feat v1.19.0: a line is never an entity of its own — walls are derived from rooms

A wall can only exist as an edge of a closed room:
- roomEdges(rooms) derives walls from room outlines, deduping shared ones, so deleting a
  room keeps the borders its neighbours still contribute and drops the rest — the rule
  falls out of the model instead of needing bookkeeping.
- Nothing is persisted while drawing: an outline you never close leaves no lines behind
  (previously every click pair was written to space.segments immediately).
- The 'Erase line' tool is gone; space.segments is stripped on every save (validation
  still tolerates it on read so a stale tab cannot fail a save).
- Dead code out: _addSegment/_removeSegmentByKey/_distToSeg/_pathSegs/_segKey.
Docs (ARCHITECTURE/TESTING/CHANGELOG) updated in the same commit. +2 tests (63).

custom_components/houseplan/const.py

@@ -11,7 +11,7 @@ PLANS_DIR = "houseplan/plans"  # relative to the HA configuration directory
 FILES_URL = "/houseplan_files/files"
 FILES_DIR = "houseplan/files"
 CONF_ADMIN_ONLY = "admin_only"
-VERSION = "1.17.2"
+VERSION = "1.19.0"
 
 DEFAULT_CONFIG: dict = {
     "spaces": [],

custom_components/houseplan/diagnostics.py

@@ -31,7 +31,6 @@ async def async_get_config_entry_diagnostics(
                 "has_plan": bool(s.get("plan_url")),
                 "rooms": len(s.get("rooms", [])),
                 "rooms_with_area": sum(1 for r in s.get("rooms", []) if r.get("area")),
-                "segments": len(s.get("segments", [])),
             }
             for s in config.get("spaces", [])
         ],

custom_components/houseplan/manifest.json

@@ -16,5 +16,5 @@
   "issue_tracker": "https://github.com/Matysh/houseplan-card/issues",
   "requirements": [],
   "single_config_entry": true,
-  "version": "1.17.2"
+  "version": "1.19.0"
 }

custom_components/houseplan/validation.py

@@ -100,6 +100,9 @@ SPACE_SCHEMA = vol.Schema(
         vol.Required("aspect"): vol.All(vol.Coerce(float), vol.Range(min=0.05, max=20)),
         vol.Required("view_box"): vol.All([vol.Coerce(float)], vol.Length(min=4, max=4)),
         vol.Required("rooms"): [ROOM_SCHEMA],
+        # Legacy: walls are derived from room outlines since v1.19.0 — a line has no
+        # independent existence. Still accepted so a stale browser tab cannot fail a save;
+        # the card strips the field on every write.
         vol.Optional("segments"): [vol.All([vol.Coerce(float)], vol.Length(min=4, max=4))],
     },
     extra=vol.ALLOW_EXTRA,

docs/ARCHITECTURE.md

@@ -111,12 +111,22 @@ If the server config is empty, the card falls back to the legacy bundle (the dac
 
 ## Markup editor (v1.4.0+)
 
-State inside the card: `_markup` (mode), `_tool` (draw/erase/delroom), `_path` (the current outline,
-vertices on the GRID_N=60 grid). Clicks on the stage → `_svgPoint`→`_snap`. Each pair of points adds a
-segment to `space.segments` (dedup by key, saved via config/set with debounce). The outline is closed
+State inside the card: `_markup` (mode), `_tool` (draw/delroom), `_path` (the current outline,
+vertices on the GRID_N=240 grid). Clicks on the stage → `_svgPoint`→`_snap`. The outline is closed
 = a click on the first vertex → area select (hass.areas) + name → room {poly}. Polygon rooms and
 rectangles are rendered uniformly (hit-test: point-in-polygon / rect).
 
+**A line is never an entity of its own (v1.19.0).** Nothing is persisted while you draw: an
+outline you never close leaves no trace. Walls are *derived* from the room outlines by
+`roomEdges(rooms)` (logic.ts) and deduped by `segKey`, so a wall shared by two rooms is emitted
+once — deleting a room therefore keeps the borders its neighbours still contribute and drops the
+rest, with no bookkeeping. The legacy `space.segments` array is stripped on every save (validation
+still tolerates it on read; see CHANGELOG v1.19.0).
+
+While drawing, the length of the current segment follows the cursor (`_fmtLen` → `segmentCm`/
+`formatLength`): metres, or feet+inches when `hass.config.unit_system` is imperial. The scale is
+per-space `cell_cm` — cm represented by one grid cell (default 5, so 240 cells ≈ 12 m).
+
 ## Integration WS API
 
