Target Size
The IS1200 can recognize fiducials at a distance up to 20x the fiducial diameter. For example, on a 40 foot tall soundstage ceiling, the fiducial diameter required to enable recognition by a camera resting on the floor would be 2 feet in diameter.
In addition, the fiducial needs a white margin of at least 25% of the fiducial diameter around the fiducial itself to provide proper contrast and enable fiducial detection. In the above example of the 2 foot diameter marker, the complete fidicual diameter (dark + white) would be (2 feet) + (2 feet * 0.25) + (2 feet * 0.25) = 3 feet total diameter.
If the IS1200 will be tilted forward to avoid camera equipment blockage, as in the case of a Technocrane or an underslung camera head, the distance increases because of the angle. For example, if the camera is tilted 45 degrees, the effective distance to the ceiling is increased by a factor of 1.4, so the target size should be increased by 40-50%. This creates a 3 foot diameter base fiducial, and a 4.5 foot diameter fiducial + white surround.
Each fiducial can have a different size, as long as the size is accurately recorded in the Intersense calibration file.
Minimum to Maximum
The IS1200 can recognize targets from distances ranging between 20x the fiducial diameter to 5x the fiducial diameter. In the above soundstage example, this means that the 2 foot diameter fiducials can be recognized to within 10 feet, or 30 feet off the ground.
Fiducial distance
At the minimum acquisition distance of 10 feet, we can then determine the total width that the IS1200 can see with its 80 degree field of view:
tan(80 deg/2) = (visible width)/2 * distance
visible width = distance * 1.67 = 16.7 feet
To maintain the correct target density at the minimum distance, 3 targets should be visible across the field of view. To keep the required white space around the target for recognition, the centers of the outermost targets should be 1.5 feet in from the edge of the visible space, leaving 3 targets over a space of 13.7 feet, or a target to target spacing of 6.8 feet.
At first glance, this would seem to produce a fiducial grid that would take over the ceiling, given 3 foot diameter markers. However, by varying the markers size and distribution, the fidicial constellation can be adjusted to fit typical camera moves.
Multiple Size Targets
Using multiple target sizes solves the near and far tracking distance problem. As shown above, a standardized 5 target pattern is used to cover a stage area.
The 2 standard sizes are 25cm diameter center targets, with 10cm diameter outer fiducials, or 50cm center targets with 20cm outer fiducials. These fit onto a 2’ x 2’ or 4’ x 4’ Gatorboard backing, respectively.