Draw it flat: an orthographic plan view

A perspective camera leans everything away from the centre of the frame. In a
system that means the orbits are ellipses whose shape depends on where they
happen to sit on screen, so two planets on the same circular orbit do not look
like they are on the same circle. Plan view, in the Display panel, swaps the
projection for a parallel one and swings to look down the plane the current
scale is read against — the galactic plane out in the field, this system's own
orbital plane inside one. Circles are circles again, wherever they are.

Both halves are the feature and neither alone is it. The projection is what
makes the shape honest; the swing is what makes it worth looking at. Orbiting
still works afterwards, so a plan is where the view starts rather than a cage.

The engine now holds both cameras and keeps them in step, rather than making
one on demand: a camera that exists only while it is being looked through is a
camera whose pose is always one swap out of date. The orthographic frustum is
derived, never stored — it is the perspective camera's own frustum at the
current orbit distance, made parallel — which is why the camera flights work
through it untouched. They move the camera; the frame follows.

Three things had to be taught that a projection had changed.

Sprites. three.js turns an angular size into a world size only when it is
compiling against a perspective camera (SpriteNodeMaterial: `camera
.isPerspectiveCamera && sizeAttenuation === false`). Under a parallel one that
step is silently skipped and every star in the field collapses to a thousandth
of a parsec. The same arithmetic is now done in the node graph behind a
uniform, so one material serves both cameras without being recompiled — and
picking follows it exactly, since a star has to be clickable where it is drawn.

Depth. A parallel camera does not back away as its frame grows, so at galactic
framing the backdrop shell and half the Milky Way sit behind its own plane. Its
depth range is symmetric about it instead, which a linear depth buffer can
afford and a perspective one could not.

And distance. Half the map was keyed on how far back the camera was pulled —
the scale ladder, the crossfade, the label radius, the range readout — which
under a parallel projection says nothing at all, because the frustum sets the
extent. They all read one honest equivalent now: the distance a perspective
camera would need to frame the same thing.

Two defects found while verifying, both mine, both from this change:

The per-frame work was computed against the camera captured at bootstrap while
the renderer drew through the other one, so after a swap every label was
projected by a camera nobody was looking through.

And the zoom limits were derived from the orbit limits, which are in whichever
unit space the view is in. Reading them on the frame the scene swaps parsecs
for astronomical units pinned the zoom at the ratio between the two, and
leaving a system landed the view three kiloparsecs out. Zoom is a plain
multiplier on a frame the distance already sets, so it is bounded by a factor.

Verified: build clean, 595/595 unit including a new spec for the projection
arithmetic, 13/13 end-to-end including two that flatten a system and check the
ladder still knows how far out it is, design detector clean, screenshots of
both scales in both projections.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_016jxMkwA2rbicdGxHosecYi

src/app/features/galaxy-system/camera-rig-controller.ts

@@ -1,4 +1,6 @@
 import * as THREE from 'three/webgpu';
+
+import { SceneCamera } from '../../core/engine/engine.service';
 import { OrbitControls } from 'three/addons/controls/OrbitControls.js';
 
 export interface CameraPose {
@@ -27,7 +29,7 @@ export class CameraRigController {
   };
 
   constructor(
-    private readonly camera: THREE.PerspectiveCamera,
+    private readonly camera: SceneCamera,
     private readonly controls: OrbitControls
   ) {}
 

src/app/features/galaxy-system/galaxy-system-scene.component.spec.ts

@@ -64,7 +64,9 @@ const EARTH: BodyRecord = {
 class FakeEngineService {
   private readonly scene = new THREE.Scene();
   private readonly camera = new THREE.PerspectiveCamera(50, 1, 0.1, 1000);
+  private readonly orthographic = new THREE.OrthographicCamera(-1, 1, 1, -1, 0.1, 1000);
   private readonly tickCallbacks = new Set<EngineTickCallback>();
+  projection: 'perspective' | 'orthographic' = 'perspective';
 
   get isInitialized(): boolean {
     return true;
@@ -78,10 +80,37 @@ class FakeEngineService {
     return this.scene;
   }
 
-  getCamera(): THREE.PerspectiveCamera {
+  getCamera(): THREE.PerspectiveCamera | THREE.OrthographicCamera {
+    return this.projection === 'orthographic' ? this.orthographic : this.camera;
+  }
+
+  getPerspectiveCamera(): THREE.PerspectiveCamera {
     return this.camera;
   }
 
