Show only the CCTV cameras in view, thinned to one per ground cell when zoomed out

The CCTV layer kept every loaded camera live: each view change touched every
icon, every notification rebuilt every camera's state, and the picker listed
the whole catalog. With the default 4,000 cameras, a continental drag ran at
152-179 fps with 18-24 frames over 30 ms (worst 112 ms). It now runs at
235 fps with none. The layer now:

- shows icons only on or near the screen, and when zoomed out keeps one camera
  per world-anchored cell (healthy feeds first), so panning keeps the same
  cameras;
- grounds and draws coverage only for the cameras in view, the active camera
  and its coverage neighbours, retrying a camera at most twice per view regime;
- reruns the view pass while the camera moves, paced by its own cost, and
  projects with one matrix per pass instead of a Cesium call per camera;
- keeps the billboard buffer-usage hint fixed, so showing or hiding a few
  icons no longer rewrites the vertex data of every billboard;
- lists the cameras in view (active camera first) in the picker, closing with
  the count of cameras outside the view, and builds the full camera list in
  the UI state only when it is read.

src/layers/cctv/rendering.js

@@ -1,5 +1,4 @@
 import * as Cesium from 'cesium';
-import { horizonOccluder } from '../../data/iconOrientation.js';
 import { isHeadingEstimated } from './headingConfidence.js';
 import {
   ACTIVE_CAMERA_COLOR,
@@ -11,8 +10,21 @@ import {
   ACTIVE_COVERAGE_EDGE,
   IDLE_COVERAGE_EDGE_MUTED,
   ACTIVE_COVERAGE_EDGE_DEPTHFAIL,
+  VIEW_CELL_PX,
+  VIEW_ICON_MARGIN,
+  VIEW_MIN_CELL_M,
 } from './policy.js';
 
+/** FNV-1a over a camera id: a stable, well-spread order for crowded cells. */
+function idHash(id) {
+  let hash = 0x811c9dc5;
+  for (let i = 0; i < id.length; i++) {
+    hash ^= id.charCodeAt(i);
+    hash = Math.imul(hash, 0x01000193);
+  }
+  return hash >>> 0;
+}
+
 export function createRendering({
   state: layerState,
   services,
@@ -138,24 +150,186 @@ export function createRendering({
    * (Google-3D regime) nothing writes far-side depth, and the billboards are now
    * always-on-top (`disableDepthTestDistance: INFINITY` — the far-zoom submerge
    * fix), so without this pass London's cluster would shine through the planet
-   * from a US viewpoint. Pure math over ≤ catalog-size points; runs on
-   * camera.moveEnd + init only (event-driven — no steady-state work).
+   * from a US viewpoint.
+   *
+   * The same pass thins the view so a large catalog stays readable and cheap:
+   * icons well off screen hide, and each camera competes for a world-anchored
+   * cell sized to VIEW_CELL_PX at its distance (healthiest first, then a stable
+   * id order). The active camera shows whenever it is above the horizon
+   * and near the screen, even in a crowded cell. The shown records become
+   * `_viewRecords`, the working set for geometry, cards and the camera picker;
+   * the array keeps its identity while the shown set is unchanged.
+   *
+   * Runs on camera settle, throttled during motion, on resize, enable and
+   * activation changes — never per frame.
+   * @returns {boolean} Whether the shown set changed.
    */
 
