add docs

docs/api-reference/viewport.md

@@ -137,6 +137,17 @@ Returns:
 * `[longitude, latitude, altitude]`
 
 
+##### `getBounds`
+
+Extracts the axis-aligned bounding box of the current visible area.
+
+* `options` (Object, optional)
+  + `options.z` (Number, optional) - To calculate a bounding volume for fetching 3D data, this option can be used to get the bounding box at a specific elevation. Default `0`.
+
+Returns:
+* `[minX, minY, maxX, maxY]` that defines the smallest orthogonal bounds that encompasses the visible region.
+
+
 ##### `getFrustumPlanes`
 
 Extract view frustum planes of the current camera. Each plane is defined by its normal `normal` and distance from

modules/geo-layers/src/tile-layer/utils.js

@@ -50,10 +50,10 @@ function getBoundingBox(viewport, zRange, extent) {
     const bounds0 = viewport.getBounds({z: minZ});
     const bounds1 = viewport.getBounds({z: maxZ});
     bounds = [
-      Math.min(bounds0[0], bounds1[0], extent[0]),
-      Math.min(bounds0[1], bounds1[1], extent[1]),
-      Math.max(bounds0[2], bounds1[2], extent[2]),
-      Math.max(bounds0[3], bounds1[3], extent[3])
+      Math.min(bounds0[0], bounds1[0]),
+      Math.min(bounds0[1], bounds1[1]),
+      Math.max(bounds0[2], bounds1[2]),
+      Math.max(bounds0[3], bounds1[3])
     ];
   } else {
     bounds = viewport.getBounds();