Calibration targets

As multiple calibrtion targets are necessary in calibration scenario, classic checkerboards are obviously not a good choice. 2D barcode is very desirable to distinguish the corners of checkerboard, one representative example is the Aprilgrid pattern used in Kalibr. In Multical, we further extend to multiple Aprilgrids with different April tags. The users could utilize multical_create_target_pdf command to create a group of pdf files, each one is an Aprilgrid, the following is the arguments of multical_create_target_pdf when --type is set to apriltag:

--nx N_COLS           The number of tags in x direction (default: 6)
--ny N_ROWS           The number of tags in y direction (default: 7)
--tsize TSIZE         The size of one tag [m] (default: 0.08)
--tspace TAGSPACING   The space between the tags in fraction of the edge
                        size [0..1] (default: 0.3)
--tfam TAGFAMILIY     Familiy of April tags ['t16h5', 't25h9', 't25h7',
                        't36h11'] (default: t36h11)
--skip-ids SKIPIDS    Space-separated list of tag ids to leave blank
                        (default: none)
--target-count TARGETCOUNT
                        number of targets

The first four arguments are same to these in Kalibr, and --target-count is added here to specify the number of targets.

As reference, the mount of tags of each families are given here, in case used number of tags excceeds the range

• 't16h5' -- 30
• 't25h9' -- 35
• 't25h7' -- 242
• 't36h11' -- 587

For example, if --nx is 6 and --ny is 7, and we choose 't36h11', then the available maximum of --target-count is int(587/(6*7))=13.

For convenience, here is a group of aprilgrid targets with 6x6 tags whose size is 0.08m, and a corresponding yaml file, which could be utilized when running multical_calibrate_sensors. Remember to revise the field 'numberTargets' to the real number of targets when calibration.