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Measure a feature

A dimension from a photo, computed by geometry from marks you place, with the uncertainty that follows from how it was captured. No model produces the number. Without a scale reference there is no number at all.

What to photograph

The item's profile names the features that decide fit and how to capture each one: for a blender or frappé maker agitator the rod's diameter and length, the width, length and span of the two coupling tabs and the head's diameter; for a power adapter the plug's two diameters and its length. Put a ruler, or any object of known length, in the same plane as the feature and photograph it straight on. For an oblique view on a flat surface, use a known rectangle instead, a card for instance, and mark its four corners.

Marking

On the item page, under Measure a feature: pick the feature, choose the photo (signed out, one of the demo set's photos is picked instead and nothing is stored), then click the marks in order on the photo: the two ends of the known length on the reference (or the four corners of the rectangle, top-left first, clockwise), then the two ends of the feature. Enter the reference's length (or width and height) in millimetres and its tolerance. For a two-point reference, confirm the photo was taken straight on; that statement is yours, and a two-point scale is only valid when it is true.

What comes back

  • Accepted: the value with its uncertainty, the method, and the basis the uncertainty was derived from: the marking precision at each end of the feature, the marking precision at each end of the reference, and the reference's own tolerance. The value is proposed on the item as an observed value (source measurement) and waits for you to accept it in the fields table.
  • Refused: no number, and the checks that failed with what to redo. A capture is refused for a short reference (too few pixels per millimetre), marks off the photo or on top of each other, a photo whose short side is under the minimum, blur or a featureless patch around any mark, a two-point capture not declared straight on, a rectangle seen at too steep an angle, or four corners that lie on one line.
  • Tolerance: when the profile's tolerance for the feature is tighter than the capture's uncertainty, the result says so and asks for a caliper or ruler reading. Enter it under Add what you know with tested as how you know it. Direct entry is always available.

Every capture is kept on the item, accepted or refused, with the prepared photo and the marks in its pixels, so a result can always be traced back and the share of unusable captures is on record.

The methods

Two-point scale. Pixels per millimetre from the reference's two marks and its known length; the feature's pixel length divided by that scale. Valid when the reference lies in the plane of the feature and the view is straight on.

Homography. The four corners of a known rectangle give the projective transform from the photo to that plane in millimetres; both feature marks are mapped through it and the distance is taken there. Valid for a feature on the same flat surface, from any angle up to the steepness limit. One plane's calibration is never applied to a feature at another depth.

Uncertainty. With a marking precision of m pixels at each end (MEASURE_MARK_PX), a reference of R pixels and L millimetres with tolerance t, and a feature of F pixels at s pixels per millimetre:

value       = F / s
uncertainty = 2m / s  +  value * (2m / R + t / L)

No default accuracy is assumed and nothing is rounded away: a short reference or a loose tolerance shows up directly in the number.

What a dimension does not prove

A matching dimension filters candidates. It never establishes electrical, structural, firmware, sealing, retention or safety fit on its own; those need their own evidence.

API

# try a capture without storing it
curl -s -X POST localhost:8000/api/measure \
  -F photo=@agitator-ruler.jpg \
  -F 'request={"feature":"rod_diameter","profile":"blender_agitator",
      "image":{"width":640,"height":480},
      "reference":{"kind":"two_point","points":[{"x":100,"y":400},{"x":500,"y":400}],"length_mm":100},
      "points":[{"x":200,"y":200},{"x":340,"y":200}],"straight_on":true}'

# the same on an item: stored, and proposed as an observed value when accepted
curl -s -X POST localhost:8000/api/items/1/measurements -F photo=@agitator-ruler.jpg -F 'request={...}'
curl -s localhost:8000/api/items/1/measurements
curl -s localhost:8000/api/items/1/measurements/1/photo -o capture.jpg

# the features a profile names
curl -s localhost:8000/api/profiles
curl -s localhost:8000/api/profiles/blender_agitator/features

Full shapes in the HTTP API; thresholds in Configuration.