Create The Perfect Calibration Pattern
Create your calibration board in three simple steps
Choose your preferred board type and configuration
Print your board
Measure your board
Tips and guidelines
Before you start, you should keep a few things in mind when building your board and calibrating your cameras.
Try to print your pattern on stiff material. You want to make sure the board does not change its shape or warp over the course of recording image data for calibration.
Make sure your pattern is as flat as possible.
You will need to record 20-30 images per camera when calibrating. Move the pattern and try to cover the entire image space with all calibration snapshots. You don't need to cover the entire image in each snapshot though. Use the example image set below for placement reference.
Make sure to tilt the calibration board around the x, y, and z axes by ~30°.
Choose your board type
With camcalib, you currently have the choice between two board types:
With AprilTag boards, you can achieve higher feature densities over ChArUco boards, so we recommend using AprilTag boards.
Your next choice is the board size. You will also be faced with a trade-off here. A larger board will allow you to show the camera many more features simultaneously, while a smaller board will be easier to stabilize mechanically. You should aim to maximize the number of visible features while minimizing board flexing.
Tip: camcalib object point optimization
Camcalib can compensate for board flexing and warping as long as the board deformation remains unchanged during the entire calibration process.
As a rule of thumb, select your board size to cover 15-40% of the image width when you place it at working distance. Note, however, that this implies you will need quite large boards (1x1m) for fish-eye lenses.
For most cases, this 6x6 AprilTag board template should work fine. For now, download the file; we will discuss printing and display in a moment. We also provide a 12x6 AprilTag board and ChArUco board templates if you feel like experimenting.
Print or display your board
A vital remark upfront: if you print, make sure your printed board is flat and does not warp while recording calibration data. You have three options if your printed board tends to flex noticeably – more than 1mm for A3 boards, less for smaller ones – with light to moderate handling.
Calibrate by placing the pattern on the ground or in a corner and moving only the camera. Do not touch or move the board during calibration and move the camera slowly. This is the preferable approach.
Print the pattern on a stiff material. Most print shops offer cost-effective aluminum-composite substrates for this.
Show the pattern on a 4k monitor. This may be an attractive option to control motion blur if you can configure the camera to avoid monitor flickering.
Measure your board
Camcalibs extrinsic calibration requires a visible length gauge. To define the length gauge, you must measure the board shape parameters with a ruler or calipers.
Caution: AprilTag and Charuco board parameters are not the same. Please read on with care.
Charuco board measures are simple to determine. Measure the lengths of one checkerboard field and one marker. For more accurate measurements, compute the square size by measuring the length of the checkerboard pattern from one end to the other and divide it by the number of fields along the measured distance.
AprilTag patterns also require two length measures. However, the tag spacing parameter is the fraction of the tag space (b) and tag size (a).
Tag size variable a in the image below
Tag spacing is the tag space b divided by the tag size a. Or tag spacing = b/a.
You should now be ready to use your new calibration board. You also have the necessary configuration parameters to calibrate your cameras with camcalib.
Please reach out to us if you should have any issues with setting up your calibration boards. We are happy to help.