 | Command | Parameters | Response |

docs/CHANGELOG.md

@@ -1,5 +1,44 @@
 # Changelog
 
+## v1.19.0 — 2026-07-16 (a line is never a thing of its own)
+**Model change.** A wall can only exist as an edge of a closed room. Consequences:
+- **Walls are derived from room outlines** (`roomEdges` in logic.ts), not stored. A wall
+  shared by two rooms is emitted once, so **deleting a room keeps the borders its neighbours
+  still contribute** — and drops the walls nobody else uses. This falls out of the model
+  instead of needing bookkeeping.
+- **An abandoned outline leaves nothing behind.** Previously every click pair was written to
+  `space.segments` immediately, so a contour you never closed left orphan lines on the plan.
+  Now nothing is persisted until the room is saved.
+- **The "Erase line" tool is gone** — there is no standalone line to erase. Mis-clicks are
+  undone with Esc / Ctrl+Z as before.
+- **`space.segments` is dropped on every save** (legacy configs shed it on first write).
+  Validation still tolerates the field so a stale browser tab cannot fail a save; diagnostics
+  no longer reports it. Lines that belonged to no room disappear on upgrade — by design.
+- Dead code removed: `_addSegment`, `_removeSegmentByKey`, `_distToSeg`, `_pathSegs`, `_segKey`.
+  `segKey(a, b, prec)` gained a precision argument (normalized coords need more than render
+  units). (+2 tests: 61 → 63.)
+
+## v1.18.1 — 2026-07-16 (fix: the drawing ruler was invisible)
+- Fix on top of v1.18.0: the length badge never showed up while drawing. It was rendered
+  inside `.devlayer`, and `.stage.markup .devlayer { display: none }` hides that whole layer
+  in markup mode (so icons do not get in the way) — the badge was in the DOM but invisible.
+  It now lives in its own `.measurelayer` (absolute, `pointer-events: none`), which markup
+  mode does not hide. Verified visually on a real drawn segment ("3.60 m" on screen).
+- Testing lesson (see docs/TESTING.md): asserting on `textContent` is not enough — a DOM
+  query passes on elements hidden by an ancestor. Check `offsetParent`/rect or look at a
+  screenshot.
+
+## v1.18.0 — 2026-07-14 (live measurements while drawing rooms + per-space scale)
+- **Ruler while drawing.** In room-markup "draw" mode, a badge follows the cursor showing the
+  length of the current segment (last placed vertex → cursor). Units come from the HA unit
+  system: metric → metres ("1.25 m"), imperial → feet+inches ("4′ 1″").
+- **Per-space scale.** New "Scale (grid cell size)" field in the space dialog — cm represented
+  by one grid cell (default **5 cm**, i.e. 240 cells ≈ 12 m). Stored as `space.cell_cm`; each
+  plan can have its own real-world size.
+- Pure helpers `segmentCm` / `formatLength` in logic.ts (unit-tested); `_fmtLen` +
+  `_renderMeasureLabel` in the card; `.measurelabel` style; i18n `space.scale_label`/`scale_unit`.
+  (+3 tests: 58 → 61 frontend.)
+
 ## v1.17.2 — 2026-07-11 (humidity badge: gate on the sensor, not the icon)
 - Fix on top of v1.17.1: the humidity `%` badge is now shown whenever the marker's primary
   entity is a humidity sensor (`device_class: humidity`), regardless of the resolved icon.

docs/STATUS.md

@@ -13,14 +13,14 @@
 
 | Item | State |
 |---|---|
-| Version | **v1.17.2** everywhere (manifest, const.py, package.json, CARD_VERSION) |
+| Version | **v1.19.0** everywhere (manifest, const.py, package.json, CARD_VERSION) |
 | GitHub | https://github.com/Matysh/houseplan-card — branch `main`, releases v1.9.3…v1.11.2 |
 | CI | `.github/workflows/validate.yml` (hacs + hassfest + frontend + backend) — **fully green** since v1.11.1; `release.yml` auto-attaches the card bundle (needs `permissions: contents: write`, fixed) |
 | HACS | Works as custom repository (id 1290210112 on the home instance). **Inclusion PR: https://github.com/hacs/default/pull/9004** (queue ≈2 months as of 2026-07). Lesson: #8995 was auto-closed by hacs-bot — the PR body MUST be their exact template with every checkbox ticked and all 3 links (release, HACS action run, hassfest run); a custom body gets closed without discussion |
 | Brands | Ships **inside the integration**: `custom_components/houseplan/brand/{icon,icon@2x,logo,logo@2x}.png` (HA ≥2026.3 local-brands mechanism). home-assistant/brands PR #10700 was auto-closed — that repo no longer accepts custom integrations |
 | Home instance | ha.jbstudio.pro (SSH port 323, key `ha_jb`), deployed v1.11.2, installed *via HACS* (custom repo) — updates flow through HACS now |
 | Localization | UI en/ru (`src/i18n.ts`), auto by `hass.locale` + `language` card option; codebase and docs are English-first (`README.ru.md` is the Russian copy) |
-| Tests | 58 frontend (node:test, incl. a 12-test buildDevices suite on a fake hass) + 10 pure backend (anywhere) + 12 HA-harness backend (CI only, py3.13; skipped locally — sandbox has py3.10) |
+| Tests | 63 frontend (node:test, incl. a 12-test buildDevices suite on a fake hass) + 10 pure backend (anywhere) + 12 HA-harness backend (CI only, py3.13; skipped locally — sandbox has py3.10) |
 
