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Distractor Anatomy
Filled-In and Phantom Features
Summary
One feature, one counterpart
The correct opening is the exact shape of one of the object's external outlines, and "exact" is a promise that runs both ways. Every feature of the outline appears in the opening, and every feature of the opening appears in the outline. A boss standing proud of the object has a matching bump in the opening; a notch cut into the object has a tongue of plate reaching into the opening to fill it. Break that correspondence anywhere and the opening is wrong, whatever its proportions.
How a feature gets filled in
Most wrong openings are shadows of a variant object. The generator rebuilds the same recipe with one feature edited and traces a true outline of the result, so a wrong answer is always the honest shadow of something that could exist — which is what makes it tempting. Two of the four available edits erase detail: one deletes a feature outright, the other flattens a corner out of the object's master profile. The blank itself is never what gets deleted, so what disappears is always a feature.
Seen from the aperture's side, that is a feature filled in. The variant has no boss, so its outline has no slot for one, and the opening you are offered is a solid unbroken edge exactly where the real object needs room.
The disc carries a cross boss standing 1.3 units above its top face. The true front outline has a matching bar-shaped extension; the imposter runs straight across the top of the disc instead. Red marks what that costs — the whole 6.8-by-1.3 band the boss needs. Present the object at that opening from any side you like and the boss meets plate: the prover rejects this opening across all 24 view-and-orientation combinations it checks. Notice what makes it easy to accept — a filled outline is simpler than the true one, and simplicity reads as clean.
The phantom cut
The same machinery breaks correspondence in the other direction. Slide a cut feature along the object, or resize it, and the variant's shadow carries a notch where this object is solid.
The disc's rim really does have one slot cut into it. The imposter has two: the dashed outline adds a second bite, 1.6 units tall and 0.8 deep, on the side where the disc is solid. That is what makes phantom cuts hard — the bite looks like it belongs, because the object taught you to expect one. You will not catch it by asking "is anything missing?", because nothing is. Ask instead whether every line in the opening answers to something on the object.
The opening that is not about fitting at all
One wrong opening plays by a different rule entirely. When the object has a hole bored through it, the generator may offer its outline traced with the holes left in — the shape you draw when you copy what you see instead of the external boundary.
That is not an opening. The material inside a bore would be an island of plate floating in mid-air with nothing holding it up. It is wrong by the external-outline rule rather than by geometry, and the passability check deliberately never runs on it. Every correct opening has its holes filled in for the same reason: the answer is always the outer boundary. At most one of these appears in an item, and only when the object has a hole to trace.
On a real item
Here is an item from the generator that serves the practice page, with the answer hidden. Walk the features before you judge any size — a slot that is not there settles an opening faster than a measurement does.
The takeaway
Walk the features in both directions. If the opening has filled in a slot the object needs, whatever needed that slot is stopped at the plate. If the opening carries a cut with nothing on the object to answer to it, the object is stopped just as hard. And if the opening has the object's bores drawn into it, it is not an opening at all — the answer is always the external outline. None of those three checks asks you to judge a length.
Practice Questions
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