SVT: align synthetic terrain horizon with the attitude horizon

The 3D terrain showed almost no sky — the horizon sat far above the attitude
horizon line. Base camera pitch was 72°, but with MapLibre's 36.87° vertical FOV
the flat horizon only reaches the attitude line (28% of the SVT box) at ~82°.
Invert the perspective to derive the camera pitch from aircraft pitch so the
synthetic horizon lands exactly on the attitude horizon and tracks it 1:1
(accounting for the 1.5× canvas scale). Raise maxPitch to 85.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

web/src/components/SVT.jsx

@@ -63,6 +63,26 @@ function runwayGeoJSON(list) {
   return { type: 'FeatureCollection', features: feats };
 }
 
+// Place the synthetic horizon exactly where the PFD attitude horizon sits, so
+// the 3D terrain and the 2D attitude agree. The attitude horizon is at 28% of
+// the SVT box at level flight and moves PITCH_PX (9 px) per degree within the
+// 706-px-tall box; the canvas is scaled 1.5× about that 28% line (to cover the
+// corners when banked). We invert MapLibre's perspective to find the camera
+// pitch that lands the flat horizon there. With the default vertical FOV of
+// 36.87°, the focal-length/height ratio is 0.5/tan(fov/2) = 1.5, independent of
+// resolution. Screen offset of the horizon above centre = f·tan(90°−pitch).
+const CANVAS_SCALE = 1.5;          // matches .svt-canvas CSS transform scale
+const FOV_FH = 1.5;                // 0.5 / tan(fov/2), fov = 36.87° (MapLibre default)
+const HORIZON0 = (270 - 74) / 706; // attitude horizon as a fraction of the SVT box (≈0.2776)
+const PX_PER_DEG = 9 / 706;        // PITCH_PX / box height — displayed horizon travel per °
+function cameraPitchForAircraft(aircraftPitchDeg) {
+  const dispFrac = HORIZON0 + PX_PER_DEG * aircraftPitchDeg;        // where the horizon must appear
+  const rawFrac = HORIZON0 + (dispFrac - HORIZON0) / CANVAS_SCALE;  // undo the 1.5× canvas scale
+  const t = (0.5 - rawFrac) / FOV_FH;                              // = tan(90°−pitch)
+  const pitch = 90 - (Math.atan(t) * 180) / Math.PI;
+  return Math.max(60, Math.min(85, pitch));
+}
+
 export default function SVT({ values }) {
   const elRef = useRef(null);
   const mapRef = useRef(null);
@@ -77,9 +97,9 @@ export default function SVT({ values }) {
         style: STYLE,
         center: [num(values.lon, -122.31), num(values.lat, 47.45)],
         zoom: 11.5,
-        pitch: 72,
+        pitch: cameraPitchForAircraft(num(values.pitch)),
         bearing: num(values.heading),
-        maxPitch: 76,           // lower max pitch = nearer horizon = less distant terrain
+        maxPitch: 85,           // horizon placement needs ~82° at level, more when pitched up
         pixelRatio: 1,          // don't render at 2× on retina — big perf/bandwidth win
         renderWorldCopies: false,
         maxTileCacheSize: 40,
@@ -154,7 +174,7 @@ export default function SVT({ values }) {
         map.jumpTo({
           center: [num(v.lon, -122.31), num(v.lat, 47.45)],
           bearing: num(v.heading),
-          pitch: Math.max(58, Math.min(76, 72 + num(v.pitch))),
+          pitch: cameraPitchForAircraft(num(v.pitch)),
           zoom,
         });
         updateTerrainAwareness(num(v.altitude, 5500));