Add design spec for E440 arc-dial scale generator pivot

docs/superpowers/specs/2026-07-28-e440-arc-dial-design.md

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+# E440 arc-dial scale generator — design
+
+## Summary
+
+Pivot the repository from generating M1730 **linear-strip** meter scales to
+generating **E440-style round-dial** meter scales, matching the hand-drawn
+reference `e440.svg`. This is a full replacement, not an added mode: the
+M1730 linear generator, its three SVG files, and its infra naming are all
+retired and replaced by E440 equivalents. Feature parity is kept wherever it
+translates naturally (scale mappings, output modes, custom labels, unit
+markup) but re-expressed in polar terms (angle/radius) instead of Cartesian
+(x-position).
+
+## Case geometry (fixed constants)
+
+The physical dial body, exactly as drawn in `e440.svg`, does not change
+between generated scales — only the tick/label/text content does. This
+mirrors how M1730's background panel rectangle was a fixed constant.
+
+- Canvas: 93.824249 × 57.215687 mm, `layer1` transform
+  `translate(-60.717741,-107.14126)` (same convention as M1730: constants are
+  expressed in this pre-translate local frame).
+- Body outline + two mounting ears + rim cutout: the existing `<path id="path2">`
+  in `e440.svg`, carried over unchanged.
+- Pivot center: local `(107.42845, 162.62819)`. Small gray guide circle
+  (r≈1.73mm) at the pivot is kept as-is.
+- Rim / tick-base radius `R0 = 43.842751` mm — ticks start here and extend
+  outward.
+- Angular sweep: `[-45.667°, +45.667°]` measured from vertical (north),
+  negative = left/low end, positive = right/high end, clockwise-positive
+  (matches the reference: 0 at the left end, 50 at the right end).
+
+## Tick generation (parametrized, 3 tiers)
+
+Same conceptual parameters as the M1730 generator (`big_ticks`,
+`small_ticks`, `label_count`), reinterpreted along the arc:
+
+- **Major ticks** — one per major interval boundary (`big_ticks + 1` of
+  them), evenly spaced in angle across the sweep. Radial length 4.7839mm,
+  stroke-width 0.7966. These are the ones eligible for numeric labels.
+- **Medium tick** — appears **only when `small_ticks` is odd**, at the exact
+  midpoint subdivision of each major interval (e.g. `small_ticks=9` →
+  10 subdivisions → the 5th is the midpoint). Radial length 3.1216mm,
+  stroke-width 0.3799. This matches the reference exactly (values 5, 15, 25,
+  35, 45 get a medium tick between the 0/10/20/30/40/50 majors).
+- **Small ticks** — every other subdivision. Radial length 1.8122mm,
+  stroke-width 0.2748.
+- If `small_ticks` is even, there's no exact midpoint — all minor
+  subdivisions render as small ticks (2-tier fallback), no medium tier.
+
+Each tick is emitted as a `<line>` from `(R0, angle)` to `(R0+length,
+angle)` in polar coordinates around the pivot (as `<line>` elements, not
+rotated `<rect>`s like the hand-drawn file — simpler and consistent with how
+M1730's code already draws ticks as lines rather than filled rotated boxes).
+
+Minor-tick angular placement under each scale mapping (linear/√/log/1/x)
+carries over directly from the M1730 code, substituting angle for x.
+
+## Labels
+
+Numeric labels are positioned at radius ≈49.86mm (just beyond the major tick
+tips) and **rotated to match their tick's angle** — following the arc, as in
+the reference (confirmed: "rotated/radial" was the chosen orientation).
+Default label font size ≈5.11mm (kept configurable, like `label_size`
+today). Label placement/count logic (`label_count`, `custom_labels`,
+`_fmt()`) is an unchanged port from the M1730 code.
+
+## Unit label and corner text
+
+- **Unit label** (e.g. `uA`): centered above the arc at local
+  `(107.035, 136.646)`, horizontal (not rotated), default size ≈9.47mm.
+  Markup support (`^`, `_`, `{a/b}`) is preserved — same parser as today.