+  get currentProjection(): 'perspective' | 'orthographic' {
+    return this.projection;
+  }
+
+  setProjection(projection: 'perspective' | 'orthographic', distanceToTarget: number): void {
+    this.projection = projection;
+    const halfHeight = Math.max(distanceToTarget, 1e-6) * Math.tan((this.camera.fov * Math.PI) / 360);
+    this.orthographic.top = halfHeight;
+    this.orthographic.bottom = -halfHeight;
+    this.orthographic.left = -halfHeight * this.camera.aspect;
+    this.orthographic.right = halfHeight * this.camera.aspect;
+    this.orthographic.zoom = 1;
+    this.orthographic.position.copy(this.camera.position);
+    this.orthographic.quaternion.copy(this.camera.quaternion);
+    this.orthographic.updateProjectionMatrix();
+  }
+
+  visibleHalfHeight(distanceToTarget: number): number {
+    return this.projection === 'orthographic'
+      ? (this.orthographic.top - this.orthographic.bottom) / (2 * this.orthographic.zoom)
+      : distanceToTarget * Math.tan((this.camera.fov * Math.PI) / 360);
+  }
+
   onTick(callback: EngineTickCallback): () => void {
     this.tickCallbacks.add(callback);
     return () => this.tickCallbacks.delete(callback);

src/app/features/galaxy-system/galaxy-system-scene.component.ts

@@ -6,7 +6,7 @@ import { OrbitControls } from 'three/addons/controls/OrbitControls.js';
 import { dateToJulianDate } from '../../shared/astro/constants';
 import { galacticCentrePositionPc, galacticToEquatorial, MILKY_WAY_ARMS, SUN_GALACTOCENTRIC_RADIUS_PC } from '../../shared/astro/galaxy';
 import { DataLoaderService } from '../../core/data/data-loader.service';
-import { EngineService } from '../../core/engine/engine.service';
+import { EngineService, SceneCamera } from '../../core/engine/engine.service';
 import { BodyRecord } from '../../shared/models/body.model';
 import { DeepSkyRecord } from '../../shared/models/deepsky.model';
 import { ExoplanetRecord } from '../../shared/models/exoplanet.model';
@@ -64,6 +64,13 @@ const LABEL_REACH_NDC = 0.3;
 /** How long the range control has to be still before the graph is rebuilt at its value. */
 const JUMP_LINK_REBUILD_DELAY_MS = 250;
 
+/**
+ * How far in or out the plan view may be zoomed from the extent its distance frames. Under a
+ * parallel projection the wheel changes the frame rather than the distance, so the orbit limits
+ * stop applying and this is what stands in for them.
+ */
+const PLAN_ZOOM_SPAN = 64;
+
 /** How many matches each routing field offers, and how little may be typed to get any. */
 const ROUTE_OPTION_COUNT = 6;
 const MIN_ROUTE_QUERY_LENGTH = 2;
@@ -362,6 +369,7 @@ export class GalaxySystemSceneComponent implements AfterViewInit, OnDestroy {
       }
     });
     effect(() => this.applyDisplay(this.display()));
+    effect(() => this.applyProjection(this.display().plan));
     // Reads both signals, so flipping the layer on and dragging the range each land here. The
     // rebuild is a quarter-second of walking the catalogue, and the range control emits per
     // pixel dragged, so it waits for the hand to settle rather than running once per pixel.
@@ -540,15 +548,18 @@ export class GalaxySystemSceneComponent implements AfterViewInit, OnDestroy {
     canvas.addEventListener('pointermove', this.handlePointerMove);
     this.observeResize(canvas);
 
-    this.unsubscribeTick = this.engine.onTick((deltaSeconds) => this.tick(camera, deltaSeconds));
+    // Asked for per frame rather than captured: the projection can be swapped underneath, and a
+    // frame computed against one camera and drawn through the other puts every label off its star.
+    this.unsubscribeTick = this.engine.onTick((deltaSeconds) => this.tick(this.engine.getCamera(), deltaSeconds));
     this.engine.start();
 
     this.ready = true;
     this.reconcileSelection(this.navigationStore.selectedStarId());
   }
 
-  private tick(camera: THREE.PerspectiveCamera, deltaSeconds: number): void {
+  private tick(camera: SceneCamera, deltaSeconds: number): void {
     this.rig?.update(deltaSeconds);
+    this.frameProjection(camera);
     this.controls?.update();
 
     // Gated on the galaxy group rather than on `currentStarId`, which is only assigned once the
@@ -586,12 +597,16 @@ export class GalaxySystemSceneComponent implements AfterViewInit, OnDestroy {
    * has pulled back from the Sun, so the scale ladder reports where the view already is instead
    * of switching it.
    */
-  private updateGalacticCrossfade(camera: THREE.PerspectiveCamera): void {
+  private updateGalacticCrossfade(camera: SceneCamera): void {
     if (!this.milkyWay) {
       return;
     }
 
-    const distancePc = camera.position.length();
+    // How much of the Galaxy is in frame, expressed as the distance a perspective camera would
+    // have to be at to show that much. Under a plan view the camera's own distance says nothing
+    // about the extent — the frustum does — so reading `position.length()` there would report a
+    // fixed scale however far the view was zoomed.
+    const distancePc = this.effectiveDistance(camera);
     this.galacticStrength = this.milkyWay.setViewerDistancePc(distancePc);
 
