Keep Navara terrain visible on night-side descents

src/globe/navara/look.js

@@ -108,24 +108,6 @@ export function buildNavaraLook(
       tone.update({ toneMapping: { mode: landscape ? AGX : ACES_FILMIC } });
       changed = true;
     }
-    // In orbit, retain some of the underlying imagery in the atmospheric
-    // shadow. Fade up with altitude so ground views keep their original light.
-    const nextFill =
-      landscape && Number.isFinite(height)
-        ? Math.min(1, Math.max(0, (height - 50_000) / 250_000))
-        : 0;
-    if (nextFill !== fill) {
-      fill = nextFill;
-      if (fillLight) fillLight.update({ ambient: { intensity: fill } });
-      else fillLight = view.addLight({ ambient: { intensity: fill } });
-      changed = true;
-    }
-    const opacity = aerial?.ref?.raw?.rawEffect?.blendMode.opacity;
-    const strength = 1 - 0.2 * fill;
-    if (opacity && opacity.value !== strength) {
-      opacity.value = strength;
-      changed = true;
-    }
     const atmosphere = view.atmosphere;
     // Choose inspection daylight once. Rotating the camera must not rotate
     // the terminator across the geography.
@@ -140,6 +122,27 @@ export function buildNavaraLook(
       place && typeof atmosphere?.getSunElevation === 'function'
         ? atmosphere.getSunElevation(place)
         : Number.NaN;
+    // Keep the map legible from orbit and when descending into darkness.
+    // At low altitude the fill increases from sunset to civil dusk.
+    const orbitalFill = Number.isFinite(height)
+      ? Math.min(1, Math.max(0, (height - 50_000) / 250_000))
+      : 0;
+    const nightFill = Number.isFinite(elevation)
+      ? Math.min(1, Math.max(0, -elevation / 6))
+      : 0;
+    const nextFill = landscape ? Math.max(orbitalFill, nightFill) : 0;
+    if (nextFill !== fill) {
+      fill = nextFill;
+      if (fillLight) fillLight.update({ ambient: { intensity: fill } });
+      else fillLight = view.addLight({ ambient: { intensity: fill } });
+      changed = true;
+    }
+    const opacity = aerial?.ref?.raw?.rawEffect?.blendMode.opacity;
+    const strength = 1 - 0.2 * fill;
+    if (opacity && opacity.value !== strength) {
+      opacity.value = strength;
+      changed = true;
+    }
     if (warmUp) {
       warmUp -= 1;
       aerial.update(warmUp ? IRRADIANCE_ON : IRRADIANCE_OFF);

src/globe/navara/look.test.mjs

@@ -193,6 +193,41 @@ test('daylight inspection restores live exposure, tone mapping and time when lef
   assert.equal(view.atmosphere.date.getTime(), 0);
 });
 
+test('the dark side retains satellite imagery down to the surface', () => {
+  const view = fakeView(0);
+  const look = warmCinematic(view);
+  const aerial = view.added.find(({ desc }) =>
+    Object.hasOwn(desc, 'aerialPerspective'),
+  );
+  const opacity = { value: 1 };
+  aerial.ref = { raw: { rawEffect: { blendMode: { opacity } } } };
+  const origin = { lat: 21.975, lng: 20.9038 };
+  const fuji = { lat: 35.2599, lng: 138.6574 };
+  let elevation = 40;
+  view.atmosphere.getSunElevation = () => elevation;
+  look.frame(33_529_370, false, 0, origin, true);
+  const sun = view.atmosphere.date;
+  const ambient = view.added.find(({ desc }) => Object.hasOwn(desc, 'ambient'));
+  elevation = -30;
+  for (const height of [2_000_000, 100_000, 50_000, 6508, 600]) {
+    look.frame(height, true, 0, fuji, true);
+    assert.equal(opacity.value, 0.8);
+    assert.equal(
+      ambient.updates.length,
+      0,
+      'the orbital fill remains at full strength',
+    );
+    assert.equal(view.atmosphere.date, sun);
+  }
+  elevation = -3;
+  look.frame(600, false, 0, fuji, true);
+  assert.equal(ambient.updates.at(-1).ambient.intensity, 0.5);
+  elevation = 10;
+  look.frame(600, false, 0, origin, true);
+  assert.equal(ambient.updates.at(-1).ambient.intensity, 0);
+  assert.equal(opacity.value, 1);
+});
+
 test('inspection sunlight stays fixed through orbit, zoom and move end', () => {
   const view = fakeView(0);
   const look = warmCinematic(view);