22

Real-Time Location Systems (RTLS)23

Local Positioning System24

Electromagnetic Trackers26

Advantages of Magnetic Trackers28

Disadvantages of Magnetic Trackers28

Electromechanical Performance Capture Suits28

Advantages of Electromechanical Body Suits30

Disadvantages of Electromechanical Body Suits30

Digital Armatures30

Facial Motion Capture Systems31

Facial Muscle and Skin Simulators32

Dynamic Blend Shapes33

Computer Vision33

Other Motion Capture Systems34

The Waldo34

Hand Trackers35

Applications of Motion Capture37

Medicine37

Sports39

Entertainment41

Video Games41

Television42

Feature Films43

Law44

Security and Defense44

Engineering45

Future Applications45

Motion Capture and Performance Animation

Motion capture is the process of recording a live motion event and translating it into usable mathematical terms by tracking a number of key points in space over time and combining them to obtain a single three-dimensional (3D) representation of the performance. In brief, it is the technology that enables the process of translating a live performance into a digital performance. The captured subject could be anything that exists in the real world and has motion; the key points are the areas that best represent the motion of the subject's different moving parts. These points should be pivot points or connections between rigid parts of the subject. For a human, for example, some of the key points are the joints that act as pivot points and connections for the bones. The location of each of these points is identified by one or more sensors, markers, or potentiometers that are placed on the subject and that serve, in one way or another, as conduits of information to the main collection device. From now on, when speaking generally about these, I will refer to them as “markers.”

Performance animation is not the same as motion capture, although many people use the two terms interchangeably. Whereas motion capture pertains to the technology used to collect the motion, performance animation refers to the actual performance that is used to bring a character to life, regardless of the technology used. To obtain it, one must go through the whole process of motion capture and then map the resulting data onto a 3D character. In short, motion capture is the collection of data that represents motion, whereas performance animation is the final product of a character driven by a performer.

There are different ways of capturing motion. Some systems use cameras that digitize different views of the performance, which are then used to put together the position of key points, each represented by one or more reflective markers. Others use electromagnetic fields or ultrasound to track a group of sensors. Mechanical systems based on linked structures or armatures that use potentiometers to determine the rotation of each link are also available. Combinations of two or more of these technologies exist, and newer technologies are also being tested, all aiming for one result: real-time tracking of an unlimited number of key points with no space limitations at the highest frequency possible with the smallest margin of error. This is the Holy Grail of motion capture and probably the mission statement of every motion capture hardware manufacturer's research department. I later discuss how each of the current technologies falls short in this respect.

History of Performance Animation in the Entertainment Field

The Rotoscope

Motion capture in the entertainment field is the descendant of rotoscoping, a technique still used by some traditional animation studios to copy realistic motion from film footage onto cartoon characters.

The rotoscope device was invented and patented by cartoonist Max Fleischer in 1915, with the intent of automating the production of cartoon films. The device projected live-action film, a frame at a time, onto a light table, allowing cartoonists to trace the frame's image onto paper. The first cartoon character ever to be rotoscoped was Koko the Clown. Fleischer's brother, Dave, acted out Koko's movements in a clown suit. Fleischer wanted to use Koko to convince the big studios to use the new process for their cartoon projects. The sale was difficult because it had taken Fleischer about a year to produce the initial 1-min cartoon using the technique, so he couldn't market it as a mass production tool. Eventually, Fleischer realized that rotoscoping would be a viable technique only for certain shots that required realistic motion.

Walt Disney Studios used some rotoscoping in 1937 to create the motion of human characters in Snow White. Snow White herself and the Prince were partially rotoscoped. The decision to use rotoscoping wasn't a matter of cost, but of realistic human motion. In fact, Snow White went tremendously over budget due to the complexity of the animation.

Rotoscoping has been adopted over the years by many cartoon studios, but few actually admit using it because many people in the animation industry consider it cheating and a desecration of the art of animation.

A two-dimensional (2D) approach, rotoscoping was designed for traditional, hand-drawn cartoons. The advent of 3D animation brought about the birth of a new, 3D way of rotoscoping. Hence, motion capture.

Brilliance

Some of the current motion capture technologies have been around for decades, being used in different applications for medical and military purposes. Motion capture in computer graphics was first used in the late 1970s and early 1980s in the form of research projects at schools such as Simon Fraser University, Massachusetts Institute of Technology, and New York Institute of Technology, but it was used in actual production only in the mid-1980s.

In late 1984, Robert Abel appeared on a talk show and was asked if he would soon be able to counterfeit people digitally. “We are a long ways from that,” he replied. “We haven't even figured out human motion, which is the basis, and that's a year away.” A week and a half later, Abel received a visit on a Friday afternoon from a creative director from Ketchum, a prominent advertising agency. The visitor brought six drawings of a very sexy woman made out of chrome. She was to have Kathleen Turner's voice and would be the spokesperson for the National Canned Food Information Council, an association formed by Heinz, Del Monte, Campbell's, and a number of big players that sold canned food. They felt they had to make a powerful statement because the idea of buying food in cans was becoming obsolete, so they wanted to do something really different and outrageous, and they wanted it to air during the Super Bowl in January 1985. “Can you do it?” asked the client. “You're certainly here a lot earlier than I would have planned,” replied Abel, and asked the client to wait until the end of the weekend for an answer.

At that time most computer graphics consisted of moving logos, landscapes, and other hard objects, and Robert Abel and Associates had already become a player in that market, along with MAGI, Triple-I (Information International, Inc.), John Whitney's Digital Productions, and PDI, all of which had their own proprietary software, because at that time there was almost no off-the-shelf animation software and whatever was available was still in its infancy. Abel's software was initially based on bits and pieces from Bell Labs, Evans and Sutherland, JPL, and other places, and was augmented over time by his group.

The next step would be to animate a digital character. “For storytelling, which was really our goal, we had to have human characters,” recalled Abel, “because nobody better than a human character is able to convey emotion and story. We come from a long line of storytellers that go back maybe 35,000 years, and although the forms may change from cave paintings to digitally made motion pictures, it's still the same thing, it's the passing on of stories.” Creating the first animated digital character would open a Pandora's Box of many new challenges, such as creating realistic skin, hair, and expression. But first they had to deal with the motion.

Abel and his team decided to lock the doors and not leave the building until Monday morning. If by then they didn't have the solution figured out, they would have to pass on the project. Robert Abel and Associates' background in shooting miniatures with motion control cameras since the late 1960s and early 1970s was the key to the solution. They knew that the answer to their problem would have to do with motion and control, except this time it would be human motion. Keyframe character animation was not an option at the time, so they decided to find a way to track the motions of a woman acting the part of the character. It made sense to shoot the woman with several cameras from different points of view, and then use this footage to create a motion algorithm.

Seven people worked throughout the weekend. “Several of us got into our underwear,” recalled Abel. “We got black adhesive dots and we put them on our bodies and we would photog...