     // Layer toggles from the dock fold in here rather than as a one-off `visible = false`:
@@ -622,7 +637,25 @@ export class GalaxySystemSceneComponent implements AfterViewInit, OnDestroy {
    * and holding the Galaxy needs a far plane a hundred thousand parsecs out, and a projection
    * spanning both has no precision left to separate one spiral arm from the next.
    */
-  private applyGalaxyDepthRange(camera: THREE.PerspectiveCamera, distancePc: number): void {
+  /**
+   * What "how far back is the camera" means, in either projection. Under perspective it is the
+   * camera's own distance from the origin; under an orthographic one it is the distance a
+   * perspective camera would need to frame the same extent, so everything keyed on it — the
+   * crossfade, the depth range, the scale ladder — goes on meaning what it meant.
+   */
+  private effectiveDistance(camera: SceneCamera): number {
+    if (this.engine.currentProjection === 'perspective') {
+      return camera.position.length();
+    }
+    const halfHeight = this.engine.visibleHalfHeight(camera.position.distanceTo(this.controls?.target ?? GALAXY_OVERVIEW_TARGET));
+    return halfHeight / Math.tan((this.engine.getPerspectiveCamera().fov * Math.PI) / 360);
+  }
+
+  private applyGalaxyDepthRange(camera: SceneCamera, distancePc: number): void {
+    // The plan view sets its own depth range, symmetric about the camera; see `frameOrthographic`.
+    if (this.engine.currentProjection === 'orthographic') {
+      return;
+    }
     const near = THREE.MathUtils.clamp(distancePc / 2000, GALAXY_NEAR_PC, GALACTIC_NEAR_PC);
     const far = THREE.MathUtils.clamp(distancePc * 8, GALAXY_FAR_PC, GALACTIC_FAR_PC);
     // Only when it has drifted enough to matter, so a slow zoom isn't rebuilding the projection
@@ -634,14 +667,14 @@ export class GalaxySystemSceneComponent implements AfterViewInit, OnDestroy {
     }
   }
 
-  private updateLabels(camera: THREE.PerspectiveCamera): void {
+  private updateLabels(camera: SceneCamera): void {
     const selectedId = this.navigationStore.selectedStarId();
     // Measured from what the camera is looking at, not from where it is. Those differ by the
     // orbit distance, so a camera-relative rule names the stars closest to the near edge of the
     // view — a ring of labels around the outside of the thing the user is actually looking at.
     const target = this.controls?.target ?? GALAXY_OVERVIEW_TARGET;
     const { x: cx, y: cy, z: cz } = target;
-    const orbitDistance = (this.controls ? camera.position.distanceTo(target) : GALAXY_OVERVIEW_POSITION.length()) * LABEL_RADIUS_TO_ORBIT_DISTANCE;
+    const orbitDistance = (this.controls ? this.effectiveDistance(camera) : GALAXY_OVERVIEW_POSITION.length()) * LABEL_RADIUS_TO_ORBIT_DISTANCE;
     const labelRadius = THREE.MathUtils.clamp(orbitDistance, MIN_LABEL_RADIUS_PC, MAX_LABEL_RADIUS_PC);
     const maxDistanceSq = labelRadius * labelRadius;
 
@@ -698,7 +731,13 @@ export class GalaxySystemSceneComponent implements AfterViewInit, OnDestroy {
    * `keepId` is exempt from both tests — it is the selection, which is about to be flown to, and
    * its label going missing mid-flight reads as the target having been lost.
    */
-  private spreadLabels(candidates: readonly LabeledPoint[], camera: THREE.PerspectiveCamera, keepId: number | string | null): LabeledPoint[] {
+  /** The frame's shape, from the canvas rather than the camera: only one of the two has it. */
+  private viewportAspect(): number {
+    const canvas = this.canvasRef().nativeElement;
+    return canvas.clientHeight > 0 ? canvas.clientWidth / canvas.clientHeight : 1;
+  }
+
+  private spreadLabels(candidates: readonly LabeledPoint[], camera: SceneCamera, keepId: number | string | null): LabeledPoint[] {
     const placed: THREE.Vector2[] = [];
     const chosen: LabeledPoint[] = [];
     const projected = new THREE.Vector3();
@@ -715,7 +754,7 @@ export class GalaxySystemSceneComponent implements AfterViewInit, OnDestroy {
         continue;
       }
 
