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lab1-pixel-size

Lab 1 · Pixel size calibration

Goal
Calibrate mm/px and verify against ground truth to within 0.05 mm.
Estimated time
30min
Level
Introductory
Prerequisites
None. This is the first lab.
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Goal

Calibrate pixel size on demo-2axis-vision and answer one question: how far is your result from the truth?

Setup

  • Machine: demo-2axis-vision
  • Prerequisite calibration: run C3 intrinsics and distortion first (do not skip it — step 5 explains why)
  • Time: about 30 minutes

Steps

  1. File → Open machine → demo-2axis-vision, wait for all devices to connect.
  2. Calibration → Intrinsics and distortion, run with defaults. Note the reprojection error (should be < 0.15 px).
  3. Calibration → Pixel size. In prepare, set the acquisition span to 50 mm and 9 poses.
  4. Record three numbers: solved.mmPerPx, truth.mmPerPx, verify.deviationMm.
  5. Change the span to 5 mm and the pose count to 25, run again. More poses, larger deviation — record it.
  6. Reset intrinsics (Calibration → Reset → intrinsics only) and run pixel size directly. Note how much the residual moved compared with how much the deviation moved.

Pass criteria

Item Criterion
Deviation in step 4 < 0.05 mm
Residual RMS in step 4 < 0.15 px
Deviation in step 5 clearly larger than step 4 (typically 3–5×)
Step 6 residual grows a little, deviation grows more than 5×

Common mistakes

  • All poses crowded into the centre of the field. Beautiful residual, bad deviation. Span matters far more than pose count.
  • Skipping distortion calibration. Step 6 makes you reproduce this on purpose.
  • Unidirectional acquisition. Approaching every pose from the same direction hides backlash (Lab 3 covers it).
  • Target parameters changed but not updated in configuration. A 0.1 % error in nominal spacing becomes a 0.1 % error in the result.

Review questions

  1. Going from 9 to 25 poses should improve precision by some factor in theory. Why was the result worse instead?
  2. In step 6 the residual RMS grew by less than 3× while the deviation grew by more than 5×. When, exactly, does a residual lie to you?
  3. On real hardware there is no truth. Design an experiment that estimates your deviation anyway. (Hint: gauge blocks, or two positionings a known distance apart.)
  4. A 0.1 % relative error in pixel size over 200 mm of travel is how many millimetres? Is that acceptable for your application?

Further reading