The scene

sceneOf(form) returns the three-dimensional scene a Form describes, as data. A viewer turns the nodes into meshes with its own materials, its own lighting and its own idea of what is selected; two viewers showing the same building disagree about all of that and about none of this.

import { sceneOf } from "@kensnzk/koyu/draw";

const scene = sceneOf(derive(model));
for (const node of scene.nodes) {
  // node.role decides the treatment; node.solid / .line / .mark is the geometry
}

No three.js, and no options

koyu declares no runtime dependency, so a scene graph cannot come out of this package. The constraint turned out to be the right shape: every viewer already wraps the result in its own group, and none of them wants another's materials.

There are no options either. Which levels, whether to draw openings, how thick a ground plate is, how far to spread the storeys apart — each is a filter over scene.nodes or a transform on the consumer's own group:

group.position.y = level.z * (spread - 1);        // exploding storeys
scene.nodes.filter((n) => n.level === shown);     // one storey

Baking any of them in would stop the scene being a projection of the Form, and two viewers would then differ by which options they passed rather than by how they draw.

The roles

role	what it is
volume	the air a space encloses, floor to ceiling
plate	a space or the site read as a horizontal face (bottom === top)
body	matter — a wall body, a column, a slab, a tread, a ramp, a leaf
edge	a line, not a body — a boundary's centreline, the site edge
mark	a seat for a symbol or a word

A plate is a face, not a slab. Form derives no thickness for the ground, and inventing one here would be a paper decision made in the wrong place; the consumer extrudes it by however much its drawing wants.

level is a fact about the numbers

Every solid carries ring, bottom and top, one z per vertex. level is true when every bottom is one number and every top is one number — a right prism. It is not a hint about materials: it is what lets a consumer take the cheap extrude path and know when it must stitch a per-vertex buffer instead, which a flight of stairs and a ramp both require.

A wall body is its footprint

Never its centreline thickened. A junction is already settled in the body: where two walls meet, one runs through and the other stops at its face, so the body runs past the end of its own centre line or stops short of it. A body node carries both — solid.ring is the matter and centre is the axis — because the axis cannot be recovered from the footprint. A viewer that thickens the centre line puts a handrail beside itself at every corner, and the bundled buildings all contain such bodies.

The ground is not levels[0]

scene.ground is the lowest level at or above z 0. A building with a basement otherwise lands its site and its landscaping on the basement.

Free words are carried, never read

facts.style, facts.name and facts.grid are values the model wrote. koyu carries them and does not interpret them: deciding that a spec containing ガラス means a transparent material is a judgement about the meaning of a word, and it belongs to the viewer that makes it.

For getting back to what was written, written and pair mean exactly what they mean on a mark — written.boundary indexes canonical boundary order, never model.boundaries.

Neighbouring pages

• Matter — the bodies the scene is enumerated from
• The marks of a plan — the same service in two dimensions
• The section — a vertical plane through the same bodies
• Scope — why presentation does not freeze