diff --git a/vendor/clouds3d.js b/vendor/clouds3d.js
index 9e620c7..cc9f213 100644
--- a/vendor/clouds3d.js
+++ b/vendor/clouds3d.js
@@ -335,7 +335,7 @@ function drawTileAt(lng, perp, dx, dz, centerTileX, centerTileY) {
     // N flight) lands at small canvasY (= screen top after flip).
     // For perp axis (X): keep the natural sign so world east (dx>0)
     // ends up at canvasX > halfPerp area (right side of canvas).
-    var canvasX = (perp + halfPerp + 0.5) * GROUND_TILE_SIZE;
+    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);