Match Move

Directors and cinematographers hate to lock off the camera. They love to swoop the
camera through the scene, boom it up over a crowd, or swing it around the heroic
character. This is fine if the shot is all live action, but it wreaks havoc if the shot
requires CGI to be mixed with the live action.
If the live-action camera is moving, then the CGI camera must also move so that the CGI will match the changing perspective of the live action. Not only must the CGI camera move, but it must move
in perfect sync with the live action camera or the CGI element will drift and squirm
in the frame and not appear to be locked with the rest of the shot. Special matchmove
programs are needed to pull off this bit of cinematic magic. An example of a
match-move shot is shown in Figure  where the camera orbits around the liveaction
soldier. The soldier is in an all CGI environment.

Match move is a two-step process.

First,
the live-action plate is analyzed by the match-move program. A plate is simply a shot that is intended to be composited with another shot. The match-move program correlates as many features as it can between
frames and tracks them over the length of the shot. This produces a 3D model of the terrain in the scene, as well as camera move data. The terrain model is low-detail and
incomplete, but it does provide enough information for the next step.

The second step is to give the 3D terrain information and the camera move data
to the 3D animators. They use the terrain information as a guide as to where to place
their 3D objects. The camera move data is used by the computer camera to match
the move of the live-action camera in order to render the 3D objects with a matching
perspective that changes over the length of the shot. If the terrain information
or camera move data is off in any important way, then the 3D objects will scoot and
squirm rather than appear to be fi rmly planted on the ground.

After all the 3D elements are rendered, the live-action plate and the CGI arrive
at the digital compositor for fi nal color correction and compositing. Of course, the
compositor is expected to fix any small (or large) lineup problems in the CGI or live
action. The match move game can be played in two fundamental ways: Live action
can be placed in a CGI environment, or CGI can be placed in a live-action environment, like King Kong in New York City. Either way you work it, match move is truly one of the great wonders of visual effects.