fix: a bridge member is a true-metre object, not a scene-unit floor

size(metres, floor) took floor in scene units, tuned at 94 m to the unit. On one California (1,919 m/unit) those floors always won: a 21 m deck half became 173 m and the Golden Gate read as a highway. The same bug the metre conversion fixed for the Vincent Thomas at 391 m/unit, reintroduced by the floor. Floors are metres now — the Bay-board values, so that board does not move. Two boards twenty times apart draw the same 21 m half. Lamp pitch follows.
2026-08-24 22:48:17 -07:00
parent ce85867de9
commit 2c59b2cfce
3 changed files with 112 additions and 26 deletions
@@ -61,10 +61,10 @@
 * `socal.ts` asked for this in a comment: bridge members used to be constants in
 * scene units tuned at San Francisco's 94 m per unit, so at Los Angeles' 391 the
 * Vincent Thomas came out four times too heavy. Every cross-section here is
- * `metres / metresPerUnit` instead — with a minimum, because true scale on the
- * SoCal board puts the deck of the Long Beach Gateway below one pixel and an
- * invisible bridge is a worse answer than a chunky one. Heights are the
- * exception and stay on `world.metres()`, which carries the board's vertical
+ * `metres / metresPerUnit` instead — with a **metre** floor, because a floor in
+ * scene units is a floor that assumes one board. 0.09 units is 8.5 m on the Bay
+ * and 173 m on merged California: the Golden Gate's deck became a highway.
+ * Heights stay on `world.metres()`, which carries the board's vertical
 * exaggeration: a deck has to sit at the same exaggerated height as the hills it
 * lands on or it lands inside them.
 */
@@ -272,8 +272,9 @@ function stations(world: BridgeWorld, path: LatLng[], spacing: number): Station[
 * hangers are quantised to, and a coarser one leaves the Golden Gate with nine
 * hangers a span and a cable made of straight lines.
 */
-function stationSpacing(deckHalf: number): number {
-  return Math.max(deckHalf * 4, 0.28);
+function stationSpacing(deckHalf: number, perUnit: number): number {
+  // 26 m was 0.28 scene units on the Bay board, the floor this was tuned on.
+  return Math.max(deckHalf * 4, 26 / perUnit);
 }

 /** A point offset laterally from a station, at a given height. */
@@ -524,7 +525,15 @@ function layout(world: BridgeWorld, bridge: Bridge): BridgeLayout {
  // `world.metres`, which carries the board's vertical exaggeration. See the
  // module comment for why those two are not the same conversion.
  const perUnit = world.metresPerUnit;
-  const size = (metres: number, floor: number) => Math.max(metres / perUnit, floor);
+  /**
+   * Cross-section in metres, never thinner than `floorM` metres.
+   *
+   * The second argument used to be scene units, tuned at 94 m/unit. On a board
+   * 1,919 m to the unit those floors always won: a 21 m deck half became 173 m.
+   * Floors are the metres those old constants measured on the Bay, so that
+   * board does not move and every other board draws the same steel.
+   */
+  const size = (metres: number, floorM: number) => Math.max(metres, floorM) / perUnit;
  // Three of these are wider than the real member, and the reason is the
  // board's vertical exaggeration. San Francisco draws height at 3.6× and distance at
  // 1×, so a tower at true scale is 227 m tall and 12 m thick — a 62:1 needle
@@ -536,13 +545,13 @@ function layout(world: BridgeWorld, bridge: Bridge): BridgeLayout {
  // saddle) and where nothing else needs the room. The deck follows at 42 m
  // against a true 27, which is the same allowance `socal.ts` records for its
  // landmarks: at this scale a true-width deck is under two pixels.
-  const deckHalf = size(21, 0.09);
-  const deckDepth = size(14, 0.04);
-  const legWidth = size(20, 0.05);
-  const legDepth = size(46, 0.1);
-  const cableRadius = size(8, 0.015);
-  const hangerHalf = size(1.9, 0.005);
-  const pierHalf = size(8, 0.025);
+  const deckHalf = size(21, 8.5);
+  const deckDepth = size(14, 3.8);
+  const legWidth = size(20, 4.7);
+  const legDepth = size(46, 9.4);
+  const cableRadius = size(8, 1.4);
+  const hangerHalf = size(1.9, 0.5);
+  const pierHalf = size(8, 2.4);

  const deckY = world.metres(bridge.deckHeight);
  const towerY = world.metres(bridge.towerHeight);
@@ -564,7 +573,7 @@ function layout(world: BridgeWorld, bridge: Bridge): BridgeLayout {
  const footing = (ground: number, margin: number) =>
    Math.max(ground, seabed) - world.metres(margin);