 ## Recent milestones (details in CHANGELOG.md)
 

docs/TESTING.md

@@ -63,7 +63,14 @@ Run the *core flows* (marked ★ below) in each environment at least once per mi
 
 ## Room markup editor ★
 
-- [ ] Grid appears; dots snap; segments draw pair-by-pair; shared walls reused
+- [ ] Grid appears; dots snap; the outline draws pair-by-pair; shared walls reused
+- [ ] Ruler: while drawing, the length of the current segment follows the cursor
+      (metres, or feet+inches on an imperial HA); scale = space "cm per cell" (default 5)
+- [ ] A line cannot exist on its own: start an outline, do NOT close it, leave markup —
+      no lines are left behind (nothing was written to the config)
+- [ ] Deleting a room removes its walls, EXCEPT those shared with a neighbouring room
+      (the neighbour still yields them); deleting the neighbour too removes them as well
+- [ ] There is no "Erase" tool in the markup toolbar (removed in v1.19.0)
 - [ ] Esc / Ctrl+Z removes the last dot (and its line); Reset clears the path
 - [ ] Closing the contour (click the first dot, ≥4 points) opens the room dialog
 - [ ] Room dialog: area list shows only unassigned areas; picking an area prefills the name

package.json

@@ -1,6 +1,6 @@
 {
   "name": "houseplan-card",
-  "version": "1.17.2",
+  "version": "1.19.0",
   "description": "Interactive house plan Lovelace card for Home Assistant",
   "license": "MIT",
   "type": "module",

src/houseplan-card.ts

@@ -12,7 +12,8 @@ import {
   type IconRule, type CompiledIconRule,
 } from './rules';
 import {
-  lqiColor, snapToGrid, segKey as segKeyOf, samePoint, pointInPolygon, markerIdForBinding,
+  lqiColor, snapToGrid, samePoint, pointInPolygon, markerIdForBinding,
+  segmentCm, formatLength, roomEdges,
   averageLqi, fitView, declump, safeUrl, resolveTapAction, floorsOf, type FloorInfo,
   spaceDisplayOf, roomFillColor, DEFAULT_ROOM_COLOR, DEFAULT_ROOM_OPACITY,
   DEFAULT_TEMP_MIN, DEFAULT_TEMP_MAX, type SpaceDisplay,
@@ -26,14 +27,14 @@ import './space-card';
 import { cardStyles } from './styles';
 import { langOf, t, type I18nKey } from './i18n';
 
-const CARD_VERSION = '1.17.2';
+const CARD_VERSION = '1.19.0';
 const LS_KEY = 'houseplan_card_layout_v1';
 const LS_CFG = 'houseplan_card_cfg_v1'; // cache of the server config+layout for instant rendering
 const LS_ZOOM = 'houseplan_card_zoom_v1';
 const NORM_W = 1000; // width of the render space for normalized configs
 
 const GRID_N = 240; // grid points across the plan width (half the previous step; old nodes are a subset of the new ones, positions are preserved)
-type MarkupTool = 'draw' | 'erase' | 'delroom';
+type MarkupTool = 'draw' | 'delroom';
 
 const fireEvent = (node: EventTarget, type: string, detail?: unknown) => {
   const ev = new Event(type, { bubbles: true, composed: true }) as any;
@@ -79,7 +80,6 @@ class HouseplanCard extends LitElement {
   private _markup = false;
   private _tool: MarkupTool = 'draw';
   private _path: number[][] = []; // current outline (render units, vertices snapped to the grid)
-  private _pathSegs: (string | null)[] = []; // keys of the segments added by outline steps
   private _cursorPt: number[] | null = null;
   private _areaSel = '';
   private _nameSel = '';
@@ -132,6 +132,7 @@ class HouseplanCard extends LitElement {
     fillMode: 'none' | 'lqi' | 'light' | 'temp';
     tempMin: number;
     tempMax: number;
+    cellCm: number;                // real-world cm represented by one grid cell
     busy: boolean;
   } | null = null;
   private _keyHandler = (e: KeyboardEvent) => this._onKey(e);
@@ -219,32 +220,12 @@ class HouseplanCard extends LitElement {
     }
   }
 
