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calib-targets-core

calib-targets-core provides shared geometric types and utilities. It is intentionally small and purely geometric; it does not depend on any particular corner detector or image pipeline.

Rectified grid view Global rectification output from the chessboard pipeline.

Core data types

  • Corner: raw corner observations from your detector.
    • position: image-space pixel coordinates.
    • orientation: dominant grid orientation in radians, defined modulo pi.
    • orientation_cluster: optional cluster index (0 or 1) if clustering is used.
    • strength: detector response.
  • GridCoords: integer grid indices (i, j) in board space.
  • LabeledCorner: a detected corner with optional grid coordinates and logical ID.
  • TargetDetection: a collection of labeled corners for one board instance.
  • TargetKind: enum for Chessboard, Charuco, or CheckerboardMarker.

These types are shared across all detectors so downstream code can be target-agnostic.

Orientation clustering

ChESS corner orientations are only defined modulo pi. The clustering utilities help recover two dominant grid directions:

  • cluster_orientations: histogram-based peak finding followed by 2-means refinement.
  • OrientationClusteringParams: histogram size, separation thresholds, outlier rejection.
  • compute_orientation_histogram: debug visualization helper.
  • estimate_grid_axes_from_orientations: a lightweight fallback when clustering fails.

Chessboard detection uses these helpers to label corners by axis and to reject outliers.

Homography and rectification

Homography is a small wrapper around a 3x3 matrix with helpers for DLT estimation and point mapping:

  • estimate_homography_rect_to_img: DLT with Hartley normalization for N >= 4 point pairs.
  • homography_from_4pt: direct 4-point estimation.
  • warp_perspective_gray: warp a grayscale image using a homography.

For mapping rectified pixels back to the original image, core defines:

  • RectifiedView: a rectified grayscale image and its mapping info.
  • RectToImgMapper: either a single global homography or a per-cell mesh map.

Higher-level crates (notably chessboard) wrap these utilities for global or mesh rectification.

Image utilities

GrayImage and GrayImageView are lightweight, row-major grayscale buffers with bilinear sampling helpers:

  • sample_bilinear: float sampling with edge clamp to 0.
  • sample_bilinear_u8: u8 sampling with clamping to 0..255.

These utilities are used by rectification and marker decoding.

Conventions recap

  • Coordinate system: origin at top-left, x right, y down.
  • Grid coordinates: i right, j down, and grid indices refer to corners.
  • Quad order: TL, TR, BR, BL in both source and destination spaces.

If you build on core types, stick to these conventions to avoid subtle alignment bugs.