Target points specified in square units and in millimeters can describe the same image geometry. Their translation coordinates need the corresponding scale.
Define the target coordinate scale
Camera calibration uses object-point coordinates paired with image-point observations. The object-coordinate unit supplies the length scale of translation quantities in that model. Specifying target points in arbitrary square intervals does not, by itself, establish a millimeter unit for a subsequently reported translation. Primary reference: OpenCV object-point coordinates and calibration geometry.
For an original worksheet, retain the target identity, coordinate origin and physical square size when known. Keep pixel-based intrinsic quantities separate from object-coordinate lengths. A correct-looking projection can coexist with an undocumented length scale, so the units must accompany the model.
Rescale a fictional geometry
Consider an ideal projection x = fX/Z with f = 800 pixel units. X = 2 square units and Z = 5 square units give x = 320. If each square interval is defined as 20 mm, X becomes 40 mm and Z becomes 100 mm, still giving x = 320.
Uniformly scaling both object position and translation leaves this projection unchanged. A translation of 5 square units means 100 mm only after the stated square-size conversion. This original ideal calculation is not a software fit result and establishes no accuracy for a fabricated target or real camera.
Preserve scale in the handoff
Prepare an original model record listing target coordinates, their units and any physical scale conversion. If a target size changes or a model is exchanged, review the declared scale before interpreting translation values as mechanical distances. Keep the underlying image observations and target definition together.
This guide supplies no HM calibration target dimensions or measurement performance. Its decision is whether physical length has been assigned through a documented target-coordinate scale. Pixel intrinsics and object-coordinate translations should retain their different unit definitions when the calibration record is used elsewhere.
Customer Questions
What is five fictional square units in millimeters?
One hundred, using the stated twenty-millimeter interval.
Does uniform object-scale conversion change the example's projection?
No. The ratio X/Z remains the same.
Does an arbitrary square unit mean a millimeter?
No. Its physical scale must be documented.
Primary References
These references support the technical principles discussed in this guide. The worked examples and review questions are educational.
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