   function refreshHorizonCulling() {
     if (
+      !layerState._enabled ||
       !layerState._viewer ||
       layerState._viewer.isDestroyed() ||
       !layerState._records.length
     )
-      return;
-    const occluder = horizonOccluder(layerState._viewer.camera);
+      return false;
+    const { camera, scene } = layerState._viewer;
+    const width = scene.canvas.clientWidth || scene.canvas.width || 0;
+    const height = scene.canvas.clientHeight || scene.canvas.height || 0;
+    const minX = -width * VIEW_ICON_MARGIN;
+    const maxX = width * (1 + VIEW_ICON_MARGIN);
+    const minY = -height * VIEW_ICON_MARGIN;
+    const maxY = height * (1 + VIEW_ICON_MARGIN);
+    // Ground metres one pixel covers per metre of distance from the viewer.
+    const fovy = Number.isFinite(camera.frustum?.fovy)
+      ? camera.frustum.fovy
+      : Math.PI / 3;
+    const metresPerPixelPerMetre =
+      (2 * Math.tan(fovy / 2)) / Math.max(1, height);
+    const viewer = camera.positionWC;
+    // The horizon test of Cesium's EllipsoidalOccluder, inlined: this loop
+    // visits every camera in the catalog.
+    const inverseRadii = Cesium.Ellipsoid.WGS84.oneOverRadii;
+    const cx = viewer.x * inverseRadii.x;
+    const cy = viewer.y * inverseRadii.y;
+    const cz = viewer.z * inverseRadii.z;
+    const limbSquared = cx * cx + cy * cy + cz * cz - 1;
+    const activeId = layerState._activeCameraId;
+    const project = screenProjector(scene, camera, width, height);
+    const pass = (layerState._viewPassId = (layerState._viewPassId || 0) + 1);
+    const shown = [];
+    const cells = new Map();
+    for (const record of layerState._records) {
+      const position = record.billboard?.position;
+      if (!position) continue;
+      const vx = position.x * inverseRadii.x - cx;
+      const vy = position.y * inverseRadii.y - cy;
+      const vz = position.z * inverseRadii.z - cz;
+      const vtDotVc = -(vx * cx + vy * cy + vz * cz);
+      if (
+        limbSquared < 0
+          ? vtDotVc > 0
+          : vtDotVc > limbSquared &&
+            (vtDotVc * vtDotVc) / (vx * vx + vy * vy + vz * vz) > limbSquared
+      )
+        continue;
+      const screen = project(position);
+      if (
+        !screen ||
+        !(screen.x >= minX && screen.x <= maxX) ||
+        !(screen.y >= minY && screen.y <= maxY)
+      )
+        continue;
+      const dx = position.x - viewer.x;
+      const dy = position.y - viewer.y;
+      const dz = position.z - viewer.z;
+      const cellM =
+        VIEW_CELL_PX *
+        metresPerPixelPerMetre *
+        Math.sqrt(dx * dx + dy * dy + dz * dz);
+      if (record.camera.id === activeId || !(cellM >= VIEW_MIN_CELL_M)) {
+        record.viewPass = pass;
+        shown.push(record);
+        continue;
+      }
+      const key = worldCellKey(record, cellM);
+      const rank = viewRank(record);
+      const holder = cells.get(key);
+      if (!holder || rank < holder.rank) cells.set(key, { record, rank });
+    }
+    for (const { record } of cells.values()) {
+      record.viewPass = pass;
+      shown.push(record);
+    }
     for (const record of layerState._records) {
       const bb = record.billboard;
       if (!bb) continue;
-      const visible = occluder.isPointVisible(bb.position);
+      const visible = record.viewPass === pass;
       if (bb.show !== visible) bb.show = visible;
     }
+    const previous = layerState._viewRecords;
+    const changed =
+      previous.length !== shown.length ||
+      previous.some((record) => record.viewPass !== pass);
+    if (changed) layerState._viewRecords = shown;
+    return changed;
+  }
+
+  /**
+   * Canvas coordinates for world positions. In 3D the pass multiplies by one
+   * view-projection matrix per call instead of Cesium's per-point
+   * cartesianToCanvasCoordinates, which dominated the pass on a large
+   * catalog; other scene modes keep Cesium's projection.
+   * @returns {(position: Cesium.Cartesian3) => ({x: number, y: number}|null)}
+   */
+  function screenProjector(scene, camera, width, height) {
+    const out = layerState._scratchViewScreen;
+    const projection = camera.frustum?.projectionMatrix;
+    if (
+      scene.mode !== Cesium.SceneMode.SCENE3D ||
+      !projection ||
+      !camera.viewMatrix
+    )
+      return (position) => scene.cartesianToCanvasCoordinates(position, out);
+    const m = Cesium.Matrix4.multiply(
+      projection,
+      camera.viewMatrix,
+      layerState._scratchViewProjection,
+    );
+    return (p) => {
+      const w = m[3] * p.x + m[7] * p.y + m[11] * p.z + m[15];
+      const z = m[2] * p.x + m[6] * p.y + m[10] * p.z + m[14];
+      // Behind the eye or before the near plane: Cesium does not draw it.
+      if (!(w > 0) || z < 0) return null;
+      out.x =
+        ((m[0] * p.x + m[4] * p.y + m[8] * p.z + m[12]) / w + 1) * 0.5 * width;
+      out.y =
+        (1 - (m[1] * p.x + m[5] * p.y + m[9] * p.z + m[13]) / w) * 0.5 * height;
+      return out;
+    };
+  }
+
+  /**
+   * A grid cell fixed to the ground: `cellM` rounded up to a power of two
+   * metres, so the cell (and its winner) only changes when the camera's
+   * distance crosses a doubling: a level pan keeps it, a tilted pan or a zoom
+   * can move cameras near the edge of a step to the next one.
+   */
+  function worldCellKey(record, cellM) {
+    const level = Math.ceil(Math.log2(cellM));
+    const { lat, lon } = record.camera;
+    // A camera's key changes only with the level (or an edited position), so
+    // it is kept per level.
+    if (
+      record.viewCellLevel === level &&
+      record.viewCellLat === lat &&
+      record.viewCellLon === lon
+    )
+      return record.viewCellKey;
+    const latStep = 2 ** level / 111_320;
+    const row = Math.floor(lat / latStep);
+    const rowLat = Cesium.Math.toRadians((row + 0.5) * latStep);
+    const lonStep = latStep / Math.max(0.05, Math.cos(rowLat));
+    record.viewCellLevel = level;
+    record.viewCellLat = lat;
+    record.viewCellLon = lon;
+    record.viewCellKey = `${level}:${row}:${Math.floor(lon / lonStep)}`;
+    return record.viewCellKey;
+  }
+
+  /** Lower ranks win a crowded cell: healthy feeds first, then a stable id order. */
+  function viewRank(record) {
+    const status = layerState._healthById.get(record.camera.id)?.status;
+    const tier =
+      status === 'ok'
+        ? 0
+        : status === 'degraded'
+          ? 1
+          : status === 'offline'
+            ? 3
+            : 2;
+    record.viewHash ??= idHash(record.camera.id);
+    return tier * 2 ** 32 + record.viewHash;
   }
 
