MATCHMOVING

What is Match moving?

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

2D vs. 3D

Match moving has two forms. Some compositing programs, such as Shake, Adobe After Effects, and Discreet Combustion, include two-dimensional motion tracking capabilities. Two dimensional match moving only tracks features in two-dimensional space, without any concern to camera movement or distortion. It can be used to add motion blur or image stabilization effects to footage. This technique is sufficient to create realistic effects when the original footage does not include major changes in camera perspective.

Three-dimensional match moving tools make it possible to extrapolate three-dimensional information from two-dimensional photography. These tools allow users to derive camera movement and other relative motion from arbitrary footage. The tracking information can be transferred to computer graphics software and used to animate virtual cameras and simulated objects. Programs capable of 3D match moving include:

Automatic vs. interactive tracking

There are two methods by which motion information can be extracted from an image. Interactive tracking, sometimes referred to as "supervised tracking", relies on the user to follow features through a scene. Automatic tracking relies on computer algorithms to identify and track features through a shot. The tracked points movements are then used to calculate a "solution". This solution is composed of all the camera's information such as the motion, focal length, and lens distortion.

The advantage of automatic tracking is that the computer can create many points faster than a human can. A large number of points can be analyzed with statistics to determine the most reliable data. The disadvantage of automatic tracking is that, depending on the algorithm, the computer can be easily confused as it tracks objects through the scene. Automatic tracking methods are particularly ineffective in shots involving fast camera motion such as that seen with hand-held camera work and in shots with repetitive subject matter like small tiles or any sort of regular pattern where one area is not very distinct. This tracking method also suffers when a shot contains a large amount of motion blur, making the small details it needs harder to distinguish.

The advantage of interactive tracking is that a human user can follow features through an entire scene and will not be confused by features that are not rigid. A human user can also determine where features are in a shot that suffers from motion blur; it is extremely difficult for an automatic tracker to correctly find features with high amounts of motion blur. The disadvantage of interactive tracking is that the user will inevitably introduce small errors as they follow objects through the scene, which can lead to what is called "drift".

Professional-level motion tracking is usually achieved using a combination of interactive and automatic techniques. An artist can remove points that are clearly anomalous and use "tracking mattes" to block confusing information out of the automatic tracking process. Tracking mattes are also employed to cover areas of the shot which contain moving elements such as an actor or a spinning ceiling fan.