-  const list = stations(world, bridge.path, stationSpacing(deckHalf));
+  const list = stations(world, bridge.path, stationSpacing(deckHalf, perUnit));
  const towerAt = bridge.towers.map((tower) => nearestStation(world, list, tower));
  const reaches = classify(list, towerAt, bridge.towerHeight, perUnit);
  const runs = [
@@ -580,11 +589,11 @@ function layout(world: BridgeWorld, bridge: Bridge): BridgeLayout {
  // has something to land on, and a deck held rigidly at 67 m simply disappeared
  // into the Presidio bluff with no visible touchdown.
  const total = list[list.length - 1]?.along ?? 0;
-  const rampLength = Math.min(total * 0.18, Math.max(deckHalf * 12, 1.2));
+  const rampLength = Math.min(total * 0.18, Math.max(deckHalf * 12, 113 / perUnit));
  const deckTopAt = (station: Station): number => {
    const fromEnd = Math.min(station.along, total - station.along);
    if (fromEnd >= rampLength) return deckY;
-    const landing = Math.max(station.ground + deckDepth + size(4, 0.01), 0);
+    const landing = Math.max(station.ground + deckDepth + size(4, 0.9), 0);
    if (landing >= deckY) return deckY;
    const t = rampLength <= 0 ? 1 : fromEnd / rampLength;
    // Smoothstep, so the deck leaves the shore tangentially rather than kinking.
@@ -758,7 +767,7 @@ export function buildBridge(
  }

  // ---- Cables and hangers ----
-  const hangerSpacing = Math.max(deckHalf * 3.4, size(60, 0.22));
+  const hangerSpacing = Math.max(deckHalf * 3.4, size(60, 21));
  for (const chain of runs) {
    for (const side of [-1, 1] as const) {
      const lateral = side * deckHalf;
@@ -852,7 +861,7 @@ export function buildBridge(
  // One every so often under an approach, and none at all where the ground has
  // already risen to the deck: that is what walks the Bay Bridge onto Yerba
  // Buena instead of standing it on stilts over the top of the island.
-  const pierSpacing = Math.max(deckHalf * 9, size(240, 0.7));
+  const pierSpacing = Math.max(deckHalf * 9, size(240, 66));
  for (const reach of reaches) {
    if (reach.kind !== "approach") continue;
    let last = -Infinity;
@@ -895,12 +904,16 @@ export interface BridgePlan {
  towerStations: number[];
  stationCount: number;
  deckLength: number;
+  /** Deck half-width in true metres. Same on every board; see `bridgeScale.test.ts`. */
+  deckHalfMetres: number;
 }

 export function planBridge(world: BridgeWorld, bridge: Bridge): BridgePlan {
  const perUnit = world.metresPerUnit;
-  const deckHalf = Math.max(15 / perUnit, 0.075);
-  const list = stations(world, bridge.path, stationSpacing(deckHalf));
+  // Same 21 m half as `layout`. A second, smaller number here meant the
+  // classifier sampled a different deck from the one that was drawn.
+  const deckHalf = Math.max(21, 8.5) / perUnit;
+  const list = stations(world, bridge.path, stationSpacing(deckHalf, perUnit));
  const towerAt = bridge.towers.map((tower) => nearestStation(world, list, tower));
  const reaches = classify(list, towerAt, bridge.towerHeight, perUnit);
  return {
@@ -911,6 +924,7 @@ export function planBridge(world: BridgeWorld, bridge: Bridge): BridgePlan {
    towerStations: towerAt,
    stationCount: list.length,
    deckLength: list[list.length - 1]?.along ?? 0,
+    deckHalfMetres: deckHalf * perUnit,
  };
 }

@@ -981,7 +995,8 @@ export interface BridgeLights {
 * whole Vincent Thomas.
 */
 const DECK_LAMP_SPACING_M = 50;
-const DECK_LAMP_MIN_SPACING = 0.16;
+/** 0.16 scene units was 15 m on the Bay; a scene-unit min is 307 m on California. */
+const DECK_LAMP_MIN_SPACING_M = 15;

