9.5 KiB
BearConnector.step — examination
Scope. One file was supplied and it contains one object: the male. Everything below is measured from that single solid. Earlier drafts of this note reasoned about a female pocket and a mating pair — those objects were never supplied, so any statement about them was speculation and has been removed. The clearance, the fit, and the pocket's legibility are all unassessed.
Measured, not eyeballed. Imported into the Design tab's own OpenCascade kernel
(import_step → one valid closed solid), topology queried, geometry checked numerically.
Flat drawing: artifacts/shots/bear-flat.png. Viewport: artifacts/shots/bear-02-zoom.png.
File: AP242 Edition 2, ST-Developer. 1 MANIFOLD_SOLID_BREP, 1 CLOSED_SHELL.
Size: 83.06 × 66.69 × 17.27 mm. Faces: 30 — 24 planar + 6 cylindrical.
Curves: 69 lines + 12 circles. No splines, spheres, tori or cones.
Relief: only four Z levels — 0, 3.00, 10.66, 17.27.
What is right, and precisely so
The sloping ridge is implemented exactly as briefed. From (0.00, 18.40, 17.27) to (0.00, 46.72, 10.66): 28.3 mm long, 6.61 mm drop, 13.1° slope, and both ends sit dead on x = 0.00. It breaks 180° rotation on its own.
20.0° uniform draft on all four snout flanks, identical to within 0.1°:
(0,−0.94,0.342) (0.936,0.08,0.342) (0,0.94,0.342) (−0.936,0.08,0.342). That is a real,
deliberate lead-in — it self-centres into a matching pocket, and it demoulds and prints.
The eyes are exactly symmetric: Ø9.87 at x = ±16.43, y = 48.01, matching to 0.01 mm. Someone mirrored those on purpose.
The mating feature is extremely economical: only five edges exist above the 3 mm plate — the ridge plus two flank edges at each end. Base plate is exactly 3.00 mm.
The low-poly constraint is honoured. All six cylinders are outline rounds and eye holes; none of them is a mating surface.
The asymmetry is deliberate, and it is complete
Correction. A first pass read the left/right differences as an unfinished mirror. That was wrong: the asymmetry is intentional. Tested properly — every candidate self-symmetry, in the part's own centred frame, with a generous 0.1 mm tolerance:
| operation | edges mapped onto the part |
|---|---|
| identity | 81 / 81 — 100 % |
| mirror about x = 0 (left/right) | 0 / 81 |
| mirror about y = 0 (top/bottom) | 0 / 81 |
| rotate 180° about Z | 0 / 81 |
| rotate 90° about Z | 0 / 81 |
| mirror about the diagonal | 0 / 81 |
The symmetry group is trivial. No rigid motion or reflection maps this part onto itself, so every partial view determines the orientation uniquely — you never need to see the whole face to know which way round it goes. That is the strongest possible result for a keying interface and it is exactly what the earlier abstract glyph work kept failing to achieve: a symmetric shape seen at a grazing angle, or half-occluded, gives an ambiguous read.
Does it let you GRASP the orientation? Measured, not asserted.
Unique-in-principle and graspable-at-a-glance are different claims. The symmetry table proves the first. For the second, the front-on picture (outline + eyes + mouth, filled) was rasterised and compared against its own mirror and its own 180° rotation — the two ways a person can get it wrong.
By size (percentage of pixels that differ):
| width | vs mirror | vs rotated 180° |
|---|---|---|
| 16 px | 20.7 % | 26.0 % |
| 24 px | 21.9 % | 30.9 % |
| 32 px | 23.0 % | 28.1 % |
| 48 px | 22.4 % | 30.6 % |
| 80 px | 24.7 % | 31.0 % |
| 160 px | 23.6 % | 31.0 % |
The curve is flat. The full signal is already there at 16 pixels and more resolution adds nothing. That is the whole result: the orientation cue lives at low spatial frequency, carried by the overall shape rather than by any detail. It therefore survives distance, blur, poor light, peripheral vision, a small print and a low-resolution screen. It is the exact opposite of the abstract disc glyph, whose roll cue was a small high-frequency feature and died at a grazing angle.
Partial views — a claim I made and then withdrew. I ran a masked-window test and concluded that a single quarter of the face was enough to read the orientation. That test was invalid and the conclusion is wrong. It compared a window of the original against the same window of the mirrored and rotated versions — which silently hands the observer the registration. It assumes you already know that the patch you are looking at is the top-left quarter, which is exactly the thing you would not know if you could only see a quarter.
