# Wall thickness — the spec

Status: **implemented / evolving** (post beta.4 redesign). Visual reference:
[docs/assets/wall-thickness-reference.png](assets/wall-thickness-reference.png)
(look at wall bodies only).

Owner intent: thick walls form one continuous hatched body (seamless L and T),
grow both ways from the room centreline, fills/light stay inside the inner
contour, displayed area is the clean floor, and sun wedges narrow through the
opening tunnel.

Code: `src/wall-thickness.ts`, render in `src/houseplan-card.ts` /
`src/space-render.ts`. Sun: `src/sun.ts`. Tests: `test/wall-thickness.test.mjs`,
`demo/smoke_wall_thickness.mjs`.

## 1. Model

Per space: `walls: [{ key, cm }]`. Key = quantised midpoint + direction
(modulo 180°). Config always stores centimetres. Open boundaries refuse
thickness. One physical stretch has one thickness (atomic collinear spans when
neighbours overlap only partially).

Degrade unmatched keys silently on write. Resize / undo / scale re-key all
touched spans in the same transaction and keep `walls` in the resize snapshot.

## 2. Growth (centreline ±½)

Every thick wall grows **half outward and half inward** from the polygon edge
(outer and shared alike). Silhouette is wider than the polygon by `cm/2` on
outer walls. Paper and the content frame grow under that outer half.

## 3. Body render

Production body is the **ring** `outset(poly, half) − inset(poly, half)` per
room, **union**ed across rooms so shared and T junctions show one continuous
body with no internal seams (as in the reference plan). The body is painted in
two layers: a solid fill from global `settings.fill_colors.wall_fill` (default
opaque white, with its own opacity) **under** the diagonal hatch whose stroke
matches the wall outline. Neither replaces the other. Mitre joins; bevel when
the mitre spike exceeds `MITRE_LIMIT × thickness`.

Openings cut the body full-depth; jambs cap the cut; window glass mid-tunnel;
door swing from the **inner face**. Association uses wall direction ≈ opening
angle (mod 180°), then nearest span — never a perpendicular neighbour at a T.

## 4. Floor, fills, light, area

- **Inner contour** = `inset(poly, half[])`.
- Room fills and glow are clipped to the inner contour (not painted into the
  wall hatch).
- Displayed **m²** = area of the inner contour (clean floor). Wall-length
  rulers and opening anchors stay on the centreline.
- With no thickness, inner = poly (parity with pre-thickness behaviour).

## 5. Sun

Wedges do not draw through wall bodies. The window is a tunnel of depth `cm`:
effective lit width ≈ `max(0, L − d·tan(|α|))` where `α` is the ray angle from
the inward normal. Clip to the receiving room’s inner contour. `d = 0` keeps
the previous wedge. `hide_openings` hides the symbol only.

## 6. Tool / hooks / i18n

Plan-editor tool «Wall thickness»; hover whole wall; cm/in from HA; empty/0
clears; apply-to-room. Hooks: `data-hp="wall"`. i18n en/ru.

**Draw with thickness.** The Draw toolbar carries a session thickness field
(default **15 cm**, or inches when HA is imperial). Closing a new room outline
writes that cm onto every new edge that does not already have one; shared
stretches that already carry a neighbour's thickness are left alone. Empty / 0
leaves the room thin. Live thick preview follows the rubber-band while drawing.
Split does not use this field. The Wall thickness tool remains for later edits.

## 7. Out of scope

Decor-line thickness, per-side finish, auto-from-backdrop, plan-wide default.

## 8. Testing

Unit: ring closed at corners; half-out; inner area; atomic partial shared;
angle-aware opening; rekey after edge/scale. Browser: seamless frame; fill not
in hatch; m² drops with thickness; real resize/undo keeps walls; sun narrows
with thickness + obliqueness; nav mode restores after `can_write`.