-  /** Remove the last placed point (and its line, if it was added by that step). */
+  /** Remove the last placed point. An unfinished outline is never persisted. */
   private _undoPoint(): void {
     if (!this._path.length) return;
-    if (this._path.length === 1) {
-      this._path = [];
-      this._pathSegs = [];
-      return;
-    }
-    const segKey = this._pathSegs[this._pathSegs.length - 1];
-    this._pathSegs = this._pathSegs.slice(0, -1);
-    if (segKey) this._removeSegmentByKey(segKey);
     this._path = this._path.slice(0, -1);
   }
 
-  private _removeSegmentByKey(key: string): void {
-    const sp = this._curSpaceCfg;
-    if (!sp?.segments) return;
-    const idx = this._segments.findIndex(
-      (s) => this._segKey([s[0], s[1]], [s[2], s[3]]) === key,
-    );
-    if (idx >= 0) {
-      sp.segments.splice(idx, 1);
-      this._saveConfig();
-    }
-  }
-
   public static getConfigElement() {
     return document.createElement('houseplan-card-editor');
   }
@@ -944,6 +925,18 @@ class HouseplanCard extends LitElement {
     return NORM_W / GRID_N;
   }
 
+  /** cm represented by one grid cell for the current space (default 5). */
+  private get _cellCm(): number {
+    const v = Number(this._curSpaceCfg?.cell_cm);
+    return Number.isFinite(v) && v > 0 ? v : 5;
+  }
+
+  /** Human-readable length of a segment (render units) using the HA unit system. */
+  private _fmtLen(a: number[], b: number[]): string {
+    const cm = segmentCm(a, b, this._gridPitch, this._cellCm);
+    return formatLength(cm, this.hass?.config?.unit_system?.length === 'mi');
+  }
+
   private get _curSpaceCfg(): any {
     return this._serverCfg?.spaces.find((s: any) => s.id === this._space);
   }
@@ -953,11 +946,15 @@ class HouseplanCard extends LitElement {
     return sp ? NORM_W / sp.aspect : NORM_W;
   }
 
-  /** Segments of the current space in render units. */
+  /**
+   * Walls of the current space in render units — DERIVED from the room outlines.
+   * There is no standalone "line" entity: every wall belongs to a closed room, and a
+   * wall shared with a neighbour survives deleting either room (the other still yields it).
+   */
   private get _segments(): number[][] {
     const sp = this._curSpaceCfg;
     const H = this._spaceH;
-    return (sp?.segments || []).map((s: number[]) => [s[0] * NORM_W, s[1] * H, s[2] * NORM_W, s[3] * H]);
+    return roomEdges(sp?.rooms || []).map((s) => [s[0] * NORM_W, s[1] * H, s[2] * NORM_W, s[3] * H]);
   }
 
   private _toggleMarkup(): void {
@@ -986,12 +983,17 @@ class HouseplanCard extends LitElement {
     return samePoint(a, b);
   }
 
-  private _segKey(a: number[], b: number[]): string {
-    return segKeyOf(a, b);
+  /**
+   * Walls are derived from rooms, so the legacy per-space `segments` array is dead
+   * weight: drop it on every save. Configs written before v1.19.0 shed it on first write.
+   */
+  private _dropLegacySegments(): void {
+    for (const sp of this._serverCfg?.spaces || []) delete (sp as any).segments;
   }
 
   private _saveConfig = debounce(() => {
     if (!this._serverCfg) return;
+    this._dropLegacySegments();
     this.hass
       .callWS({ type: 'houseplan/config/set', config: this._serverCfg, expected_rev: this._cfgRev })
       .then((r: any) => {
@@ -1008,33 +1010,6 @@ class HouseplanCard extends LitElement {
       });
   }, 500);
 
