129 lines
7.7 KiB
Markdown
129 lines
7.7 KiB
Markdown
# 3D Flyover — architecture reference
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Deep detail behind `TECHNIQUE.md`: provider loading, the camera-path pipeline, per-frame camera math, and
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the proven terrain values. Both landscape and city modes have been forward-tested through Remotion.
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## 1. Provider initialization
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Create the Viewer with `baseLayer: false`, UI widgets disabled, and
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`contextOptions.webgl.preserveDrawingBuffer: true`. Never hide the credit display.
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### Landscape
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Add MapTiler `satellite-v2` with `UrlTemplateImageryProvider`, then load
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`terrain-quantized-mesh-v2` with `CesiumTerrainProvider.fromUrl({requestVertexNormals: true})`.
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MapTiler supplies both datasets; no Cesium ion token is required.
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### City
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Do not add MapTiler. Hide the globe, then add:
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```ts
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viewer.scene.globe.show = false;
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const tileset = await Cesium.Cesium3DTileset.fromUrl(
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`https://tile.googleapis.com/v1/3dtiles/root.json?key=${GOOGLE_KEY}`,
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{showCreditsOnScreen: true, maximumScreenSpaceError: 4},
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);
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viewer.scene.primitives.add(tileset);
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```
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Lower `maximumScreenSpaceError` improves refinement at substantial download/render cost. Start at
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`4` for a hero landmark and `6–8` for wider urban shots. Enforce the Google 30-second promotional
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video ceiling in the component.
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Load Cesium from the CDN after setting `window.CESIUM_BASE_URL`; the tested version is `1.143`.
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## 2. The camera path — structure & generation
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Four properties matter, in order:
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1. **Continuous curvature** — the camera must flow through curves, never "fly straight, snap to a new
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heading, fly straight." The component applies three passes of **Chaikin corner cutting** to every
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supplied path. Each pass replaces a segment with quarter and three-quarter points, rounding a
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corner into a curve. Three passes are the default; four is softer, two is tighter.
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2. **Constant ground speed** — precompute cumulative distance along the rounded curve and interpolate
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by arc length. Do not animate by source-point index; unequal source spacing creates speed bumps.
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3. **Minimized amplitude** — for detailed landscape centerlines, dampen the prepared path toward its
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straight start→end chord by a fixed fraction `DAMP` (0 = dead straight, 1 = full river). This is
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the single swerve-amplitude knob.
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4. **Enough length** — `PATHKM ≥ TRAVEL_KM + 2·LOOK_AHEAD_KM` so the look-ahead aim never clamps.
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`../scripts/prep-cesium-path.mjs` does: clip a window of the source centerline → resample to even
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arc-length spacing (0.1 km) → moving-average smooth (±2.8 km window, 2 passes) → dampen toward the chord
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(`DAMP=0.45`). Validate with the heading-delta probe it prints — deltas should be small and change
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_gradually_ (water-wars: `3,0,-1,-3,-4,1,7,9,5,-5,-12,-7,…`). Big jumps = corners = bad.
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The component then applies Chaikin smoothing to this prepared route, or directly to a short
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hand-authored city route. Keep city control points sparse and intentional; smoothing cannot rescue a
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zig-zagging route that crosses the subject repeatedly.
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**Source data:** OSM via Overpass (~0.3 km vertex spacing) — Natural Earth is too coarse for inner
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gorges. overpass-api.de is often busy → mirror `overpass.kumi.systems`.
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## 3. Camera animation per frame (position, heading, pitch, bank)
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Walk the path by **arc length** (precompute cumulative distances once). Every frame:
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- **Position** = the point at `dCam` km along the path, altitude `lerp(ALT_START, ALT_END, prog)`.
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- **Heading** = bearing from the camera point to a **real point `LOOK_AHEAD_KM` further along the same
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path**. A far aim averages wiggle → smooth heading; on a curved path it leads into the bend, so the
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heading turns gently with the path. (A local-tangent aim spins the camera at every kink — don't.)
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- **Pitch** = constant. We keep a MapLibre-style param `PITCH_FROM_NADIR` (90 = horizon), then convert:
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**Cesium pitch = `-(90 - PITCH_FROM_NADIR)`** (Cesium: 0 = horizon, -90 = straight down). 76° → -14°.
