Using tracking markers
By Eric Huyton (Lacock Films Ltd)
Allowing for camera movement
Green screen shots don't have to be static, and adding a camera move can really help to make the end result more believable. If you're planning on a camera move, then you need to think about using tracking markers.
When the computer software combines the foreground and background images, it's essential that they move together as though both elements were shot by a single camera at the same time. Tracking markers are small, high contrast reference points which are placed on the green screen background in clear view of the camera.
Tracking software picks out these reference points and uses them to interpret how the camera moved when you filmed the foreground element against green screen. The computer can then replicate an identical camera move around the backplate element so that both parts of the composited scene look like they were filmed at the same time and in the same place.
Where to get tracking markers
Sometimes, a simple piece of camera tape will suffice but these black and white "coin" markers work well and you can make your own or download them here.
How to place tracking markers
It's absolutely essential to place the tracking markers correctly. 10 or 15 minutes spent rehearsing the camera move and action and adjusting the marker positions can literally save days of time in post production.
Guidelines for placing tracking markers
- Markers do not have to be in any particular layout but should be spaced reasonably far apart
- There should be a minimum of 7 in shot at any one time
- Don't place them right at the edges of the frame
- If the studio is large, place markers on the back wall and add some in the middle distance
- Avoid markers passing behind the foreground object(s) *
- Especially avoid markers being near the actor's hands as these tend to move a lot
- Record the distance from the lens to the distant and mid distance markers
- Markers will have to be painted out in post, so don't over do it and use too many
* Tracking markers will be painted out in post production, which is pretty straight forward when they are surrounded by green, but gets harder if they approach or collide with a foreground object. In reality, it's almost impossible to keep all the markers clear of the foreground objects during a move, but keeping this to a minimum will save a lot of time in post.