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Walls as boundaries

boundary /L1/a /L1/b is not a line that places a wall. It is a line that declares that a relation — a boundary — exists between two spaces. The wall centerline itself is written nowhere; it is derived as the shared edge of the two regions.

From that single fact a set of rules falls out together. There is no need to memorise three separate things. Remember that it is a relation and the rest follows. The notation is in boundary.

Substance rides on the relation​

A boundary grants the seat of a shared surface between two spaces, and substance sits on that seat.

  • Horizontally — wall, curtain wall, railing, glass partition, open
  • Vertically — floor, ceiling, slab, roof, void, stair, ramp, lift, escalator

Thickness, specification, fire rating, sound rating, slope, top height — every value belonging to the substance is held by the relation. Openings are placed as intervals along it.

Why not put it on the space? Because a relation grants a seat and a space's attributes do not. Put substance on the space side and the derivation has to work out which surface it lands on, and that is where ambiguity enters. A state in which the slab between two storeys can be claimed separately by the space below as its ceiling and by the space above as its floor is not a source of truth. With a relation, the owner is uniquely determined.

That decision has a consequence. The exterior must be a space. For a roof to be a relation the sky must exist as the other space; for a ground-bearing floor to be a relation the ground must. This is why space /out exterior appears in .muro files.

Rule 1 — no boundary may be written between spaces that do not touch​

A relation declared where no segment can be derived is not a state that holds.

grid X 0 3600 7200
grid Y 0 4000 8000
level L1 0 h:2400 slab:150
space /L1/a room X1..X2 Y1..Y2
space /L1/b room X2..X3 Y2..Y3
boundary /L1/a /L1/b
✖ b1.muro:line 6: The spaces do not touch, so no boundary can be derived: /L1/a | /L1/b

These two rooms meet at a corner, but "touch" in koyu means sharing an edge that has length. A single corner point is not contact.

Rule 2 — one relation may split into several segments​

A boundary with a space that has no region — the exterior, typically — is the perimeter of the room minus the stretches where it meets other spaces. That usually falls on several sides. One relation, several segments.

So an opening in an external wall must say which side it is on.

grid X 0 3600
grid Y 0 4000
level L1 0 h:2400 slab:150
space /L1/living room X1..X2 Y1..Y2
space /out exterior
boundary /L1/living /out
  door w:900
✖ b2.muro:line 7: There is more than one boundary segment; pick an edge with edge:N/E/S/W (/L1/living | /out)

Compass directions come from the coordinate system. X is positive east, Y is positive north, so N=+Y, S=−Y, E=+X, W=−X. And edge is the side as seen from the rectangle of a — the space written first.

grid X 0 3600
grid Y 0 4000
level L1 0 h:2400 slab:150
space /L1/living room X1..X2 Y1..Y2
space /out exterior
boundary /L1/living /out t:150
  door w:900 edge:S

Swap the order of the two paths and edge:S picks a different side. The convention is in orientation, and choosing an edge is covered in positions.

Rule 3 — the same pair of spaces may not carry two boundaries​

A relation has identity. Two lines for one relation means two answers to one question.

grid X 0 3600 7200
grid Y 0 4000
level L1 0 h:2400 slab:150
space /L1/a room X1..X2 Y1..Y2
space /L1/b room X2..X3 Y1..Y2
boundary /L1/a /L1/b t:120
boundary /L1/a /L1/b type:open
✖ b3.muro:line 7: Duplicate boundary: /L1/a | /L1/b (first seen at b3.muro:line 6)

Silently letting the later line win would mean whether this is a wall or an opening is decided by line order. That is not a source of truth.

Because it is a relation, its identity need not be written​

uid may be written only on spaces and aggregates (space / zone). A relation cannot carry a uid.

The reason is in the nature of a relation. A relation always lies between two spaces, so fix both ends and the relation is fixed. What the outside world wants to point at is a space; to point at a relation, point at its two ends.

And there is a matter of count. Relations outnumber spaces — the bundled mixed-use complex has 425 spaces and 1,364 boundaries. Writing a uid per relation would erode the very goal of fitting a whole building into a machine's field of view.

Identity as a whole is covered in Identity.

Ordering constraints are small​

A boundary may be written before the spaces it names — relation declarations may refer forward. Only grid and level must precede the lines that use them.

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