Surveying the Stage
To get accurate real time 3D tracking data, the 3D positions of the fiducial targets on the stage must be known to a high degree of accuracy. This is most efficiently achieved with the use of a “Total Station” 3D reflectorless surveying instrument, such as the Sokkia pictured above from ABC Surveying. The total station and Pocket PC surveying software form an automated stage survey data pipeline to generate stage calibration files.
Intersense Coordinate Reference
Previzion and the IS1200 are based around 3D coordinates to enable complex camera moves. To more easily match real world and virtual camera motion, it is important to match the physical and virtual coordinate frames.
The IS1200 uses the coordinate system shown below:
Most surveying instruments use a similar coordinate system that uses “Northing”, “Easting”, and “Elevation”:
For surveying a stage, the most straightforward method is to set the Northing direction as facing toward the main cyc background wall. The zero point for the Elevation axis is then the floor surface.
The (0, 0) point for the Northing and Easting axes is typically a physical spot on the stage floor that will remain fixed.
Northing=Positive X Axis
To save yourself a lot of pain, define the Northing axis in the total station (where you put the backsight mark) as pointing toward the main greenscreen wall at right angles to the wall.
This usually means that this axis points away from the video village. This axis will come in as the +X axis in Previzion by default. You should also create 3D sets with the +X axis pointing in this direction. This way, the +X axis always points the same way no matter what application you are in. Otherwise, it gets very confusing very quickly.
There are several techniques to align the Total Station to the stage, but the simplest is as follows:
Place the Total Station directly over the (0, 0, 0) physical spot, level it using the station’s leveling controls, and use the built in right-angle viewfinder that looks straight down the station’s vertical axis to the ground to make sure the station is precisely over the (0, 0, 0) mark.
Measure the height from the floor to the center of the station’s tilt axis. This is then input into the station using the station’s interface.
Turn on the station’s laser, and point it to a point somewhere on the physical northing axis.
Set the horizontal angle of the station to 0.000 at this reference direction.
Now, the station will measure Northing, Easting, and Elevation coordinates of the point where the laser marker is visible.