-      const point = new THREE.Vector2(projected.x * camera.aspect, projected.y);
+      const point = new THREE.Vector2(projected.x * this.viewportAspect(), projected.y);
       if (!isKept && placed.some((other) => other.distanceTo(point) < LABEL_MIN_SEPARATION_NDC)) {
         continue;
       }
@@ -744,7 +783,7 @@ export class GalaxySystemSceneComponent implements AfterViewInit, OnDestroy {
    * their rim to the card's near edge. Screen-space work done here, once per frame, because the
    * body moves every frame and the card's height depends on its content.
    */
-  private updateSelectionMark(camera: THREE.PerspectiveCamera): void {
+  private updateSelectionMark(camera: SceneCamera): void {
     const leader = this.leaderRef().nativeElement;
     const member = this.systemGroup.visible && this.cardBodyId !== null ? this.systemRenderer?.members.find((candidate) => candidate.id === this.cardBodyId) : undefined;
     if (!member) {
@@ -759,7 +798,7 @@ export class GalaxySystemSceneComponent implements AfterViewInit, OnDestroy {
     const card = this.objectCardRef()?.nativeElement.querySelector('[data-testid="object-card"]');
     const canvas = this.canvasRef().nativeElement;
     const projected = world.clone().project(camera);
-    if (!card || projected.z > 1) {
+    if (!card || projected.z > 1 || projected.z < -1) {
       leader.setAttribute('visibility', 'hidden');
       return;
     }
@@ -838,7 +877,7 @@ export class GalaxySystemSceneComponent implements AfterViewInit, OnDestroy {
    * moved along the ring where a HUD panel already holds that place. `null` where the whole
    * neighbourhood of that bearing is covered.
    */
-  private neighbourRingPosition(camera: THREE.PerspectiveCamera, direction: THREE.Vector3): THREE.Vector3 | null {
+  private neighbourRingPosition(camera: SceneCamera, direction: THREE.Vector3): THREE.Vector3 | null {
     const bearing = this.ringBearing.copy(direction).applyQuaternion(this.ringInverse.copy(camera.quaternion).invert());
     // A neighbour behind the camera keeps the side it is on, which is still the way to turn to
     // bring it round.
@@ -866,7 +905,7 @@ export class GalaxySystemSceneComponent implements AfterViewInit, OnDestroy {
    * happened to be in front and stay silent about the rest. What survives is the half of the
    * direction a viewer can act on: which way to turn to face it.
    */
-  private neighbourLabels(camera: THREE.PerspectiveCamera): LabeledPoint[] {
+  private neighbourLabels(camera: SceneCamera): LabeledPoint[] {
     this.refreshReservedBoxes();
     return this.neighbours.flatMap(({ star, distancePc, direction }) => {
       const position = this.neighbourRingPosition(camera, direction);
@@ -893,7 +932,7 @@ export class GalaxySystemSceneComponent implements AfterViewInit, OnDestroy {
    * — five a second — any camera movement would drag them off the ring and snap them back. This
    * runs every frame and costs four vector operations.
    */
-  private updateNeighbourRing(camera: THREE.PerspectiveCamera): void {
+  private updateNeighbourRing(camera: SceneCamera): void {
     if (!this.systemGroup.visible || this.neighbours.length === 0) {
       return;
     }
@@ -916,7 +955,7 @@ export class GalaxySystemSceneComponent implements AfterViewInit, OnDestroy {
    * Moons are left out entirely: they sit within a marker's width of their planet at system
    * framing, so their labels could only ever print on top of it.
    */
-  private updateSystemLabels(camera: THREE.PerspectiveCamera): void {
+  private updateSystemLabels(camera: SceneCamera): void {
     const renderer = this.systemRenderer;
     if (!renderer) {
       this.labelOverlay?.update([]);
@@ -960,6 +999,84 @@ export class GalaxySystemSceneComponent implements AfterViewInit, OnDestroy {
   }
 
