Last Modified: December 3, 2014
Circular Fiducial Markers
Fiducial markers are optical patterns that can be identified by machine vision algorithms to determine a reference point.
Barcodes store numbers. QR codes store data strings. And fiducial markers store position references.
The Intersense IS1200 identifies a specific type of fiducial marker. It is called Circular Data Matrix. These patters function both as a unique circular barcode, with a unique number, as well as a precise fiducial marker, with a specific positional reference.
In the interest of simplification, we will refer to each fiducial circle as a target.
And each board with 5 or more markers as a target board.
The IS1200 can recognize fiducial targets 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.
Intersense Position Noise
At target heights over 20′ (6 meters) from the floor, the Intersense will exhibit noise in the position tracking of about +/- 1 cm. The visibility of this ranges from not very noticeable if the background is virtual video plates located 10 meters away, to very noticeable if the virtual 3D background object is very close to camera.
To remove this, enable position clamping.
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.
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.
Distance min = 5* Diameter fiducial
Distance max = 20* Diameter fiducial
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.
The targets don’t necessarily need to be on the ceiling of a stage. It can be on the walls or the floor for that matter.
With all the above mentioned in mind, Lightcraft has created “floor targets” or “portable target boards.” By using a variety of sizes on a single board printed on vinyl, you can have the flexibility of putting targets on the floor.
We use a durable heavy-duty printable vinyl that, when protected with a spray fixer, can be used in multiple locations. The only requirement is that the portable target board be laid out on a flat surface.
For the 150cm_Floor-Board, we use 45 fiducial patterns at three different sizes:
- (5) 30-centimeter fiducial patterns;
- (12) 15-centimeter fiducial patterns; and
- (28) 7.5-centimeter fiducial patterns.
- max camera height: 19′
- min camera height: 1′ 5″
- max “usable” distance from edge of board: 5 feet (at a height of 5’6″)
For information about surveying Lightcraft’s floor targets, go to Using the Portable Target Boards.