ArchLang

23. The plan-symbol drawing language ​

  • Status: Accepted
  • Date: 2026-10
  • Scope: how every fixture symbol, door, window, fence, balcony rail and vertical-circulation glyph is drawn: pens, tones, curves, dashes and what a redraw may move. No language syntax change, no new theme key.

Context ​

The symbols drew correct facts badly. Every mark on a symbol was one tone (furnitureStroke, a pale grey chosen to sit behind the walls), and the finer of its two pens fell below what prints (about 0.1 mm on paper), so a WC's outline and its flush button, a sofa's frame and its cushion joints, read alike and the symbols read washed out next to the walls. Round things — a WC bowl, a basin, a tub — were tessellated polygons that faceted when zoomed. A door's open leaf was a single line, its swing was dashed like a hidden edge, and a window had no exterior sill to say which side was outside.

A redraw could not be a free hand. The glyph layer feeds more than the picture: the renderer mirrors a symbol only if it is handed (src/elements/glyph-chirality.ts), describe --facts symmetry reports a symbol's hand through the same probe (handed(), src/analyze/symmetry.ts), the drawing budget counts what each glyph draws (ElementDef.drawCost), and the permanent pins of test/fixture-byte-identity.test.ts (group 1) hold the shared path byte for byte.

Decision ​

1. A hierarchy of five, from the pens and tones that already exist. Wall (heavy) > door leaf and glazing > a symbol's OUTLINE (thin, in the derived symbol ink) > its DETAIL (extraThin, in furnitureStroke) > ground texture. Tone follows weight inside the glyph layer: GlyphCtx.tone maps thin to the ink and extraThin to furnitureStroke, and every factory in src/elements/glyph-lib.ts reads its colour from there, so the two cannot disagree inside one symbol. The outline is the first node a glyph emits, which makes it the node annotate marks data-arch-primary.

2. The finest pen is the ISO 128 floor. extraThin is EXTRA_THIN_RATIO (13/18) of thin, so on a sheet it lands on 0.13 mm, the thinnest pen that reproduces in print (src/scene.ts).

3. True curves. A round outline is a path of straight edges and minor arcs, each at most 120° (roundedRectPath, ovalPath — the four-centre oval — and bulgeArc), which every SVG and DXF draw as true arcs (SVG A, a DXF ARC; PDF lowers them to Béziers). The tessellated helpers stay in glyph-lib.ts, and no glyph draws with them.

4. A dash means hidden or above the cut plane, never decoration. An upper cabinet, a range hood, a wardrobe rail behind a closed door, a garage door's overhead projection and a pocket door's cavity are dashed; a hinged door's swing, at floor level, is a solid extraThin arc. The open leaf is a slab built from doorSwing's own points (src/elements/door.ts), so it cannot disagree with W_SWING_OBSTRUCTED. A window is double glazing plus an exterior sill on the side the floor probe (RenderCtx.floorAt) finds no floor.

5. No shadows. A drop shadow is ink outside the footprint (every glyph suite holds the symbol inside it), it has a direction, so it breaks the D4 equivariance of a placed instance and makes every symmetric symbol handed, and it implies a height, while heights draw nothing.

6. The symbol ink is derived, not a theme key. symbolInk mixes furnitureStroke toward wallStroke by SYMBOL_INK_MIX (3/7, the simplest fraction that clears WCAG 4.5:1 on every built-in theme's roomFill, measured by test/glyph-lib.test.ts). It follows any theme (a light wall ink lightens it), keeps a style furniture { stroke … } override in the author's hue, and leaves the defaults alone: the labelled rectangle an unknown word draws paints with furnitureStroke/furnitureFill and is a group-1 permanent pin, so those defaults cannot move. Vertical-circulation outlines use the same ink (src/elements/vertical-glyphs.ts).

7. What a redraw may move. A drawing change moves SVG (and the PNG/ASCII goldens and examples/*.svg that follow it); over the corpus describe(), lint() and compile().diagnostics stay byte-identical. The one fact a drawing can move is the opt-in describe --facts symmetry, because a piece's hand is read off its drawn symbol: a symbol may never GAIN handedness where it was symmetric, and it may lose it only through LOST_HANDEDNESS with the reason its old handed mark was decorative (test/glyph-chirality.test.ts). Such a loss can legitimately move that fact: two unmirrored copies of the family standing in mirror positions no longer break the reflection with their hand, so the plan gains an axis. The target for a family catalogued symmetric is exactness under every quarter-turn and mirror of a square footprint; pool_table is a known exception (docs/backlog.md D.6). It is pinned for the pilots in test/glyph-chirality.test.ts and by each module's sweep where one exists (glyphs-living, glyphs-outdoor, and plant/meeting_table in glyphs-misc).

Alternatives rejected ​

  • Shadows or a raised-object offset: decision 5.
  • Floor-material tints under furniture or rooms: a fill implies a material the source never named, and it competes with the ground textures that do name one.
  • A new theme key for the outline ink: every built-in and registered theme would need a value, and a style furniture override would leave the outline in the old colour unless the author also learnt the new key. A derived mix does neither.
  • Level of detail by pen size (fewer marks at 1:100): a glyph's count must not depend on the sheet, because the drawing budget counts each glyph once at canonical pens and test/drawing-budget.test.ts holds the estimate at or above what is drawn on every sheet. Counts are clamped functions of the footprint instead.

Consequences ​

  • The example SVGs, the scene and snapshot suites, the PNG goldens and the SVG-bearing byte-identity rows moved once; each re-measured row was proved SVG-only.
  • A symmetric symbol can read handed through an ulp or a decimal rounding tie alone, which flips describe --facts symmetry. That is why shape builders decide structure from radii and extents, never from derived coordinates, and why the handedness key reads numbers through keyNum (a 1e-7 mm nudge off the fmt4 ties).
  • Redraws are held by the handedness survey and grid, the D4 sweeps and the containment checks, not by eye alone; the working checklist is .claude/rules/glyphs.md.
  • Open items found during the redraw are in docs/backlog.md, section D.