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The Silhouette Identity
Silhouettes: Objects as Shadows
Summary
The shadow model
Stand an object in front of a wall and shine a light on it. The patch of darkness on the wall is a silhouette: a flat shape that keeps the object's outline and throws away everything else. Keyholes is a section about silhouettes, and once you see it that way the whole section becomes one question asked fifteen times.
One detail decides whether the model works: the light has to be parallel. A nearby bulb throws rays that spread, so the shadow comes out larger than the object and its size depends on how far away the wall is. Parallel rays cast a shadow that is exactly the object's own size, every time. That is the projection the PAT uses: its instructions for the view questions spell out that views are drawn without perspective, "along parallel lines of sight." It is also why every drawing in an item can share one scale.
What the shadow discards
A silhouette records an outline and discards depth. Whatever lies along your line of sight — how thick the object is, what sits behind the front face, which surfaces step up or down — collapses into a single flat shape. Two objects of wildly different bulk can cast identical shadows.
Look at what survives each direction. From the front, this object shows a deep bite out of its right edge and a step at its top left. From the end, it is a base slab with a single wall standing on it. From directly above it is a plain rectangle — the bite, the step and the wall all vanish, because none of them changes the outline you see looking straight down.
That vanishing act is the point. The features did not go anywhere; they simply do not reach past the outline in that direction. Which lines in the original drawing mean a cut and which mean a surface mark is a reading skill from the previous lesson — here, all that matters is the boundary each direction produces.
A second object, the same behavior. Its top view is a rectangle with one corner notched away, its front view is a low bar carrying an upright wall, and its end view is a plain rectangle. Three directions, three unrelated-looking shapes, one solid.
Why passing through is a shadow question
The official rules do the rest of the work. You may turn the object any way you like before it enters, but once it has started through it may not be turned or rotated, and it must pass completely through. A pass is therefore a straight push in one fixed direction.
On a straight push, the only thing that has to fit through the opening is the outline the object presents along the direction of travel — its silhouette. Depth along that direction never has to fit through anything; it just follows. A bar ten units long with a small square cross section slides through a small square hole, because the opening only ever meets that cross section. Volume is not the question. Outline is.
So the correct aperture is not merely similar to a view of the object. It is the exact silhouette of one of the object's primary views — top, front, or end. That is the promise the instructions make when they say the opening is always the exact shape of the appropriate external outline.
One of three shadows
The correct opening is the object's top outline.
Here is a different object again, drawn as a live item from the practice generator with the answer marked. It is a slab whose top is cut into steps, with a rectangular bite out of one corner. The keyed opening is its exact top outline: the footprint's boundary, interrupted only by that bite — six corners in all. The steps leave no trace, because looking straight down is precisely the direction that flattens their height away. An opening is always that external boundary; had a hole run clean through the object, the opening would close it up rather than carry it. The other four openings are not this object's outline in any direction.
The takeaway
An orthographic view is a shadow cast by parallel light: it keeps the outline and discards depth, at the object's true size. Because a pass is a straight push, only that outline has to fit, which makes passability a question about shape and never about bulk. The correct aperture is the exact silhouette of the top, front, or end view — so your real job is to produce those three shadows, which is the next topic.
Practice Questions
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