Ensure tiles have overlap applied

lib/output.js

@@ -575,7 +575,7 @@ function tile (options) {
         if (options.overlap > this.options.tileSize) {
           throw is.invalidParameterError('overlap', `<= size (${this.options.tileSize})`, options.overlap);
         }
-        this.options.tileOverlap = tile.overlap;
+        this.options.tileOverlap = options.overlap;
       } else {
         throw is.invalidParameterError('overlap', 'integer between 0 and 8192', options.overlap);
       }

package.json

@@ -63,7 +63,8 @@
     "Jordan Prudhomme <[email protected]>",
     "Ilya Ovdin <[email protected]>",
     "Andargor <[email protected]>",
-    "Paul Neave <[email protected]>"
+    "Paul Neave <[email protected]>",
+    "Brendan Kennedy <[email protected]>"
   ],
   "scripts": {
     "install": "(node install/libvips && node install/dll-copy && prebuild-install) || (node-gyp rebuild && node install/dll-copy)",

test/unit/tile.js

@@ -91,6 +91,29 @@ const assertGoogleTiles = function (directory, expectedTileSize, expectedLevels,
   });
 };
 
+// Verifies tiles at specified level in a given output directory are > size+overlap
+const assertTileOverlap = function (directory, tileSize, done) {
+  // Get sorted levels
+  const levels = fs.readdirSync(directory).sort((a, b) => a - b);
+  // Select the highest tile level
+  const highestLevel = levels[levels.length - 1];
+  // Get sorted tiles from greatest level
+  const tiles = fs.readdirSync(path.join(directory, highestLevel)).sort();
+  // Select a tile from the approximate center of the image
+  const squareTile = path.join(directory, highestLevel, tiles[Math.floor(tiles.length / 2)]);
+
+  sharp(squareTile).metadata(function (err, metadata) {
+    if (err) {
+      throw err;
+    } else {
+      // Tile with an overlap should be larger than original size
+      assert.strictEqual(true, metadata.width > tileSize);
+      assert.strictEqual(true, metadata.height > tileSize);
+      done();
+    }
+  });
+};
+
 describe('Tile', function () {
   it('Valid size values pass', function () {
     [1, 8192].forEach(function (size) {
@@ -297,7 +320,9 @@ describe('Tile', function () {
           assert.strictEqual(2225, info.height);
           assert.strictEqual(3, info.channels);
           assert.strictEqual('undefined', typeof info.size);
-          assertDeepZoomTiles(directory, 512 + (2 * 16), 13, done);
+          assertDeepZoomTiles(directory, 512 + (2 * 16), 13, function () {
+            assertTileOverlap(directory, 512, done);
+          });
         });
     });
   });