   /**
@@ -204,6 +378,8 @@ export function createRendering({
     const activeId = activeRecord?.camera.id || null;
     const coverageVisible =
       parts.geometry.buildCoverageVisibleSet(activeRecord);
+    // Drawn coverage needs geometry too (geometryQueue's working set).
+    layerState._coverageVisibleIds = coverageVisible;
     const coverageOn = layerState._coverageMode !== 'off';
     const viewshedOn = layerState._coverageMode === 'viewshed';
     if (coverageOn) {
@@ -215,8 +391,13 @@ export function createRendering({
     for (const record of layerState._records) {
       const isActive = record.camera.id === activeId;
       if (record.billboard) {
-        record.billboard.color = cameraColorFor(record);
-        record.billboard.scale = isActive ? 1.25 : 1.0;
+        // Unchanged writes still dirty the collection, which then rewrites
+        // every icon's vertices: across a large catalog, write only changes.
+        const color = cameraColorFor(record);
+        if (!Cesium.Color.equals(record.billboard.color, color))
+          record.billboard.color = color;
+        const scale = isActive ? 1.25 : 1.0;
+        if (record.billboard.scale !== scale) record.billboard.scale = scale;
         // disableDepthTestDistance stays POSITIVE_INFINITY for every billboard
         // (set at creation) — see the field-test far-zoom submerge fix there.
       }