-  /** Add a segment (render units) to the space skeleton (no duplicates). true = new. */
-  private _addSegment(a: number[], b: number[]): boolean {
-    const sp = this._curSpaceCfg;
-    if (!sp) return false;
-    const H = this._spaceH;
-    const key = this._segKey(a, b);
-    const exists = this._segments.some((s) => this._segKey([s[0], s[1]], [s[2], s[3]]) === key);
-    if (exists) return false;
-    sp.segments = sp.segments || [];
-    sp.segments.push([a[0] / NORM_W, a[1] / H, b[0] / NORM_W, b[1] / H]);
-    this._saveConfig();
-    return true;
-  }
-
-  private _distToSeg(p: number[], s: number[]): number {
-    const [x, y] = p;
-    const [x1, y1, x2, y2] = s;
-    const dx = x2 - x1;
-    const dy = y2 - y1;
-    const len2 = dx * dx + dy * dy || 1;
-    let t = ((x - x1) * dx + (y - y1) * dy) / len2;
-    t = Math.max(0, Math.min(1, t));
-    const px = x1 + t * dx;
-    const py = y1 + t * dy;
-    return Math.hypot(x - px, y - py);
-  }
-
   private _pointInRoom(p: number[], r: RoomCfg): boolean {
     if (r.poly) return pointInPolygon(p, r.poly);
     return (
@@ -1045,26 +1020,6 @@ class HouseplanCard extends LitElement {
   private _markupClick(ev: MouseEvent): void {
     if (!this._markup) return;
     const raw = this._svgPoint(ev);
-    if (this._tool === 'erase') {
-      const sp = this._curSpaceCfg;
-      if (!sp?.segments?.length) return;
-      const segs = this._segments;
-      let best = -1;
-      let bestD = this._gridPitch * 0.5;
-      segs.forEach((s, i) => {
-        const d = this._distToSeg(raw, s);
-        if (d < bestD) {
-          bestD = d;
-          best = i;
-        }
-      });
-      if (best >= 0) {
-        sp.segments.splice(best, 1);
-        this._saveConfig();
-        this.requestUpdate();
-      }
-      return;
-    }
     if (this._tool === 'delroom') {
       const space = this._spaceModel();
       const room = [...space.rooms].reverse().find((r) => this._pointInRoom(raw, r));
@@ -1078,17 +1033,15 @@ class HouseplanCard extends LitElement {
       this.requestUpdate();
       return;
     }
-    // draw: clicks on grid points, pairs of points get connected with a line
+    // draw: clicks on grid points build the outline. Nothing is written to the config
+    // until the contour closes — an abandoned outline leaves no lines behind.
     const pt = this._snap(raw);
     if (!this._path.length) {
       this._path = [pt];
-      this._pathSegs = [];
       return;
     }
     const last = this._path[this._path.length - 1];
     if (this._samePt(pt, last)) return; // repeated click on the same point
-    const added = this._addSegment(last, pt);
-    this._pathSegs = [...this._pathSegs, added ? this._segKey(last, pt) : null];
     this._path = [...this._path, pt];
     // closing the outline: a click on the first vertex → the save dialog
     if (this._path.length >= 4 && this._samePt(pt, this._path[0])) {
@@ -1138,7 +1091,6 @@ class HouseplanCard extends LitElement {
     });
     this._saveConfig();
     this._path = [];
-    this._pathSegs = [];
     const boundArea = this._areaSel;
     this._areaSel = '';
     this._nameSel = '';
@@ -1174,7 +1126,6 @@ class HouseplanCard extends LitElement {
 