 /** Lamp height above the deck slab, and the head light's clearance over the saddle. */
 const DECK_LAMP_HEIGHT_M = 12;
@@ -999,7 +1014,7 @@ export function bridgeLights(world: BridgeWorld, bridge: Bridge): BridgeLights {
  // is set by what a *cable* needs and is three times coarser under an approach,
  // so one lamp per station would thin out over exactly the causeways that have
  // nothing else to look at.
-  const spacing = Math.max(DECK_LAMP_SPACING_M / world.metresPerUnit, DECK_LAMP_MIN_SPACING);
+  const spacing = Math.max(DECK_LAMP_SPACING_M, DECK_LAMP_MIN_SPACING_M) / world.metresPerUnit;
  const lift = world.metres(DECK_LAMP_HEIGHT_M);
  const total = list[list.length - 1]?.along ?? 0;
  const kerb = deckHalf * 0.92;
@@ -0,0 +1,66 @@
+/**
+ * A bridge member is a true-metre object, and every board has to agree.
+ *
+ * `size(metres, floor)` used to take `floor` in scene units, tuned at 94 m to
+ * the unit. On the merged California board (1,919 m/unit) those floors always
+ * won: a 21 m deck half became 173 m, and the Golden Gate read as a highway.
+ * The floor is metres now. This is the property that would have caught it:
+ * two boards twenty times apart draw the same deck width on the ground.
+ */
+
+import assert from "node:assert/strict";
+import test from "node:test";
+
+import { planBridge } from "../../engine/bridges.ts";
+import type { Bridge } from "../../engine/types.ts";
+import type { World } from "../../engine/world.ts";
+
+const GOLDEN_GATE: Bridge = {
+  name: "Golden Gate Bridge",
+  path: [
+    [37.8025, -122.4752],
+    [37.8155, -122.4783],
+    [37.825, -122.479],
+    [37.8375, -122.4806],
+  ],
+  towers: [
+    [37.8155, -122.4783],
+    [37.825, -122.479],
+  ],
+  towerHeight: 227,
+  deckHeight: 67,
+  sag: 0.55,
+  color: 0xc0442c,
+};
+
+function board(latScale: number): World {
+  const centre = { lat: 37.7749, lng: -122.4194 };
+  const lngScale = latScale * Math.cos((centre.lat * Math.PI) / 180);
+  const metresPerUnit = 111_320 / latScale;
+  return {
+    project(lat: number, lng: number): [number, number] {
+      return [(lng - centre.lng) * lngScale, -(lat - centre.lat) * latScale];
+    },
+    groundAt(): number {
+      return 0;
+    },
+    metres(value: number): number {
+      return value / metresPerUnit;
+    },
+    metresPerUnit,
+  } as unknown as World;
+}
+
+test("the Golden Gate's deck is the same width on a 94 m board and a 1,919 m one", () => {
+  const metro = planBridge(board(1180), GOLDEN_GATE);
+  const state = planBridge(board(58), GOLDEN_GATE);
+  assert.ok(
+    Math.abs(metro.deckHalfMetres - state.deckHalfMetres) < 0.5,
+    `deck half is ${metro.deckHalfMetres.toFixed(1)} m on the Bay and ` +
+      `${state.deckHalfMetres.toFixed(1)} m on California`,
+  );
+  assert.ok(
+    metro.deckHalfMetres > 15 && metro.deckHalfMetres < 30,
+    `deck half measures ${metro.deckHalfMetres.toFixed(1)} m; that is not a highway`,
+  );
+});
@@ -179,12 +179,17 @@ test("a coarser board gets a smaller lamp, because it draws a smaller bridge", (
  // San Francisco at 94 m to the unit against Los Angeles at 391. A sprite size
  // in scene units tuned on the first would be several times the width of the
  // deck on the second, which is the bug `socal.ts` asked the kit to stop having.
+  // The floor is metres now, so both boards draw the same 21 m half and the
+  // coarser one is smaller in scene units, not clamped to 0.09 (173 m on CA).
  const fine = bridgeLights(board(1180, 3.6), GOLDEN_GATE);
  const coarse = bridgeLights(board(285, 2.2), GOLDEN_GATE);
  assert.ok(coarse.scale < fine.scale, "a coarser board draws a narrower deck");
-  assert.ok(coarse.scale >= 0.09, "…but never below the legibility floor");
-  // The run still reaches both ends of the crossing; it is the pitch floor that
-  // keeps a 391 m/unit board from putting four lamps on a whole bridge.
+  const fineM = fine.scale * (111_320 / 1180);
+  const coarseM = coarse.scale * (111_320 / 285);
+  assert.ok(
+    Math.abs(fineM - coarseM) < 0.5,
+    `deck half is ${fineM.toFixed(1)} m on the Bay and ${coarseM.toFixed(1)} m on SoCal`,
+  );
  assert.ok(triples(coarse.deck).length >= 8, "a coarse board still gets a run, not a handful");
 });