   /** Refreshes the readout panel for whichever scale the view is currently at. */
+  /** A zoom carried from one unit space into the other would frame nothing recognisable. */
+  private resetZoom(): void {
+    const camera = this.engine.getCamera();
+    if ((camera as THREE.OrthographicCamera).isOrthographicCamera) {
+      (camera as THREE.OrthographicCamera).zoom = 1;
+    }
+  }
+
+  /**
+   * Keeps the plan view's frame, its zoom limits and its star sizes in step with the camera.
+   *
+   * All three are functions of how far the camera is orbiting from its target, which is the one
+   * thing the flights already animate — so a system entered, left or flown between reframes
+   * itself under this projection with the easing the perspective flights have, and no camera
+   * rig knows anything about it.
+   */
+  private frameProjection(camera: SceneCamera): void {
+    if (!this.controls) {
+      return;
+    }
+    if (this.engine.currentProjection !== 'orthographic') {
+      this.starField?.setProjection(null);
+      this.hostRings?.setProjection(null);
+      return;
+    }
+    const distance = camera.position.distanceTo(this.controls.target);
+    this.engine.frameOrthographic(distance);
+    // Zoom is what a wheel moves under this projection, so the orbit clamps have to be restated
+    // as the zoom levels that frame the same extents.
+    // A plain multiplier on the frame the distance already sets, bounded by a factor rather than
+    // by the orbit limits: those are in whichever unit space the view is in, and reading them on
+    // the frame the scene swaps from parsecs to astronomical units pins the zoom at the ratio
+    // between the two — which is how leaving a system used to land the view three kiloparsecs out.
+    this.controls.minZoom = 1 / PLAN_ZOOM_SPAN;
+    this.controls.maxZoom = PLAN_ZOOM_SPAN;
+    const halfHeight = this.engine.visibleHalfHeight(distance);
+    this.starField?.setProjection(halfHeight);
+    this.hostRings?.setProjection(halfHeight);
+  }
+
+  /**
+   * Switches between the perspective view and the plan: an orthographic projection looking down
+   * the plane the current scale is read against — the galactic plane out here, this system's own
+   * orbital plane inside one.
+   *
+   * Both halves matter and neither alone is "2D". The projection is what makes a circle a circle
+   * wherever it sits in the frame instead of an ellipse that leans away from the centre; the
+   * swing to face the plane is what makes that worth looking at. Orbiting still works afterwards,
+   * so the plan is where a plan view starts, not a cage.
+   */
+  private applyProjection(plan: boolean): void {
+    if (!this.controls || !this.rig || this.engine.currentProjection === (plan ? 'orthographic' : 'perspective')) {
+      return;
+    }
+    const camera = this.engine.getCamera();
+    const target = this.controls.target.clone();
+    const distance = camera.position.distanceTo(target);
+
+    this.engine.setProjection(plan ? 'orthographic' : 'perspective', distance);
+    const next = this.engine.getCamera();
+    // OrbitControls holds one camera for the lifetime of the gesture state it keeps; handing it
+    // the other one keeps the target, the damping and the pointer bindings it already has.
+    this.controls.object = next;
+    this.rig = new CameraRigController(next, this.controls);
+    next.position.copy(camera.position);
+    next.up.copy(camera.up);
+
+    if (plan) {
+      // Straight down the plane's normal, from where the camera already was.
+      const normal = this.systemGroup.visible && this.systemRenderer ? new THREE.Vector3(0, 0, 1).applyQuaternion(this.systemRenderer.referenceFrame) : new THREE.Vector3(0, 0, 1);
+      next.position.copy(target).add(normal.multiplyScalar(distance));
+      next.up.set(0, 1, 0).applyQuaternion(this.systemGroup.visible && this.systemRenderer ? this.systemRenderer.referenceFrame : new THREE.Quaternion());
+    }
+    next.lookAt(target);
+    this.controls.update();
+    this.applyDisplay(this.display());
+  }
+
   /**
    * Shows or hides the layers that hold still between frames: the label layer and the system
    * view's orbits and grid. The galaxy grids and deep-sky shell are crossfaded every frame
@@ -978,7 +1095,7 @@ export class GalaxySystemSceneComponent implements AfterViewInit, OnDestroy {
     }
   }
 
-  private updateHud(camera: THREE.PerspectiveCamera): void {
+  private updateHud(camera: SceneCamera): void {
     const star = this.currentStarId === null ? undefined : this.starsById.get(this.currentStarId);
 
     if (this.systemGroup.visible && star) {
@@ -998,11 +1115,11 @@ export class GalaxySystemSceneComponent implements AfterViewInit, OnDestroy {
         ...(luminosity !== null ? [{ label: 'Luminosity', value: formatLuminosity(luminosity), derived: true }] : [])
       ]);
       this.hudNote.set('Orbits propagated from published elements to the current date.');
-      this.hudRange.set(formatAu(camera.position.distanceTo(this.controls?.target ?? GALAXY_OVERVIEW_TARGET)));
+      this.hudRange.set(formatAu(this.engine.visibleHalfHeight(camera.position.distanceTo(this.controls?.target ?? GALAXY_OVERVIEW_TARGET)) / Math.tan((this.engine.getPerspectiveCamera().fov * Math.PI) / 360)));
       return;
     }
 
-    this.hudRange.set(formatParsecs(camera.position.length()));
+    this.hudRange.set(formatParsecs(this.effectiveDistance(camera)));
 
     if (this.galacticStrength >= GALACTIC_LEVEL_THRESHOLD) {
       this.hudEyebrow.set('Galactic Scale');
@@ -1065,13 +1182,13 @@ export class GalaxySystemSceneComponent implements AfterViewInit, OnDestroy {
     }
   };
 
-  private handleGalaxyClick(pointerNdc: THREE.Vector2, camera: THREE.PerspectiveCamera): void {
+  private handleGalaxyClick(pointerNdc: THREE.Vector2, camera: SceneCamera): void {
     if (!this.starField) {
       return;
     }
     // Screen-space rather than a raycast: the star field billboards in the vertex shader, so
     // its CPU-side geometry is a single quad at the origin. See `StarFieldRenderer.pickAt`.
-    const starId = this.starField.pickAt(pointerNdc, camera);
+    const starId = this.starField.pickAt(pointerNdc, camera, this.viewportAspect());
     if (starId !== undefined) {
       this.navigationStore.selectStar(starId);
     }
@@ -1302,7 +1419,10 @@ export class GalaxySystemSceneComponent implements AfterViewInit, OnDestroy {
     // the wider of the two — and against the camera this scene actually has, so the margin holds
     // whatever the window shape. Computed before the star, because how far away the star will be
     // seen from is what decides how big its halo has to be to stay visible.
-    const viewport = { fovDegrees: camera.fov, aspect: camera.aspect };
+    // Framed against the perspective camera whichever is active: the framing distance is what
+    // the orthographic frustum is then sized from, so both projections show the same extent.
+    const framingCamera = this.engine.getPerspectiveCamera();
+    const viewport = { fovDegrees: framingCamera.fov, aspect: framingCamera.aspect };
     const framingDistance = systemFramingDistanceAu(this.systemRenderer.gridOuterRadiusAu, viewport);
     const frameRadiusAu = systemFrameRadiusAu(framingDistance, viewport);
 