   private _cancelPath(): void {
     this._path = [];
-    this._pathSegs = [];
     this._cursorPt = null;
     this._roomDialog = false;
   }
@@ -1509,6 +1460,7 @@ class HouseplanCard extends LitElement {
         showBorders: disp.showBorders, showNames: disp.showNames,
         roomColor: disp.color, roomOpacity: disp.opacity, fillMode: disp.fill,
         tempMin: disp.tempMin, tempMax: disp.tempMax,
+        cellCm: Number(sp.cell_cm) > 0 ? Number(sp.cell_cm) : 5,
         busy: false,
       };
     } else {
@@ -1518,6 +1470,7 @@ class HouseplanCard extends LitElement {
         showBorders: false, showNames: false,
         roomColor: DEFAULT_ROOM_COLOR, roomOpacity: DEFAULT_ROOM_OPACITY, fillMode: 'none',
         tempMin: DEFAULT_TEMP_MIN, tempMax: DEFAULT_TEMP_MAX,
+        cellCm: 5,
         busy: false,
       };
     }
@@ -1573,7 +1526,6 @@ class HouseplanCard extends LitElement {
           aspect: d.source === 'draw' ? drawAspect : 1.414,
           view_box: [0, 0, 1, 1],
           rooms: [],
-          segments: [],
         };
         cfg.spaces.push(sp);
       } else {
@@ -1602,6 +1554,7 @@ class HouseplanCard extends LitElement {
         temp_min: Number.isFinite(d.tempMin) ? Math.min(d.tempMin, d.tempMax) : DEFAULT_TEMP_MIN,
         temp_max: Number.isFinite(d.tempMax) ? Math.max(d.tempMin, d.tempMax) : DEFAULT_TEMP_MAX,
       };
+      sp.cell_cm = Number.isFinite(d.cellCm) && d.cellCm > 0 ? d.cellCm : 5;
       await this._saveConfigNow();
       this._spaceDialog = null;
       if (d.mode === 'create') this._space = sp.id;
@@ -1652,6 +1605,7 @@ class HouseplanCard extends LitElement {
   user's retry starts from the fresh config instead of hitting the same
   conflict again. */
   private async _saveConfigNow(): Promise<void> {
+    this._dropLegacySegments();
     try {
       const r = await this.hass.callWS({
         type: 'houseplan/config/set', config: this._serverCfg, expected_rev: this._cfgRev,
@@ -1690,6 +1644,7 @@ class HouseplanCard extends LitElement {
       showBorders: false, showNames: false,
       roomColor: DEFAULT_ROOM_COLOR, roomOpacity: DEFAULT_ROOM_OPACITY, fillMode: 'none',
       tempMin: DEFAULT_TEMP_MIN, tempMax: DEFAULT_TEMP_MAX,
+      cellCm: 5,
       busy: false,
     };
   }
@@ -2008,6 +1963,9 @@ class HouseplanCard extends LitElement {
               ? space.rooms.map((r) => this._renderRoomLabel(r, space, view, disp))
               : nothing}
           </div>
+          ${this._markup && this._tool === 'draw' && this._path.length && this._cursorPt && !this._contourClosed
+            ? html`<div class="measurelayer">${this._renderMeasureLabel(view)}</div>`
+            : nothing}
           </div>
           ${this._zoom > 1
             ? html`<div class="zoombadge">${Math.round(this._zoom * 100)}%</div>`
@@ -2128,6 +2086,15 @@ class HouseplanCard extends LitElement {
     >${r.name}</div>`;
   }
 
+  /** Length badge that follows the cursor while drawing the current segment. */
+  private _renderMeasureLabel(view: { x: number; y: number; w: number; h: number }): TemplateResult {
+    const a = this._path[this._path.length - 1];
+    const b = this._cursorPt!;
+    const left = ((b[0] - view.x) / view.w) * 100;
+    const top = ((b[1] - view.y) / view.h) * 100;
+    return html`<div class="measurelabel" style="left:${left}%;top:${top}%">${this._fmtLen(a, b)}</div>`;
+  }
+
   private _roomCenter(r: RoomCfg): number[] {
     if (r.poly) {
       const n = r.poly.length;
@@ -2174,10 +2141,6 @@ class HouseplanCard extends LitElement {
         title=${this._t('title.markup_add')}>
         <ha-icon icon="mdi:vector-polyline-plus"></ha-icon>${this._t('markup.add')}
       </button>
-      <button class="btn ${this._tool === 'erase' ? 'on' : ''}" @click=${() => (this._tool = 'erase')}
-        title=${this._t('title.markup_erase')}>
-        <ha-icon icon="mdi:eraser"></ha-icon>${this._t('markup.erase')}
-      </button>
       <button class="btn ${this._tool === 'delroom' ? 'on' : ''}" @click=${() => (this._tool = 'delroom')}
         title=${this._t('title.markup_delroom')}>
         <ha-icon icon="mdi:delete-outline"></ha-icon>${this._t('markup.delete')}
@@ -2402,6 +2365,16 @@ class HouseplanCard extends LitElement {
               </select>`
             : nothing}
 
+          <label>${this._t('space.scale_label')}</label>
+          <div class="colorrow">
+            <input class="namein tempin" type="number" min="0.1" step="0.1" .value=${String(d.cellCm)}
+              @input=${(e: Event) => {
+                const n = parseFloat((e.target as HTMLInputElement).value);
+                this._spaceDialog = { ...d, cellCm: Number.isFinite(n) && n > 0 ? n : d.cellCm };
+              }} />
+            <span class="opl">${this._t('space.scale_unit')}</span>
+          </div>
+
           <label class="dispsection">${this._t('space.display_section')}</label>
           <label class="srcrow">
             <input type="checkbox" .checked=${d.showBorders}

