Last Modified: December 3, 2014
This involves the basic set up of the Intersense fiducial markers that have been printed at an appropriate size and in sufficient numbers so they can be recognized by the tracking camera and mounted to the ceiling or walls depending on the nature of the shot and other set pieces involved.
What is the overall size of the soundstage? An overhead view is valuable for planning target locations.
Stage Ceiling Height
The tracking targets have to be sized so that their diameter is between 1/5 and 1/20 of the maximum distance form the Intersense camera, In the event that the camera is mounted on a crane or Jib we often will have 2 different sized targets mounted on the same panel so that the camera can have an increased range of vertical movement. More details are here.
Fiducial Target Construction
It is important that the fiducial is mounted to a flat smooth surface, usually spray mounted on 1/2″ Gatorboard, although we have applied them to plywood panels and even printed them on transparency film which was mounted to the spacelights on a stage. These Gatorboard panels can easily be screwed in place, clamped or tie wrapped to the grid or ceiling.
The orientation of the markers is quite flexible, as we calculate the surface normal when we survey them, and record that data along with the exact position for each target.
Tracking Target Mounting
Care must be taken in the mounting of the targets to the ceiling or lighting grid so that a minimum of 4 targets are visible at any time. We tend to err on the side of caution and will overpopulate the stage with targets by a factor of 2 or 3 x what is required in order to be able to cope with the inevitable occlusions (blocking of targets) that can happen with light, flags and wires during a hectic shoot.
The tracking camera has roughly a 80 degree field of view and is adjustable so it can be used on under-slung heads or other support systems that may interfere with its view of the markers.
For instance we often will put a row of markers just above the green screen in order to extend the field of view when the camera is pointed down at an extreme angle.
Tracking Target Survey
The survey of the mounted targets on the stage is usually done with a surveyor’s tool called a total station. A large stage with 200-300 targets can take approximately 3 hours to complete. It is very important the the targets are mounted securely and that they don’t move after the survey has been made. The more accurate the measurement of the position of the targets is measured the more accurate the camera tracking will be.