+- **Corner labels** replace M1730's single `range_label` / colored bar
+  (which E440 doesn't have): two independent fields, **left** and **right**,
+  each up to 2 lines, anchored at the reference's `text58`/`text61`
+  positions (bottom-left start-anchored, bottom-right end-anchored, ≈3.57mm
+  font size, ≈4.46mm line spacing). A field's second line is only drawn if
+  provided; no truncation beyond 2 lines (extra input past line 2 is
+  ignored).
+
+## Fonts
+
+Single font selector, unchanged mechanism from today (any `.ttf` dropped in
+`fonts/` is auto-discovered and offered in the dropdown). Default font
+becomes `alte-din-1451-mittelschrift.regular` (already present in
+`fonts/`). The `.gepraegt` (embossed) variant and the existing GOST-family
+fonts remain selectable but are not auto-paired to any specific element —
+whatever the user picks in the dropdown is used for unit, labels, and corner
+text alike, exactly like the current single-selector behavior.
+
+## Scale mappings, output modes, custom labels
+
+All four mappings (linear, √, log, 1/x) are kept, each reusing its existing
+`value_to_x`/`major_value` math from `web/app.py`, renamed conceptually to
+`value_to_angle`. Dual output mode (path-outlines vs. embedded live text)
+and custom label override are both kept unchanged in behavior, just applied
+to the new geometry.
+
+## File deliverables
+
+- `e440.svg` → renamed/tidied into `E440-Scale_template.svg` (update
+  `sodipodi:docname`, keep as the Inkscape master with the same reference
+  values: unit `uA`, range 0–50, `Text`/`Left` and `Text`/`Right` corner
+  placeholders).
+- `E440-Scale_paths.svg` — same content with all text converted to outline
+  `<path>`s via fontTools (mirrors how `M1730-Scale_paths.svg` was derived).
+- `E440-Scale_text.svg` — same content with the font embedded as base64
+  `@font-face`, `sodipodi:docname` set to `E440-Scale.svg` (the "final
+  named file", matching the old convention).
+- Old `M1730-Scale_template.svg`, `M1730-Scale_paths.svg`,
+  `M1730-Scale_text.svg` are deleted.
+
+## `web/app.py` changes
+
+- `generate_svg(...)` rewritten around polar geometry: new constants
+  (`PIVOT_X`, `PIVOT_Y`, `R0`, `ANGLE_MIN`, `ANGLE_MAX`, tick lengths/widths
+  per tier, label radius) replace the linear `ZERO_X`/`MAX_X`/`BAR_Y`/tick
+  height constants.
+- `range_label` parameter replaced by `left_label` / `right_label` (each a
+  string, `\n`-split into up to 2 lines).
+- Body/case SVG markup (the fixed `<path>` for the dial body + pivot guide
+  circle) is emitted as a fixed literal, same pattern as today's fixed
+  background `<path>`.
+- Existing helper functions (`_glyph_svg`, `_measure`, `_parse_markup`,
+  `_text_to_paths`, `_fmt`, the fraction/sup/sub renderer) are reused
+  unchanged — they're geometry-agnostic.
+- Form (`web/templates/index.html`) fields updated: drop "Range label", add
+  "Left label" / "Right label" (2-line text areas), everything else
+  (unit, min/max, mapping, major/minor tick counts, label count/size,
+  custom labels, font, output mode) stays.
+
+## Rebranding
+
+Full rename to E440 branding across infra and docs:
+- Docker image tag `m1730-scale` → `e440-scale`.
+- `k8s/ingress.yaml` route `works.baumann.gr/M1730-generator` →
+  `works.baumann.gr/E440-generator` (and its stripPrefix middleware).
+- `README.md` and `CLAUDE.md` rewritten to describe the E440 arc-dial
+  project: new file roles, new coordinate constants, new tick-tier rules,
+  new default font, new form fields.
+
+## Testing / verification
+
+No automated test suite exists in this repo (consistent with "no build,
+lint, or test tooling" in CLAUDE.md). Verification is: run the Flask app
+locally, generate a scale reproducing the exact reference parameters
+(unit `uA`, 0–50, 5 major intervals, 9 minor per interval, labels every
+major, corner texts `Text`/`Left` and `Text`/`Right`), and visually diff the
+output against `E440-Scale_template.svg` (tick angles/positions, label
+placement, corner text placement) to confirm the geometry model is correct
+before considering the port done.