Orientation and the a side

N, E, S and W are words about axes.

Word	Axis	Direction
N	+Y	north
S	−Y	south
E	+X	east
W	−X	west

They are not a compass needle. If the building is not oriented to true north, these four words still mean +X / −X / +Y / −Y in the coordinate system that grid X and grid Y set up. How coordinates are spelled is in positions and regions.

koyu 1.0
name 方位の例
unit mm

grid X 0 6400
grid Y 0 5600

level L1 0 h:2700 slab:200

space /L1/room room X1..X2 Y1..Y2 name:居室 daylight:1
space /out exterior name:外部

boundary /L1/room /out t:150
  window w:2400 h:1500 edge:S name:南窓
  window w:1200 h:1500 edge:E name:東窓
  door   w:900  h:2100 edge:N name:勝手口

The room above is a single rectangle X1..X2 Y1..Y2, so its boundary with the exterior falls into four segments. edge:S is the Y1 side (y = 0), edge:N is the Y2 side (y = 5600), edge:W is the X1 side and edge:E is the X2 side.

Why the mapping is what it is

The outline of a derived region runs counter-clockwise. Walking that vertex list and reading only the axis-parallel edges, an edge running +x is the S face, −x is N, +y is E, and −y is W. So N names the +Y face, S the −Y face, E the +X face and W the −X face.

A diagonal edge has no compass direction. Writing edge: on a boundary realised by a line narrows nothing — the segments of a boundary that carries a drawn line are not filtered by edge: at all.

What edge: selects

edge: narrows the segments of a boundary to those facing that way. It can be written in three places.

Where	Effect
on the boundary line	limits that boundary to one face from the start
on a door / window line	picks which face the opening sits on
on a seg line	the same

A boundary with the exterior normally has several segments. What is left of a room's perimeter after removing the stretches that face other spaces becomes the segments — four of them if all four sides are open. Placing an opening there without choosing a face stops.

There is more than one boundary segment; pick an edge with edge:N/E/S/W (/L1/a | /out)

That is OPN05 for openings and SEG05 for seg. Conversely, if there is no segment facing that way at all, it is OPN04 / SEG04.

The a side — whose face is being named

The compass word in edge: is read from the shape of the space written first on the boundary line (the a side).

space /L1/a room X1..X2 Y1..Y2
space /L1/b room X2..X3 Y1..Y2

boundary /L1/a /L1/b edge:E     # a's east face — the wall between the two rooms

Write the same wall the other way round and the word flips.

No shared edge on edge:E: /L1/b | /L1/a (they actually touch on W)

With /L1/b first, the wall between the rooms lies to b's west, so it is edge:W. The order of a and b matters for exactly two things — edge and swing. Everything else (where the segments fall, how area is split, which side a drawn line keeps) is independent of the order.

When the a side has no region, the word is read from the side that does. In boundary /out /L1/room edge:N the N is the north face of /L1/room, because the exterior has no shape to read a face from.

Which way the door opens — hinge and swing

Two words decide it. One is a word about axes, the other about the relation.

hinge is the jamb — which end of the segment the hinge sits on.

Segment	Accepted values	If omitted
horizontal (an N / S face)	W / E	the west end (the lower coordinate)
vertical (an E / W face)	S / N	the south end (the lower coordinate)
diagonal	—	pinned to the start end

Getting the axis wrong is OPN01 — hinge:N cannot go on a horizontal segment. Segments always run in ascending coordinate order, so "the start" is the west or south end.

swing is which side the leaf opens toward — a or b. It is not a compass word. Omitted, the door opens toward a if a has a region, otherwise toward b. The actual sense of rotation is taken from the component pointing at the centre of the nearest part of the shape on the side it opens toward.

Openings with style:sliding or style:auto have no arc — they slide toward the hinge side.

The axis of at: follows the face

On a horizontal segment (N / S) an absolute position is an X grid reference; on a vertical one (E / W) it is a Y reference.

The door position X1+1000 is on the wrong axis: a vertical segment takes a Y grid line

That is OPN07 / SEG07. A ratio (at:0.3) works on either.

Vertical circulation uses the same four words

The value of stair:, ramp: and escalator: is the direction of travel going up, spelled with the same four words.

space /B1/st stair X3..X3+2600 Y2..Y2+5400 name:避難階段 stair:N form:return

stair:N means "rises toward +Y". Riser count, tread depth and slope are never written — they are derived from the region and the storey height. A lift has no direction, so it is written lift:1. A value that is none of the four is RUN02.