Checkerboard Corner Detection

Camera calibration with checkerboards relies on detecting a regular grid of high-contrast corners and matching them to known 3D points on the calibration plate. Each black–white intersection is a strong corner: image intensity changes sharply in two orthogonal directions, and the 3D layout of these corners (typically a flat grid in the $z=0$ plane) is known precisely. Once these 2D–3D correspondences are available, standard calibration methods (e.g., Zhang’s method) estimate intrinsic parameters (focal length, principal point, distortion) and extrinsic pose for each image.

Corner detectors like Harris or FAST respond wherever there is a local 2D intensity structure that is not well explained by a single edge or flat region. Checkerboard “X-corners” are ideal: they are stable under small viewpoint changes and support sub-pixel localization by fitting a local intensity model or optimizing a local cost function. The extra difficulty in checkerboard detection is not finding some corners, but selecting only those that form a consistent grid pattern in the right order, and refining their positions to sub-pixel accuracy for high-precision calibration.

1. Background Knowledge (key concepts)

• Corner detection (Harris / FAST)

• A corner is a point where image intensity changes significantly in both $x$ and $y$ directions.

• Harris uses the second-moment matrix of gradients in a local window and looks at its eigenvalues; both large ⇒ corner.

• FAST uses a circle test around the pixel for rapid detection: if a contiguous arc of pixels is all brighter or darker than the center by a threshold, it flags a corner.

• Checkerboard structure

• Corners lie on roughly straight rows and columns with constant spacing (up to perspective distortion).

• Each valid corner belongs to exactly four squares with alternating black/white pattern (an “X-corner”).

• Geometrically, the set of corners lies on a planar grid that becomes a projective transform (homography) of a regular grid in the image.

• Sub-pixel refinement

• Initial corner detection gives integer-pixel coordinates.

• For calibration, we refine to sub-pixel by:

• Fitting a quadratic to the corner response map, or

• Fitting an intensity model to the local patch, or

• Using built-in routines like cv::cornerSubPix.

• This reduces reprojection error and improves calibration stability.

2. Algorithm / Approach Pattern

The overall pattern is:

1. Detect many raw corners using a generic detector (Harris or FAST).
2. Filter and group these corners into a structure that matches a checkerboard:

• Enforce geometric constraints (grid-like rows/columns, roughly uniform spacing, planarity).
• Optionally, validate the local intensity pattern (black–white–black–white around the corner).

3. Order the corners into a consistent $(i, j)$ grid (e.g., top-left to bottom-right).
4. Refine positions of the selected corners to sub-pixel precision.
5. (Outside this problem) Feed these 2D points with known 3D grid coordinates to a calibration routine.

This is an example of: generic detection → pattern-specific filtering → geometric reasoning → local refinement.

3. Step-by-Step Strategy

Step 1: Preprocessing (optional but helpful)

• Convert to grayscale.
• Optionally apply:
• Slight Gaussian blur to reduce noise.
• Contrast normalization (e.g., histogram equalization) if illumination is poor.

Step 2: Corner detection

• Use Harris or FAST (often available in libraries):

# Example with OpenCV-like API (conceptual)
gray = to_grayscale(img)
corners = harris_detect(gray) # or fast_detect(gray)

• Keep:
• A response map (Harris score) or list of keypoints.
• Apply non-maximum suppression and a threshold to remove weak / duplicate corners.

Step 3: Initial filtering and clustering

Goal: remove corners that obviously cannot belong to the checkerboard.