src/i18n/en.json

@@ -27,13 +27,11 @@
   "title.configure_space": "Configure space",
   "title.add_space": "Add space",
   "title.markup_add": "Add a room: connect grid dots with lines until the outline closes",
-  "title.markup_erase": "Erase a line: click the line",
   "title.markup_delroom": "Delete a room: click inside the room",
   "title.no_area_room": "Decorative room without an HA area (e.g. a hallway)",
   "title.choose_area": "Select a Home Assistant area",
   "title.need_plan": "Upload a floor-plan image",
   "markup.add": "Add",
-  "markup.erase": "Erase",
   "markup.delete": "Delete",
   "markup.hint_points": "points: {n} · Esc/Ctrl+Z — undo a dot · close the outline by clicking the first one",
   "markup.hint_start": "click a grid dot to start the outline",
@@ -152,6 +150,8 @@
   "import.progress": "Floor {i} of {n}",
   "import.done": "Spaces created. Outline the rooms: click grid dots and close the contour.",
   "btn.skip": "Skip",
+  "space.scale_label": "Scale (grid cell size)",
+  "space.scale_unit": "cm per cell",
   "space.display_section": "Display",
   "space.show_borders": "Always show room borders",
   "space.show_names": "Show room names (drag to move)",

src/i18n/ru.json

@@ -27,13 +27,11 @@
   "title.configure_space": "Настроить пространство",
   "title.add_space": "Добавить пространство",
   "title.markup_add": "Добавить комнату: соединяйте точки сетки линиями до замкнутого контура",
-  "title.markup_erase": "Стереть линию: клик по линии",
   "title.markup_delroom": "Удалить комнату: клик внутри комнаты",
   "title.no_area_room": "Декоративная комната без привязки к зоне (например, холл)",
   "title.choose_area": "Выберите зону Home Assistant",
   "title.need_plan": "Загрузите подложку (план этажа)",
   "markup.add": "Добавить",
-  "markup.erase": "Стереть",
   "markup.delete": "Удалить",
   "markup.hint_points": "точек: {n} · Esc/Ctrl+Z — убрать точку · замкните контур кликом по первой",
   "markup.hint_start": "кликните точку сетки, чтобы начать контур",
@@ -152,6 +150,8 @@
   "import.progress": "Этаж {i} из {n}",
   "import.done": "Пространства созданы. Обведите комнаты: кликайте по точкам сетки и замкните контур.",
   "btn.skip": "Пропустить",
+  "space.scale_label": "Масштаб (размер клетки сетки)",
+  "space.scale_unit": "см на клетку",
   "space.display_section": "Отображение",
   "space.show_borders": "Всегда отображать границы комнат",
   "space.show_names": "Отображать названия комнат (перетаскиваются)",

src/logic.ts

@@ -13,10 +13,60 @@ export function snapToGrid(v: number, pitch: number): number {
   return Math.round(v / pitch) * pitch;
 }
 
-/** Canonical key of a segment (independent of direction). */
-export function segKey(a: number[], b: number[]): string {
+/** Real-world length (cm) of a segment given the grid pitch (render units per cell) and cm per cell. */
+export function segmentCm(a: number[], b: number[], gridPitch: number, cellCm: number): number {
+  const cells = Math.hypot(b[0] - a[0], b[1] - a[1]) / gridPitch;
+  return cells * cellCm;
+}
+
+/** Format a length (cm) for display: metric metres ("1.25 m") or imperial feet+inches ("4′ 1″"). */
+export function formatLength(cm: number, imperial: boolean): string {
+  if (imperial) {
+    const totalIn = cm / 2.54;
+    let ft = Math.floor(totalIn / 12);
+    let inch = Math.round(totalIn - ft * 12);
+    if (inch === 12) { ft += 1; inch = 0; }
+    return `${ft}′ ${inch}″`;
+  }
+  return `${(cm / 100).toFixed(2)} m`;
+}
+
+/**
+ * Canonical key of a segment (independent of direction).
+ * `prec` = decimals used to compare coordinates: the default 1 suits render units,
+ * normalized (0..1) coordinates need more (see roomEdges).
+ */
+export function segKey(a: number[], b: number[], prec = 1): string {
   const [p, q] = a[0] < b[0] || (a[0] === b[0] && a[1] <= b[1]) ? [a, b] : [b, a];
-  return `${p[0].toFixed(1)},${p[1].toFixed(1)}-${q[0].toFixed(1)},${q[1].toFixed(1)}`;
+  return `${p[0].toFixed(prec)},${p[1].toFixed(prec)}-${q[0].toFixed(prec)},${q[1].toFixed(prec)}`;
+}
+
+/**
+ * Wall segments derived from room outlines (normalized coordinates in and out).
+ *
+ * A line has no independent existence on the plan: it can only be an edge of a closed
+ * room. Shared walls are emitted once, which is what makes deleting a room keep the
+ * borders its neighbours still contribute — the neighbour's polygon still yields them.
+ */
+export function roomEdges(rooms: any[]): number[][] {
+  const out: number[][] = [];
+  const seen = new Set<string>();
+  for (const r of rooms || []) {
+    let pts: number[][] | null = null;
+    if (r?.poly?.length) pts = r.poly;
+    else if (r && r.x != null && r.y != null && r.w != null && r.h != null)
+      pts = [[r.x, r.y], [r.x + r.w, r.y], [r.x + r.w, r.y + r.h], [r.x, r.y + r.h]];
+    if (!pts || pts.length < 3) continue;
+    for (let i = 0; i < pts.length; i++) {
+      const a = pts[i];
+      const b = pts[(i + 1) % pts.length];
+      const k = segKey(a, b, 5);
+      if (seen.has(k)) continue;
+      seen.add(k);
+      out.push([a[0], a[1], b[0], b[1]]);
+    }
+  }
+  return out;
 }
 