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- **Bank (roll)** = lean _into_ the turn — the helicopter tell. Measure turn rate as the bearing change
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between `aim` and a point `2·LOOK_AHEAD_KM` ahead; `roll = clamp(dH · BANK_GAIN, ±MAX_BANK)`. Because
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the path is smooth, `dH` changes gradually → the bank eases in and out, never jerks.
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```ts
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const setCamera = (C, viewer, prog) => {
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const dCam = Math.min(TRAVEL_KM, PATHKM - LOOK_AHEAD_KM * 2) * prog;
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const cam = alongPath(dCam); // arc-length point
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const aim = alongPath(dCam + LOOK_AHEAD_KM); // heading target (real point on the path)
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const aim2 = alongPath(dCam + LOOK_AHEAD_KM * 2); // turn-rate probe → bank
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const heading = bearing(cam, aim);
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let dH = bearing(aim, aim2) - heading; while (dH > Math.PI) dH -= 2*Math.PI; while (dH < -Math.PI) dH += 2*Math.PI;
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viewer.camera.setView({
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destination: C.Cartesian3.fromDegrees(cam[0], cam[1], lerp(ALT_START, ALT_END, prog)),
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orientation: { heading, pitch: C.Math.toRadians(-(90 - PITCH_FROM_NADIR)), roll: clamp(dH * BANK_GAIN, -MAX_BANK, MAX_BANK) },
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});
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};
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```
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> **Cesium vs MapLibre conventions (gotcha):** Cesium heading is radians, 0 = north, clockwise. Pitch
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> 0 = horizon, negative = down (MapLibre is the inverse). Roll positive = bank right; tune the sign by eye.
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## 4. The feel — proven water-wars values
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| Param | Value | Meaning |
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| ------------------------ | ---------------------- | ------------------------------------------------------------------------------------ |
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| `TRAVEL_KM` | 13 | How far the camera travels. **Speed = `TRAVEL_KM / durationSeconds`.** |
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| duration | 24 s (720 f @30) | 13 km / 24 s ≈ **0.54 km/s** — a slow, peaceful glide. 8 s felt "extremely rushed". |
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| `ALT_START → ALT_END` | 4600 → 4300 m ASL | Absolute (terrain-independent). Inside the corridor walls → fly _through_, not over. |
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| `LOOK_AHEAD_KM` | 1.5 | Heading smoothness vs responsiveness. |
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| `PITCH_FROM_NADIR` | 76° | Stare-ahead down the corridor (90 = level). → Cesium -14°. |
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| `MAX_BANK` / `BANK_GAIN` | 0.13 rad (~7.5°) / 0.6 | Helicopter lean into turns. |
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| `verticalExaggeration` | 1.1 | Subtle terrain drama. |
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| `DAMP` (prep) | 0.45 | Swerve amount: higher = weavier, lower = straighter. |
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**A slow camera renders fast.** At 0.54 km/s the camera moves ~18 m/frame, so tiles stay cached and each
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`settle()` returns almost immediately; the 720-frame render completed in one pass (no chunk-rendering).
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## 5. The complete component
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The full, runnable component is `../assets/CesiumFlythrough.tsx` — read it directly. Its shape:
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- `loadCesium()` — inject `CESIUM_BASE_URL` + the CDN `Cesium.js`, resolve when loaded.
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- init effect — build the Viewer (§1), `setCamera(…, 0)`, `await settle(viewer)`, `continueRender`.
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- `settle(viewer)` — loop `viewer.render()` until `globe.tilesLoaded` for landscapes or
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`tileset.tilesLoaded` for cities is stable for ~8 ticks (cap ~600).
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- per-frame effect — `delayRender({timeoutInMilliseconds: 60000})` → `setCamera(prog)` → `settle()` → `continueRender`.
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## 6. Render
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```bash
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bunx remotion still src/index.ts <Comp> out.png --frame=N --gl=angle --timeout=180000 # validate framing/bank first
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bunx remotion render src/index.ts <Comp> out.mp4 --gl=angle --concurrency=1 --timeout=180000
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```
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`--gl=angle` is mandatory. Use `--concurrency=1`; `settle()` already serializes tile loading.
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