calib-targets-marker

calib-targets-marker targets a checkerboard marker board: a chessboard grid with three circular markers near the center. The detector is grid-first and works with partial boards.

Detected circle markers and aligned grid overlay.

Detection pipeline

1. Chessboard detection: run calib-targets-chessboard to obtain grid-labeled corners (partial boards are allowed).
2. Per-cell circle scoring: for every valid square cell, warp the cell to a canonical patch and score a circle by comparing a disk sample to an annular ring.
3. Candidate filtering: keep the strongest circle candidates per polarity.
4. Circle matching: match candidates to the expected layout (cell coordinates + polarity).
5. Grid alignment estimation: derive a dihedral transform + translation from detected grid coordinates to board coordinates when enough circles agree.

Key types

• MarkerBoardDetector: main entry point.
• MarkerBoardLayout: rows/cols plus the three expected circles (cell coordinate + polarity).
• MarkerBoardParams: layout + chessboard/grid graph params + circle score + match settings.
• MarkerBoardDetectionResult:
  • detection: TargetDetection labeled as CheckerboardMarker.
  • circle_candidates: scored circles per cell.
  • circle_matches: matched circles (with offsets).
  • alignment: optional GridAlignment from detected grid coords to board coords.
  • alignment_inliers: number of circle matches used for the alignment.

Parameters

MarkerBoardLayout defines the board and marker placement:

• rows, cols: inner corner counts.
• cell_size: optional square size in your world units (when set, target_position is populated).
• circles: three MarkerCircleSpec entries with cell (top-left corner indices) and polarity.

MarkerBoardParams configures detection:

• chessboard: ChessboardParams (defaults to completeness_threshold = 0.05 to allow partial boards).
• grid_graph: GridGraphParams for neighbor search constraints.
• circle_score: per-cell circle scoring parameters.
• match_params: candidate filtering and matching thresholds.
• roi_cells: optional cell ROI [i0, j0, i1, j1].

CircleScoreParams controls scoring:

• patch_size: canonical square size in pixels.
• diameter_frac: circle diameter relative to the square.
• ring_thickness_frac: ring thickness relative to circle radius.
• ring_radius_mul: ring radius relative to circle radius.
• min_contrast: minimum accepted disk-vs-ring contrast.
• samples: samples per ring for averaging.
• center_search_px: small pixel search around the cell center.

CircleMatchParams controls matching:

• max_candidates_per_polarity: top-N candidates to keep per polarity.
• max_distance_cells: optional maximum distance for a match.
• min_offset_inliers: minimum agreeing circles to return an alignment.

Notes

• Cell coordinates (i, j) refer to square cells, expressed by the top-left corner indices. The cell center is at (i + 0.5, j + 0.5).
• alignment maps detected grid coordinates into board coordinates using a dihedral transform and translation.