 /** Point equality within a tolerance. */

src/styles.ts

@@ -236,6 +236,25 @@ export const cardStyles = css`
       z-index: 1;
     }
     .roomlabel:active { cursor: grabbing; }
+    .measurelayer {
+      position: absolute;
+      inset: 0;
+      pointer-events: none;
+    }
+    .measurelabel {
+      position: absolute;
+      transform: translate(12px, -150%);
+      font-size: 12px;
+      font-weight: 600;
+      padding: 1px 6px;
+      border-radius: 6px;
+      background: rgba(0, 0, 0, 0.72);
+      color: #fff;
+      white-space: nowrap;
+      pointer-events: none;
+      user-select: none;
+      z-index: 3;
+    }
     .rlabel {
       fill: var(--hp-muted);
       font-size: 15px;

test/logic.test.mjs

@@ -3,6 +3,7 @@ import assert from 'node:assert/strict';
 import {
   lqiColor, snapToGrid, segKey, samePoint, pointInPolygon, markerIdForBinding, averageLqi,
   fitView, declump, safeUrl, resolveTapAction, floorsOf, subst, spaceDisplayOf, roomFillColor,
+  segmentCm, formatLength, roomEdges,
 } from '../test-build/logic.js';
 import {
   iconFor, compileIconRules, isValidPattern, iconFromDeviceClasses,
@@ -258,3 +259,41 @@ test('spaceDisplayOf: temp bounds default to 20..25 and accept overrides', () =>
   assert.equal(o.tempMin, 18.5);
   assert.equal(o.tempMax, 23);
 });
+
+test('segmentCm: cells scaled by cm-per-cell', () => {
+  assert.equal(segmentCm([0, 0], [30, 40], 10, 5), 25); // 50 units / pitch 10 = 5 cells * 5cm
+  assert.ok(Math.abs(segmentCm([0, 0], [240, 0], 1000 / 240, 5) - 288) < 1e-9);
+});
+
+test('formatLength: metric metres with 2 decimals', () => {
+  assert.equal(formatLength(25, false), '0.25 m');
+  assert.equal(formatLength(125, false), '1.25 m');
+  assert.equal(formatLength(0, false), '0.00 m');
+});
+
+test('formatLength: imperial feet + inches, with inch rollover', () => {
+  assert.equal(formatLength(124.46, true), '4′ 1″');
+  assert.equal(formatLength(30.48, true), '1′ 0″');
+  assert.equal(formatLength(29.464, true), '1′ 0″');
+});
+
+test('roomEdges: a line exists only as a room edge; polygons and rects both yield walls', () => {
+  const sq = { poly: [[0, 0], [1, 0], [1, 1], [0, 1]] };
+  assert.equal(roomEdges([sq]).length, 4);          // closed outline → 4 walls
+  assert.equal(roomEdges([{ x: 0, y: 0, w: 1, h: 1 }]).length, 4); // legacy rect room
+  assert.equal(roomEdges([]).length, 0);            // no rooms → no lines at all
+  assert.equal(roomEdges([{ poly: [[0, 0], [1, 1]] }]).length, 0); // not a closed room → nothing
+});
+
+test('roomEdges: a wall shared by two rooms is emitted once, and survives deleting either room', () => {
+  const left = { id: 'a', poly: [[0, 0], [0.5, 0], [0.5, 1], [0, 1]] };
+  const right = { id: 'b', poly: [[0.5, 0], [1, 0], [1, 1], [0.5, 1]] }; // shares x=0.5 wall
+  const both = roomEdges([left, right]);
+  assert.equal(both.length, 7); // 4 + 4 - 1 shared, deduped regardless of direction
+  const shared = (segs) => segs.some((s) => s[0] === 0.5 && s[2] === 0.5);
+  assert.ok(shared(both));
+  // deleting 'left' → the shared wall stays, because 'right' still contributes it
+  assert.ok(shared(roomEdges([right])));
+  // deleting both → no lines remain
+  assert.equal(roomEdges([]).length, 0);
+});