Boundary segments — where a wall stands
There is no operation that places a wall. A wall's position emerges from the relation between the convex pieces of two spaces. The boundary line says what lies between those two, not where a line runs.
form.boundaries[0].segment
// { x1: 3600, y1: 0, x2: 3600, y2: 4500, horizontal: false, edgeOfA: "E" }
One boundary can produce several segments. Form holds one FormBoundary per segment, all sharing the same ref.
Edge orientation
Walk the counter-clockwise vertex list of a piece and read only the axis-parallel edges.
| Travelling | Face |
|---|---|
| +x | S |
| −x | N |
| +y | E |
| −y | W |
So N = +Y, S = −Y, E = +X, W = −X. A diagonal edge has no orientation.
Two edges "face each other" when their orientations are opposite (N↔S / E↔W) and their fixed coordinates differ by at most EPS.
edgeOfA is the orientation seen from a, the space written first. boundary /L1/a /L1/b and boundary /L1/b /L1/a are two spellings of the same relation, but edge: names opposite faces in them.
Deriving the segments — the order matters
Segments are decided in this order. The first branch that applies settles it.
- If either end space does not exist, there is no segment
- If
typeisstair/shaft/void, there is no segment (a vertical boundary has no wall) - If there is a drawn line, that line is the realisation of the boundary. Neither collinear merging nor
edge:filtering applies - If both sides have regions, take the shared edges. Different levels means no segment (no wall stands between levels)
- If only one side has a region, take the outline. The counterpart is "every space on the same level other than these two". If the other side is the outside (a derived exterior wall), take instead what no declared boundary already covers
- If neither side has a region, there is no segment
Results from 4 and 5 go through collinear merging, and finally edge: narrows them by "the face as seen from a".
Shared edges
Take the axis-parallel edges of A's pieces and B's pieces that face each other and share an overlapping interval. An overlap at or below EPS is not a shared edge — touching at a point is not touching. Segments run in ascending coordinate order and carry the orientation seen from a.
Outline
For each edge of each piece, subtract the intervals covered by facing edges of the counterpart. Other pieces of the same space count as counterparts too, so an L written as two rectangles grows no wall along its internal seam. The order of subtraction does not affect the result.
An outline splits into several faces. That is why placing an opening onto the exterior needs edge:N/E/S/W (OPN05).
Collinear merging
Segments lying on the same line are joined into one. The grouping key is (orientation, fixed coordinate, face as seen from a), and the fixed coordinate must match exactly. Within a group, segments are sorted ascending; a gap at or below EPS extends the run, a larger one breaks it.
Back-to-back edges are never merged. An N face and an S face on the same line have different orientations, so they stay separate segments.
Merging is what lets a single window span several rectangles and still sit on one segment.
The interval a drawn line realises
One design line is shared by several boundaries. The wall of a through-passage has as many boundaries as there are units along it. If every boundary realised the full length of the line, the plan would carry the same wall several times over.
To prevent that, the line is cut at the edges of both spaces' pieces; from the midpoint of each interval two points are taken PROBE (5mm) away along the normal, and only the intervals where left and right are exactly a and b are kept. If one side has no region, the side that does being alone there makes it a boundary. The test is symmetric in a and b.
PROBE is the lower bound on the resolution of shape — a space narrower than that can be assigned to neither side, and the boundary produces no segment at all.
Output segments keep the canonical endpoint order (ascending resolved coordinate) and carry no face seen from a. Therefore edge: on a boundary that has a line has no effect.
Passability
Whether a segment exists and whether a person can pass are different questions.
type | Passable |
|---|---|
wall | only if there is a door. A window does not pass |
open | always |
stair | always (vertical passage) |
shaft | no — continuous as space, but people do not go through |
void | no — there is no floor |
air:1 is about enclosure, not about passage. A rail wall is wall + air:1 with no door, so it is impassable automatically.
passable(boundary) returns this. It says nothing about pass or fail — "can you get out" is answered separately by the judgement face.
The rest of the perimeter
Parts of a space's outline face neither another space nor a declared boundary. envelopeGaps(model, space) returns them as segments, and that is where the default exterior wall stands.
The counterpart of such a wall is the outside, and it is not a space. It carries the reserved spelling outside, which no space can collide with because every space path begins with /. Declaring space /out outside:1 does not replace it — it names a part of the outside, and a boundary written to that name wins over the default wherever it reaches.
Suppression is by run, not by pair. Between two spaces one declaration suppresses the whole pair (defaults). The outside is not a pair — it is whatever the rest of the perimeter faces — so there is nothing to suppress as a unit. Each declared boundary takes the runs it reaches and the default takes what is left, which is why writing edge:S alone still leaves a wall standing on N, E and W.
Which spaces are in the population. Every space that has a region and a level, except three: a space declared outside:1 (it is the outside), a semi-outdoor space (its openness was declared, with open or air:1), and a space under a site:1 zone (paving is not a room). A void is not excluded — where one reaches the edge of the building the outer wall passes it exactly as it passes a room.
That population cannot chase its own tail: a semi-outdoor space is one with an open or air:1 boundary to the outside, and every wall derived here is a plain wall, so running the derivation can never change who is in it.
A run left unnamed is reported. The wall is there, so nothing is missing from the shape; what is missing is which outside it looks at, and that is BND08, a warning.
Neighbouring pages
- Regions — where the edges come from
- Matter — thickness and z land on the segment
- boundary — how to write one
- orientation — the N/E/S/W convention
- BND diagnostics — when no segment can be derived
- BND08 — a face onto the outside that nobody named