ArUco Decoding Details

This chapter expands on the marker decoding path in calib-targets-aruco. The decoder is grid-first: it samples expected square cells and reads bits in rectified space (or per-cell quads).

Per-cell decoding

scan_decode_markers_in_cells reads marker bits in their own cells given an existing grid of square corners, without warping the full image. ChArUco detection drives this path: only valid grid cells are decoded, and the per-cell work parallelises trivially.

Sampling model

• Bits are sampled on a regular grid inside the marker area.
• The marker area is defined by marker_size_rel, with an extra inset from inset_frac.
• A per-marker threshold (Otsu) is computed from sampled intensities.

Tuning knobs

• inset_frac controls how far inside the marker area bits are sampled. Lower values capture more of the marker; higher values are more robust to thin black borders bleeding into the bit grid.
• min_border_score is the minimum “frame looks like a marker border” score required to accept a cell. Higher values reject ambiguous cells.
• dedup_by_id collapses repeated decodes of the same dictionary ID across cells.
• marker_size_rel is the marker side relative to the enclosing chessboard cell and must match the physical board spec.