October 19, 2019

3D Motion Tracking

In cinematography, match moving is a cinematic technique that allows the insertion of computer graphics into live-action footage with correct position, scale, orientation, and motion relative to the photographed objects in the shot. The term is used loosely to describe several different methods of extracting camera motion information from a motion picture. Sometimes referred to as motion tracking or camera solving, match moving is related to rotoscoping and photogrammetry.


Motion capture transfers the movement of an actor to a digital character. Systems that use tracking cameras (with or without markers) can be referred to as “optical,” while systems that measure inertia or mechanical motion are “non-optical.” An example of the latter is the XSens MVNinertial capture suit worn by Seth Rogan playing the alien in Paul. Other tech has emerged lately, like Leap Motion’s finger-tracking depth camera system and MYO’s wristbands, which detect muscle activity in the hands and wrists. Project Tango from Google is being used mostly for mapping, but with its Kinect-like depth sensors, it also has the potential for mocap.

Optical systems work by tracking position markers or features in 3D and assembling the data into an approximation of the actor’s motion. Active systems use markers that light up or blink distinctively, while passive systems use inert objects like white balls or just painted dots (the latter is often used for face capture). Markerless systems use algorithms frommatch-moving software to track distinctive features, like an actor’s clothing or nose, instead of markers. Once captured, motion is then mapped onto a virtual “skeleton” of the animated character using software like Autodesk’s MotionBuilder. The result? Animated characters that move like real-life performers.

It’s difficult to predict how an actor’s movement will translate to an animated character, so “virtual cinematography,” developed by James Cameron for Avatar, is often used. In a nutshell, that shows the digital character moving with the actor in real time — on a virtual set — so the director can see a rough version of the “performance.” That involves plenty of math, but computers and graphics cards are now fast enough to pull it off. The video below from Weta Digital for The Hobbit: The Desolation of Smaug illustrates the process.


Are you looking to get into 3D animation and/or motion capture? There’s lots of free stuff! Autodesk will let you try most of its programs without restrictions free for 30 days (students get it free for three years) —MotionBuilder and Maya or 3DS Max are good places to start. Autodesk also has plenty of tutorials and tips in its Area section. If you have a couple of Sony PS Eye cams or a Kinect lying around, you can also play with a free trial from iPi Soft. To skip that part and try motion files that have already been captured, Carnegie Mellon University has thousands of them here, and you can use a viewer like this to preview them. If you’re just a fan of 3D and mocap, you can check the sites for digital effects and gaming shops like Weta Digital, EA’s Capture Lab, Digital Domain and of course,Industrial Light & Magic.

Skills & Education

A career as a 3-D motion tracker requires training in visual effects techniques and appropriate software. A college degree in computer animation or related field is the typical path toward employment. Visual effects artists, whatever their specialties are, must have a solid education in composition, lighting, perspective, and similar artistic aspects of cinematography. While the computer lifts most of the burden of mathematical calculations off the artist, it is beneficial to understand the algorithms at work in the motion tracking process. The techniques of the craft are constantly evolving and becoming more sophisticated, so the more the artist grasps of the equations involved, the more he or she is able to contribute to the advancement of motion tracking in cinema.

What to Expect

The role of the 3-D motion tracker is considered an entry-level position within a visual effects studio. A rookie may be recruited directly out of college based on his or her VFX reel or may graduate to the role from that of render wrangler. On the job, the work of the other artists and animators further down the post-production pipeline is dependent on the motion tracker’s initial process; this means the artist must show a keen attention to detail and a meticulous work ethic. A motion tracker who can deliver clean tracks efficiently will prove invaluable to the studio, thereby moving another step closer to promotion. Career advancement will depend on your specific skills and artistic or technical talent, but related careers include rotoscope artist, match mover, lighting artist, texture artist, and animator.


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