@@ -1339,6 +1459,7 @@ export class GalaxySystemSceneComponent implements AfterViewInit, OnDestroy {
     this.controls!.minDistance = SYSTEM_MIN_DISTANCE_AU;
     this.controls!.maxDistance = SYSTEM_MAX_DISTANCE_AU;
 
+    this.resetZoom();
     this.rig!.setImmediate({ position: direction.clone().multiplyScalar(SYSTEM_ENTRY_DISTANCE_AU), target: new THREE.Vector3(0, 0, 0) });
 
     // Arrives along whichever direction the approach came from, then swings round to look down
@@ -1389,6 +1510,7 @@ export class GalaxySystemSceneComponent implements AfterViewInit, OnDestroy {
     this.controls!.minDistance = GALAXY_MIN_DISTANCE_PC;
     this.controls!.maxDistance = GALAXY_MAX_DISTANCE_PC;
 
+    this.resetZoom();
     this.rig!.setImmediate({ position: starPc.clone().add(direction.clone().multiplyScalar(GALAXY_APPROACH_DISTANCE_PC)), target: starPc });
 
     if (isSwitchingSystems) {

src/app/features/galaxy-system/host-star-rings.ts

@@ -1,5 +1,5 @@
 import * as THREE from 'three/webgpu';
-import { color, float, instancedBufferAttribute, smoothstep, uniform, uv, vec2 } from 'three/tsl';
+import { color, float, instancedBufferAttribute, mix, modelViewMatrix, smoothstep, uniform, uv, vec2, vec4 } from 'three/tsl';
 
 import { StarRecord } from '../../shared/models/star.model';
 
@@ -40,6 +40,9 @@ export class HostStarRings {
   private readonly geometry: THREE.InstancedBufferGeometry;
   private readonly material: THREE.SpriteNodeMaterial;
   private readonly opacity = uniform(RING_PEAK_OPACITY);
+  /** 1 under a perspective camera, 0 under an orthographic one. See `setProjection`. */
+  private readonly perspective = uniform(1);
+  private readonly orthographicScale = uniform(float(0));
 
   constructor(hosts: readonly StarRecord[], accent: number) {
     const positions = new Float32Array(hosts.length * 3);
@@ -47,9 +50,13 @@ export class HostStarRings {
     this.geometry = createQuadGeometry(hosts.length);
 
     this.material = new THREE.SpriteNodeMaterial({ transparent: true, depthWrite: false });
-    this.material.sizeAttenuation = false;
-    this.material.positionNode = instancedBufferAttribute(new THREE.InstancedBufferAttribute(positions, 3), 'vec3');
-    this.material.scaleNode = float(RING_SIZE_PX * PIXELS_TO_ANGULAR_SIZE);
+    // As in the star field: the angular-to-world conversion is done here rather than by
+    // `sizeAttenuation: false`, which three.js applies only under a perspective camera.
+    this.material.sizeAttenuation = true;
+    const position = instancedBufferAttribute<'vec3'>(new THREE.InstancedBufferAttribute(positions, 3), 'vec3');
+    this.material.positionNode = position;
+    const viewDepth = modelViewMatrix.mul(vec4(position, 1)).z.negate();
+    this.material.scaleNode = float(RING_SIZE_PX * PIXELS_TO_ANGULAR_SIZE).mul(mix(this.orthographicScale, viewDepth, this.perspective));
     this.material.colorNode = color(accent);
     // Opaque on the ring's centreline, falling to nothing one stroke-width either side.
     const distanceFromRing = uv().sub(vec2(0.5)).length().sub(RING_RADIUS_UV).abs();
@@ -62,6 +69,12 @@ export class HostStarRings {
     this.count = hosts.length;
   }
 
