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Capture Tips

Good calibration starts with good captures. This page covers the three most important factors and how to avoid the most common mistakes.

Use a large board

The ChArUco board must be clearly legible in every camera at the same time. If the board is too small, corners appear only a few pixels wide and detection becomes unreliable.

Guidelines:

  • Print the board as large as your space and printer allow.
  • If a camera shows fewer than 10 detected corners in ChArUcal's grid view, the board is probably too small for that camera.
  • For high-resolution cameras that are far from the board, go bigger.

Tip

Laminating the board or mounting it on rigid foam board prevents warping, which would introduce subtle distortions into the calibration.

Maximize overlap

All cameras must see the board simultaneously and completely. Partial visibility is fine for individual corners, but the more corners that appear in multiple cameras at once, the more accurate the inter-camera stitching will be.

How to do it:

  • Place the board at the point where all camera views overlap, usually the center-front of the rig.
  • Hold the board flat and level, so its real-world plane matches ChArUcal's planar homography assumption.
  • Before saving a capture, check the grid view in the CLI. Every camera should show the majority of corners detected (shown in green). Red or yellow means poor overlap.
  • Take 3-5 captures from slightly different positions and angles. This gives ChArUcal more point correspondences to work with and reduces sensitivity to any single noisy detection.

Ensure good lighting

Shadows and reflections are the leading causes of missed or inaccurate corner detections.

Checklist:

  • Avoid direct sunlight on the board's surface. Bright spots wash out the ArUco marker patterns and ruin corner detection.
  • Use matte paper or matte lamination instead of glossy. Glossy boards create specular highlights that the detector mistakes for saturated markers.
  • Light the board evenly. Side lighting that casts shadows across the squares will hide corners near the shadow edge.
  • Avoid backlight. If a window or lamp is behind the board, the camera will expose for the background and leave the board dark.
  • Check exposure. Open the camera preview before capturing. The board should be neither blown out (all white) nor muddy (all gray).

What happens if calibration is poor?

Symptom Likely cause
Cameras don't align at edges Too few shared corners between cameras
Real-world scale is wrong square_length or marker_length entered incorrectly
Output is rotated or skewed Board was tilted at a steep angle during capture
Flickering seams at runtime RANSAC accepted an unstable homography — recapture

See Troubleshooting for more detailed diagnostics.