Add #flight=N|NE|E|SE|S|SW|W|NW URL parameter for corridor direction

Adds 8 compass directions for the tile-build axis. Camera position,
corridor rotation, and refetch math all read from window.__flightDir.

Test URLs:
  #flight=N   north  (default, +Z, current behaviour)
  #flight=E   east   (+X)
  #flight=S   south  (-Z)
  #flight=W   west   (-X)
  #flight=NE  northeast (diagonal)
  #flight=SE  southeast
  #flight=SW  southwest
  #flight=NW  northwest

Renders with no hash unchanged. Each direction rotates the
corridor so its long axis follows the chosen compass bearing, and
the camera drifts along that bearing.

background.html

@@ -516,6 +516,21 @@ var noiseDataUri = 'data:image/png;base64,PLACEHOLDER';
 // URL override for easy A/B testing: #renderer=clouds2d / #renderer=clouds3d
 var _urlRenderer = (location.hash.match(/renderer=(\w+)/) || [])[1];
 var renderer = _urlRenderer || (window.CONFIG && window.CONFIG.renderer) || 'vanta';
+
+// Flight direction for ground-tile corridor (N, NE, E, SE, S, SW, W, NW).
+// Default 'N' (= +Z world) so behaviour is unchanged when hash is absent.
+var _urlFlight = (location.hash.match(/flight=(\w+)/) || [])[1];
+var FLIGHT_DIRS = {
+    N:  { cos: 0.0,        sin: 1.0,        dx: 0,  dz: 1  },
+    NE: { cos: 0.7071068,  sin: 0.7071068,  dx: 0.7071068, dz: 0.7071068 },
+    E:  { cos: 1.0,        sin: 0.0,        dx: 1,  dz: 0  },
+    SE: { cos: 0.7071068,  sin:-0.7071068,  dx: 0.7071068, dz:-0.7071068 },
+    S:  { cos: 0.0,        sin:-1.0,        dx: 0,  dz:-1  },
+    SW: { cos:-0.7071068,  sin:-0.7071068,  dx:-0.7071068, dz:-0.7071068 },
+    W:  { cos:-1.0,        sin: 0.0,        dx:-1,  dz: 0  },
+    NW: { cos:-0.7071068,  sin: 0.7071068,  dx:-0.7071068, dz: 0.7071068 }
+};
+window.__flightDir = FLIGHT_DIRS[_urlFlight && FLIGHT_DIRS[_urlFlight.toUpperCase()] ? _urlFlight.toUpperCase() : 'N'];
 var useVanta = (renderer !== 'clouds2d' && renderer !== 'clouds3d');
 var effect;
 if (useVanta && typeof VANTA !== 'undefined') {

vendor/clouds3d.js

@@ -139,7 +139,11 @@ var FRAG_SRC = [
         'float drift = uTime * uSpeed * 0.12;',
         // Fixed altitude well above the cloud ceiling (y=1.5).
         'float camY = 2.2;',
-        'vec3 ro = vec3(0.0, camY, drift);',
+        // Camera position follows window.__flightDir (set from URL #flight=...).
+        // Default: drift along +Z (north). For east: drift along +X. For south: -Z, etc.
+        'float fdx = (window.__flightDir ? window.__flightDir.dx : 0.0);',
+        'float fdz = (window.__flightDir ? window.__flightDir.dz : 1.0);',
+        'vec3 ro = vec3(drift * fdx, camY, drift * fdz);',
         // Ray direction: tilt more downward so the visible ground fits
         // within the mosaic. At uv.y = 0 the ray goes ~26° below
         // horizontal, putting the visible ground at tGround < 5
@@ -281,10 +285,11 @@ function init(canvas) {
 var GROUND_TILE_URL = 'https://server.arcgisonline.com/ArcGIS/rest/services/World_Imagery/MapServer/tile/';
 var GROUND_TILE_Z = 11;
 var GROUND_TILE_SIZE = 256;
-// Camera travels in +Z (forward) and looks in +Z (also forward).
-// The corridor is aligned with both motion and look direction.
-var GROUND_TRAVEL_COS = 0.0;      // cos(90 deg)
-var GROUND_TRAVEL_SIN = 1.0;      // sin(90 deg)
+// Camera flight direction. Set from window.__flightDir (URL: #flight=N|NE|E|...).
+// Default is N (= +Z forward). Each flight direction rotates the
+// corridor so its long axis follows the camera path.
+var GROUND_TRAVEL_COS = (window.__flightDir && window.__flightDir.cos) || 0.0;
+var GROUND_TRAVEL_SIN = (window.__flightDir && window.__flightDir.sin) || 1.0;
 var GROUND_CORRIDOR_LONG = 25;    // tiles along travel direction (~3.5 min of flight at uSpeed=1)
 var GROUND_CORRIDOR_PERP = 5;     // tiles perpendicular to travel
 var GROUND_BUFFER_LONG = 11;       // refetch when this close to forward edge
@@ -375,8 +380,13 @@ function rebuildGroundMosaic(centerTileX, centerTileY) {
 // Called periodically from the render loop with the camera world
 // position. Re-centres the mosaic if we're approaching its edge.
 function maybeRefetchGround(cameraWorldX, cameraWorldZ, gl) {
-    var cameraTileX = GROUND_BASE_X + Math.floor(cameraWorldX);
-    var cameraTileY = GROUND_BASE_Y + Math.floor(cameraWorldZ);
+    var fdx = (window.__flightDir && window.__flightDir.dx) || 0;
+    var fdz = (window.__flightDir && window.__flightDir.dz) || 1;
+    // Camera position in world (X, Z) from the dominant flight direction
+    var camX = (fdx !== 0) ? cameraWorldX * fdx : 0;
+    var camZ = (fdz !== 0) ? cameraWorldZ * fdz : 0;
+    var cameraTileX = GROUND_BASE_X + Math.floor(camX);
+    var cameraTileY = GROUND_BASE_Y + Math.floor(camZ);
     // Project camera offset onto the corridor's rotated axes.
     // With GROUND_TRAVEL_COS/SIN = (0, 1) the corridor runs along
     // +Z (camera's motion + look direction).
@@ -536,7 +546,7 @@ window.clouds3dRender = function(target) {
     var drift = now * speed * 0.12;
     if (performance.now() - groundLastCheck > 1500) {
         groundLastCheck = performance.now();
-        maybeRefetchGround(drift, drift * 0.7, gl);
+        maybeRefetchGround(drift, drift, gl);
     }
 
     gl.drawArrays(gl.TRIANGLES, 0, 3);