You need to see the whole face. The cues here are relational: the big ear only means something next to the small ear, and the mouth offset only means something relative to the centreline. None of them is self-locating. Whole-face is the operating condition, and the design should be judged and used on that basis.
That does not weaken the size result above, which always used the complete silhouette: the whole face reads at 16 px. Needing all of it, and needing very little resolution of it, are compatible — and for a part held in a hand, seeing all of it is the normal case.
The signal is allocated to the right risks. The strongest cue (up to 41.7 %) guards against inserting it upside down — the mistake people actually make. The weakest (~23 %) guards the mirror case, which needs the part flipped over and which the protrusion already prevents mechanically.
It also does mechanical work beyond the ridge. The ridge alone breaks 180° rotation; the asymmetric outline additionally defeats the mirrored-part case — a mirror-image copy will not fit, so a modelling or printing mirror is caught at assembly rather than three steps later.
And for children specifically, a symmetric cartoon face reads as a mask; illustrators asymmetrise deliberately so a face reads as a character. The asymmetry is earning its keep three ways at once.
What is worth keeping in mind anyway
The ears differ by 42 % — left 8.33 mm wide (top y 65.68), right 11.81 mm (top y 66.69). Both start at the same y = 60.79, so they read as a deliberate pair rather than an error. 42 % is well above the perceptual threshold: you see it instantly. Good cue.
The mouth is a smirk — x −21.93 … 0.00, centred at x = −10.96, stopping on the centreline. A classic character device and a strong asymmetry.
The rounds are the best cue and the one safety question. All four are on the left — Ø11.71 at (−40.82, 7.38), Ø11.71 at (−34.76, 0.58), Ø10.00 at (−29.85, 60.83), Ø2.90 at (−26.70, 65.95) — and the right side is entirely sharp. This is the most locally readable cue in the design: the ears differ only by comparison (you must see both to know which is which), whereas a rounded corner tells you "this is the left" from that corner alone, by eye or by fingertip. For children assembling by feel that is the cue doing the real work.
The tension is that "sharp" on a children's part is a hazard, and the obvious safety fix — round everything — destroys the cue. The resolution is not round-vs-sharp but large-vs-small radius: keep R≈6 on the left and give the right R≈1. R1 still reads and feels sharp locally, so the cue survives, and the actual edge hazard goes away. That is the one recommendation that outlives the correction.
One measurement that does not fit the story: the outline is off-centre by 0.54 mm (left reach 40.99, right reach 42.07). A deliberate cue should be unmissable; 0.54 mm is invisible. It is probably a by-product of the other features rather than intent — worth a look, not a defect.
Two judgement calls, not defects
The snout is highest at the nose tip and slopes down toward the brow — a real bear's muzzle does the opposite. Anatomically it reads more like a beak or a horn than a snout. But mechanically it is the better choice: the nose tip enters the pocket first and does the finding. Keep it if the lead-in matters more than the likeness; flip it if "it must look like a bear" wins.
Only the male was supplied, so the clearance, the fit and the pocket are unassessed. Nothing in this note should be read as a judgement on them.
The strategic point, which is the real reason this design is good
It gives orientation a name. "Ears up, nose down" needs no legend, no convention and no documentation. Face recognition is the most robust pattern-matching humans have: it survives low resolution, poor light, partial occlusion and peripheral vision. That is exactly the robustness the abstract ridge key was reaching for, and here it comes for free.
One earlier objection does not transfer — noting it only so it is not carried over by mistake. In §8c of the design doc a female pocket measured as visually invisible — flat-shaded, a recess reads as a blank rectangle — and I concluded male/female is the wrong polarity cue. That was a viewport finding, and it does not apply to a physical part. Nobody looks into the pocket of a toy; they feel it. For a part in a child's hands, male/female is exactly the right polarity language. The earlier conclusion stands for the on-screen glyph and must not be carried over to this.
The one rule to write down now: the face and the key must never be allowed to disagree. People will trust the face over the mechanics every time. Here they agree — ridge on the centreline, ears up. If the face is ever restyled independently of the key, a user will orient by the bear and be wrong. Tie them permanently, in the model and in whatever generates it.