+  /** Which projection the rings are drawn under; see `StarFieldRenderer.setProjection`. */
+  setProjection(halfHeightWorld: number | null): void {
+    this.perspective.value = halfHeightWorld === null ? 1 : 0;
+    this.orthographicScale.value = halfHeightWorld === null ? 0 : halfHeightWorld / Math.tan((REFERENCE_FOV_DEGREES * Math.PI) / 360);
+  }
+
   /** Crossfaded with the local grid: from outside the Galaxy the rings are noise. */
   setStrength(strength: number): void {
     const clamped = THREE.MathUtils.clamp(strength, 0, 1);

src/app/features/galaxy-system/star-field-renderer.spec.ts

@@ -152,13 +152,13 @@ describe('StarFieldRenderer', () => {
     it('finds the star under the pointer', () => {
       const renderer = new StarFieldRenderer(picked, packPositions(picked));
       // Both Near and Far project to the screen centre; either is a correct hit.
-      expect([1, 2]).toContain(renderer.pickAt(new THREE.Vector2(0, 0), camera));
+      expect([1, 2]).toContain(renderer.pickAt(new THREE.Vector2(0, 0), camera, camera.aspect));
       renderer.dispose();
     });
 
     it('returns undefined when the pointer is on empty sky', () => {
       const renderer = new StarFieldRenderer(picked, packPositions(picked));
-      expect(renderer.pickAt(new THREE.Vector2(-0.9, 0.9), camera)).toBeUndefined();
+      expect(renderer.pickAt(new THREE.Vector2(-0.9, 0.9), camera, camera.aspect)).toBeUndefined();
       renderer.dispose();
     });
 
@@ -168,7 +168,7 @@ describe('StarFieldRenderer', () => {
       const behind = [star({ id: 7, x: 0, y: 0, z: 10 })];
       const renderer = new StarFieldRenderer(behind, packPositions(behind));
 
-      expect(renderer.pickAt(new THREE.Vector2(0, 0), camera)).toBeUndefined();
+      expect(renderer.pickAt(new THREE.Vector2(0, 0), camera, camera.aspect)).toBeUndefined();
       renderer.dispose();
     });
 
@@ -182,7 +182,7 @@ describe('StarFieldRenderer', () => {
 
       // Aim at where star 2 projects, and confirm we get it rather than its neighbours.
       const target = new THREE.Vector3(0, 2, -10).project(camera);
-      expect(renderer.pickAt(new THREE.Vector2(target.x, target.y), camera)).toBe(2);
+      expect(renderer.pickAt(new THREE.Vector2(target.x, target.y), camera, camera.aspect)).toBe(2);
       renderer.dispose();
     });
 
@@ -195,7 +195,7 @@ describe('StarFieldRenderer', () => {
       // Walk outward from the centre until each stops being pickable.
       const reach = (renderer: StarFieldRenderer): number => {
         let offset = 0;
-        while (offset < 1 && renderer.pickAt(new THREE.Vector2(0, offset), camera) !== undefined) {
+        while (offset < 1 && renderer.pickAt(new THREE.Vector2(0, offset), camera, camera.aspect) !== undefined) {
           offset += 0.001;
         }
         return offset;
@@ -212,13 +212,13 @@ describe('StarFieldRenderer', () => {
       const faint = [star({ id: 5, x: 0, y: 0, z: -10, magnitude: 15 })];
       const renderer = new StarFieldRenderer(faint, packPositions(faint));
 
-      expect(renderer.pickAt(new THREE.Vector2(0, 0.005), camera)).toBe(5);
+      expect(renderer.pickAt(new THREE.Vector2(0, 0.005), camera, camera.aspect)).toBe(5);
       renderer.dispose();
     });
 
     it('finds nothing in an empty field', () => {
       const renderer = new StarFieldRenderer([], new Float32Array(0));
-      expect(renderer.pickAt(new THREE.Vector2(0, 0), camera)).toBeUndefined();
+      expect(renderer.pickAt(new THREE.Vector2(0, 0), camera, camera.aspect)).toBeUndefined();
       renderer.dispose();
     });
   });

src/app/features/galaxy-system/star-field-renderer.ts

@@ -1,7 +1,8 @@
 import * as THREE from 'three/webgpu';
-import { instancedBufferAttribute, smoothstep, uv, vec2 } from 'three/tsl';
+import { float, instancedBufferAttribute, mix, modelViewMatrix, smoothstep, uniform, uv, vec2, vec4 } from 'three/tsl';
 
 import { spectralTypeToColorIndex } from '../../shared/astro/spectral';
+import { SceneCamera } from '../../core/engine/engine.service';
 import { StarRecord } from '../../shared/models/star.model';
 
 /** Apparent star diameters, in pixels at {@link REFERENCE_VIEWPORT_HEIGHT_PX}. */
@@ -169,6 +170,10 @@ export class StarFieldRenderer {
   /** How many of the catalogue's stars this field actually draws. */
   readonly drawnCount: number;
 
