cesiumjs-primitives

# CesiumJS Primitives & Geometry

> **Applies to:** CesiumJS v1.142+ (ES module imports, `??` instead of `defaultValue`)

## Architecture

The Primitive API is the low-level rendering layer beneath the Entity API, trading convenience for performance.

**Core formula:** `Primitive = GeometryInstance[] + Appearance`

- **GeometryInstance** -- positions a Geometry in world space with per-instance attributes (color, show).
- **Geometry** -- vertex data describing a shape (polygon, box, ellipsoid, etc.).
- **Appearance** -- GLSL shaders + render state + optional Material that shade the geometry.

Primitives are **immutable after first render** -- geometry cannot change, but per-instance attributes update via `primitive.getGeometryInstanceAttributes(id)`.

## Primitive

```js
import {
  Viewer, Primitive, GeometryInstance, EllipseGeometry,
  EllipsoidSurfaceAppearance, Material, Cartesian3, Math as CesiumMath,
} from "cesium";

const viewer = new Viewer("cesiumContainer");
const scene = viewer.scene;

const primitive = scene.primitives.add(new Primitive({
  geometryInstances: new GeometryInstance({
    geometry: new EllipseGeometry({
      center: Cartesian3.fromDegrees(-100.0, 40.0),
      semiMinorAxis: 250000.0,
      semiMajorAxis: 400000.0,
      rotation: CesiumMath.PI_OVER_FOUR,
      vertexFormat: EllipsoidSurfaceAppearance.VERTEX_FORMAT, // must match appearance
    }),
    id: "myEllipse", // returned by Scene.pick()
  }),
  appearance: new EllipsoidSurfaceAppearance({ material: Material.fromType("Stripe") }),
}));
```

### Key Options

| Option | Default | Purpose |
|---|---|---|
| `geometryInstances` | -- | Single instance or array |
| `appearance` | -- | Shading (Appearance subclass) |
| `show` | `true` | Toggle visibility |
| `modelMatrix` | `Matrix4.IDENTITY` | Transform all instances |
| `asynchronous` | `true` | Build geometry on web worker |
| `releaseGeometryInstances` | `true` | Free geometry after GPU upload |
| `allowPicking` | `true` | `false` saves GPU memory |
| `shadows` | `ShadowMode.DISABLED` | Cast/receive shadows |

## Batching Multiple Instances

All instances in one Primitive share a single draw call.

```js
import {
  Primitive, GeometryInstance, RectangleGeometry, EllipseGeometry,
  PerInstanceColorAppearance, ColorGeometryInstanceAttribute,
  Cartesian3, Rectangle, Color,
} from "cesium";

scene.primitives.add(new Primitive({
  geometryInstances: [
    new GeometryInstance({
      geometry: new RectangleGeometry({
        rectangle: Rectangle.fromDegrees(-140, 30, -100, 40),
        vertexFormat: PerInstanceColorAppearance.VERTEX_FORMAT,
      }),
      id: "rect",
      attributes: { color: ColorGeometryInstanceAttribute.fromColor(Color.RED.withAlpha(0.5)) },
    }),
    new GeometryInstance({
      geometry: new EllipseGeometry({
        center: Cartesian3.fromDegrees(-80, 35),
        semiMinorAxis: 200000.0,
        semiMajorAxis: 300000.0,
        vertexFormat: PerInstanceColorAppearance.VERTEX_FORMAT,
      }),
      id: "ellipse",
      attributes: { color: ColorGeometryInstanceAttribute.fromColor(Color.BLUE.withAlpha(0.5)) },
    }),
  ],
  appearance: new PerInstanceColorAppearance(),
}));
```

## Updating Per-Instance Attributes

```js
import { ColorGeometryInstanceAttribute, ShowGeometryInstanceAttribute } from "cesium";

// Wait for async geometry compilation
const removeListener = scene.postRender.addEventListener(() => {
  if (!primitive.ready) return;
  const attrs = primitive.getGeometryInstanceAttributes("rect");
  attrs.color = ColorGeometryInstanceAttribute.toValue(Color.YELLOW);
  attrs.show = ShowGeometryInstanceAttribute.toValue(true);
  removeListener();
});
```

## PrimitiveCollection

Nestable container -- `scene.primitives` is itself a PrimitiveCollection.

```js
import { PrimitiveCollection, BillboardCollection, LabelCollection } from "cesium";

const group = new PrimitiveCollection();
group.add(new BillboardCollection());
group.add(new LabelCollection());
scene.primitives.add(group);
group.show = false; // toggle all children
```

## Choosing a Vector Data Path

| Need | Use |
|---|---|
| Entity lifecycle, clustering, per-entity styling, time-dynamic values | `GeoJsonDataSource` in `cesiumjs-entities` |
| One large GeoJSON object with low overhead and primitive-level performance | `GeoJsonPrimitive` in this skill |
| Tiled vector data, 3D Tiles LOD, metadata styling, feature picking | `MVTDataProvider` in `cesiumjs-3d-tiles` |
| Fully manual high-throughput point/polyline/polygon buffers | `BufferPointCollection`, `BufferPolylineCollection`, `BufferPolygonCollection` |

## Buffer Primitive Collections (Experimental, 1.140+)

Use `BufferPointCollection`, `BufferPolylineCollection`, and `BufferPolygonCollection`
for very large vector datasets where Entity/DataSource overhead is too high. These
APIs were introduced in 1.140 (#13212) and refined through 1.142; they are
experimental and use flyweight primitive objects: reuse one `BufferPoint`,
`BufferPolyline`, or `BufferPolygon` when adding or iterating thousands of items.

```js
import {
  BlendOption,
  BoundingSphere,
  BufferPoint,
  BufferPointCollection,
  BufferPointMaterial,
  Cartesian3,
  Color,
} from "cesium";

const positions = [
  Cartesian3.fromDegrees(-75.16, 39.95),
  Cartesian3.fromDegrees(-73.98, 40.75),
];

const points = scene.primitives.add(new BufferPointCollection({
  primitiveCountMax: positions.length,
  allowPicking: true,
  blendOption: BlendOption.TRANSLUCENT,
  boundingVolume: BoundingSphere.fromPoints(positions), // world space in 1.142+
}));

const point = new BufferPoint();
const material = new BufferPointMaterial({
  color: Color.CYAN.withAlpha(0.65),
  outlineColor: Color.WHITE.withAlpha(0.9),
  outlineWidth: 2,
  size: 10,
});

positions.forEach((position, featureId) => {
  points.add({
    position,
    featureId,
    material,
  }, point);
});

const picked = scene.pick(windowPosition);
if (picked?.collection === points) {
  console.log(picked.inde