diff --git a/vendor/clouds3d.js b/vendor/clouds3d.js
index b755466..b714de2 100644
--- a/vendor/clouds3d.js
+++ b/vendor/clouds3d.js
@@ -328,8 +328,12 @@ function drawTileAt(lng, perp, dx, dz, centerTileX, centerTileY) {
     // (vertical) and perp along canvas X (horizontal). This way the
     // corridor's long axis (flight direction +Z) appears as a vertical
     // strip on canvas, matching the way depth maps on screen.
-    var canvasX = (perp + halfPerp + 0.5) * GROUND_TILE_SIZE;
-    var canvasY = (lng + halfLong + 0.5) * GROUND_TILE_SIZE;
+    // UNPACK_FLIP_Y_WEBGL=true inverts V: canvas top -> texture V=1 (top).
+    // To put lng=+12 (corridor front in N flight) at screen TOP we put it
+    // at canvas top (small canvasY). Same inversion for perp so world
+    // east (dx>0, perp<0) ends up at canvas right (large canvasX).
+    var canvasX = (-perp + halfPerp + 0.5) * GROUND_TILE_SIZE;
+    var canvasY = (-lng + halfLong + 0.5) * GROUND_TILE_SIZE;
     if (canvasX < 0 || canvasX >= groundCanvas.width || canvasY < 0 || canvasY >= groundCanvas.height) {
         return Promise.resolve(false);
     }