+  /** 1 under a perspective camera, 0 under an orthographic one. See `setProjection`. */
+  private readonly perspective = uniform(1);
+  private readonly orthographicScale = uniform(float(0));
+
   private readonly geometry: THREE.InstancedBufferGeometry;
   private readonly material: THREE.SpriteNodeMaterial;
   /** The subset of the catalogue that is drawn, and so the only set that can be clicked. */
@@ -209,10 +214,22 @@ export class StarFieldRenderer {
       depthWrite: false,
       blending: THREE.AdditiveBlending
     });
-    this.material.sizeAttenuation = false;
-    this.material.positionNode = instancedBufferAttribute(positionAttribute, 'vec3');
-    this.material.scaleNode = instancedBufferAttribute(sizeAttribute, 'float');
-    this.material.colorNode = instancedBufferAttribute(colorAttribute, 'vec3');
+    // The compensation that turns an angular size into a world size is done here rather than by
+    // `sizeAttenuation: false`, which three.js applies only when it is compiling against a
+    // perspective camera (SpriteNodeMaterial.js: `camera.isPerspectiveCamera && sizeAttenuation
+    // === false`). Under an orthographic one it is silently skipped and every star collapses to
+    // a thousandth of a parsec — invisible. Doing the same arithmetic in the node graph, behind
+    // a uniform, lets one material serve both cameras without being recompiled between them.
+    this.material.sizeAttenuation = true;
+    const position = instancedBufferAttribute<'vec3'>(positionAttribute, 'vec3');
+    const angularSize = instancedBufferAttribute<'float'>(sizeAttribute, 'float');
+    this.material.positionNode = position;
+    // Perspective: a star's world size is its angular size times how far away it is, which is
+    // exactly what the built-in does. Orthographic: distance does not set apparent size at all,
+    // the frustum does, so the same angular size is scaled by the frustum instead.
+    const viewDepth = modelViewMatrix.mul(vec4(position, 1)).z.negate();
+    this.material.scaleNode = angularSize.mul(mix(this.orthographicScale, viewDepth, this.perspective));
+    this.material.colorNode = instancedBufferAttribute<'vec3'>(colorAttribute, 'vec3');
     // Soft radial falloff so each star is a small bright core inside a halo, rather than a
     // hard-edged square. `uv` runs 0..1 across the quad, so 0.5 is its centre.
     const radius = uv().sub(vec2(0.5)).length();
@@ -224,6 +241,19 @@ export class StarFieldRenderer {
     this.object.frustumCulled = false;
   }
 
+  /**
+   * Tells the field which projection it is being drawn under.
+   *
+   * `halfHeightWorld` is half the orthographic frustum's height in world units; `null` means a
+   * perspective camera, where a star's distance sets its apparent size on its own.
+   */
+  setProjection(halfHeightWorld: number | null): void {
+    this.perspective.value = halfHeightWorld === null ? 1 : 0;
+    // The world size that subtends the same share of the viewport an angular size would under
+    // the reference field of view: `angular * halfHeight / tan(fov/2)`.
+    this.orthographicScale.value = halfHeightWorld === null ? 0 : halfHeightWorld / Math.tan((REFERENCE_FOV_DEGREES * Math.PI) / 360);
+  }
+
   /** Looks up the HYG star id for a given instance index. */
   starIdAt(instanceIndex: number): number | undefined {
     return this.stars[instanceIndex]?.id;
@@ -239,8 +269,14 @@ export class StarFieldRenderer {
    * what the user sees at every zoom level instead of being over-permissive up close and
    * sub-pixel at the far end of the camera's range.
    */
-  pickAt(pointerNdc: THREE.Vector2, camera: THREE.PerspectiveCamera): number | undefined {
-    const tanHalfFov = Math.tan((camera.fov * Math.PI) / 360);
+  pickAt(pointerNdc: THREE.Vector2, camera: SceneCamera, aspect: number): number | undefined {
+    // What a unit of angular size is worth on screen. Under perspective that is set by the
+    // field of view; under an orthographic camera the same size was already turned into a world
+    // size by `setProjection`, so it is the frustum that converts it back.
+    const perspective = (camera as THREE.PerspectiveCamera).isPerspectiveCamera;
+    const orthographic = camera as THREE.OrthographicCamera;
+    const halfHeightWorld = perspective ? 0 : (orthographic.top - orthographic.bottom) / (2 * orthographic.zoom);
+    const tanHalfFov = perspective ? Math.tan(((camera as THREE.PerspectiveCamera).fov * Math.PI) / 360) : 0;
     const projected = new THREE.Vector3();
 
     let bestIndex: number | undefined;
@@ -257,8 +293,12 @@ export class StarFieldRenderer {
 
       // A sprite square in view space projects to an ellipse in NDC: the same half-extent in y,
       // divided by the aspect ratio in x. Scaling dx by the aspect makes the comparison circular.
-      const ndcRadius = (0.5 * this.angularSizes[index]) / tanHalfFov + PICK_NDC_SLOP;
-      const dx = (projected.x - pointerNdc.x) * camera.aspect;
+      const ndcRadius =
+        (perspective
+          ? (0.5 * this.angularSizes[index]) / tanHalfFov
+          : // The world size the star is drawn at, as a fraction of the frustum's half-height.
+            (0.5 * this.angularSizes[index] * (halfHeightWorld / Math.tan((REFERENCE_FOV_DEGREES * Math.PI) / 360))) / halfHeightWorld) + PICK_NDC_SLOP;
+      const dx = (projected.x - pointerNdc.x) * aspect;
       const dy = projected.y - pointerNdc.y;
       const score = Math.hypot(dx, dy) / ndcRadius;