diff --git a/demo/_synctiles.py b/demo/_synctiles.py
index 14c1037..1707bf6 100755
--- a/demo/_synctiles.py
+++ b/demo/_synctiles.py
@@ -3,7 +3,7 @@
 # _synctiles - Generate and upload Deep Zoom tiles for test slides
 #
 # Copyright (c) 2010-2015 Carnegie Mellon University
-# Copyright (c) 2016-2025 Benjamin Gilbert
+# Copyright (c) 2016-2026 Benjamin Gilbert
 #
 # This program is free software; you can redistribute it and/or modify it
 # under the terms of version 2.1 of the GNU Lesser General Public License
@@ -21,10 +21,12 @@
 from __future__ import annotations
 
 from argparse import ArgumentParser, FileType
+from array import array
 import base64
 from collections.abc import Callable, Iterator
 from concurrent.futures import ThreadPoolExecutor, as_completed
 from dataclasses import dataclass
+import gzip
 from hashlib import md5, sha256
 from io import BytesIO
 import json
@@ -57,7 +59,7 @@
 if TYPE_CHECKING:
     from mypy_boto3_s3.service_resource import Object
 
-STAMP_VERSION = 'threads'  # change to retile without OpenSlide version bump
+STAMP_VERSION = 'sparse'  # change to retile without OpenSlide version bump
 CORS_ORIGINS = ['*']
 DOWNLOAD_BASE_URL = 'https://openslide.cs.cmu.edu/download/openslide-testdata/'
 DOWNLOAD_INDEX = 'index.json'
@@ -178,6 +180,12 @@ class ImageInfo(TypedDict):
     name: str | None
     mpp: float | None
     source: DzSource
+    sparse: dict[str, SparseLevel]
+
+
+class SparseLevel(TypedDict):
+    tiles: tuple[int, int]
+    bitmap: str
 
 
 class DzSource(TypedDict):
@@ -262,14 +270,43 @@ def upload_metadata(
         self, path: PurePath, item: Any, cache: bool = True
     ) -> None:
         self.object(path).put(
-            Body=json.dumps(item, indent=1, sort_keys=True).encode(),
+            Body=gzip.compress(
+                json.dumps(
+                    item, separators=(',', ':'), sort_keys=True
+                ).encode()
+            ),
             CacheControl=(
                 CACHE_CONTROL_CACHE if cache else CACHE_CONTROL_NOCACHE
             ),
+            ContentEncoding='gzip',
             ContentType='application/json',
         )
 
 
+class SparseMap:
+    def __init__(self, generator: Generator):
+        self._level_tiles = generator.dz.level_tiles
+        self._bitmaps = [
+            # ceil division
+            array('B', [0] * -(level_tiles[0] * level_tiles[1] // -8))
+            for level_tiles in self._level_tiles
+        ]
+
+    def set_bit(self, level: int, address: tuple[int, int]) -> None:
+        bit = self._level_tiles[level][0] * address[1] + address[0]
+        self._bitmaps[level][bit >> 3] |= 1 << (bit & 7)
+
+    def save(self) -> dict[str, SparseLevel]:
+        return {
+            str(level): {
+                'tiles': self._level_tiles[level],
+                'bitmap': base64.b64encode(bitmap.tobytes()).decode(),
+            }
+            for level, bitmap in enumerate(self._bitmaps)
+            if any(bitmap)
+        }
+
+
 @dataclass
 class Tile:
     storage: S3Storage
@@ -279,9 +316,12 @@ class Tile:
     key_name: PurePath
     cur_md5: str | None
 
-    def sync(self) -> PurePath:
+    def sync(self) -> PurePath | tuple[int, tuple[int, int]]:
         """Generate and possibly upload a tile."""
         tile = self.generator.get_tile(self.level, self.address)
+        if tile.getextrema() == ((255, 255), (255, 255), (255, 255)):  # type: ignore[no-untyped-call]
+            # completely white tile; add to sparse bitmap
+            return (self.level, self.address)
         buf = BytesIO()
         tile.save(
             buf,
@@ -352,13 +392,18 @@ def progress() -> None:
 
     # Sync tiles
     progress()
+    sparse_map = SparseMap(generator)
     for future in as_completed(
         exec.submit(Tile.sync, tile)
         for tile in Tile.enumerate(
             storage, generator, key_imagepath, key_md5sums
         )
     ):
-        key_md5sums.pop(future.result(), None)
+        result = future.result()
+        if isinstance(result, PurePath):
+            key_md5sums.pop(result, None)
+        else:
+            sparse_map.set_bit(*result)
         count += 1
         if count % 100 == 0:
             progress()
@@ -385,6 +430,7 @@ def progress() -> None:
         'name': associated,
         'mpp': mpp,
         'source': source,
+        'sparse': sparse_map.save(),
     }
 
 
@@ -403,8 +449,11 @@ def sync_slide(
 
     # Get current metadata
     try:
+        resp = storage.object(metadata_key_name).get()
         metadata: SlideMetadata | None = json.load(
-            storage.object(metadata_key_name).get()['Body']
+            gzip.open(resp['Body'])
+            if resp.get('ContentEncoding') == 'gzip'
+            else resp['Body']
         )
     except storage.NoSuchKey:
         metadata = None
@@ -629,8 +678,12 @@ def start_retile(
 
     # If the stamp is changing, mark bucket dirty
     try:
-        stream = storage.object(PurePath(METADATA_NAME)).get()['Body']
-        metadata: BucketMetadata = json.load(stream)
+        resp = storage.object(PurePath(METADATA_NAME)).get()
+        metadata: BucketMetadata = json.load(
+            gzip.open(resp['Body'])
+            if resp.get('ContentEncoding') == 'gzip'
+            else resp['Body']
+        )
         old_stamp = metadata['stamp']
     except storage.NoSuchKey:
         old_stamp = None
diff --git a/demo/index.html b/demo/index.html
index a993639..3f7c03d 100644
--- a/demo/index.html
+++ b/demo/index.html
@@ -2,7 +2,7 @@
   OpenSlide demo site
 
   Copyright (c) 2010-2015 Carnegie Mellon University
-  Copyright (c) 2016 Benjamin Gilbert
+  Copyright (c) 2016-2026 Benjamin Gilbert
 
   This program is free software; you can redistribute it and/or modify it
   under the terms of version 2.1 of the GNU Lesser General Public License
@@ -59,7 +59,7 @@
     <div id="details" class="scroll-parent"></div>
 </div>
 
-<script src="https://code.jquery.com/jquery-4.0.0.min.js" integrity="sha256-OaVG6prZf4v69dPg6PhVattBXkcOWQB62pdZ3ORyrao=" crossorigin="anonymous"></script>
+<script src="https://cdn.jsdelivr.net/npm/jquery@4.0.0/dist/jquery.min.js" integrity="sha256-OaVG6prZf4v69dPg6PhVattBXkcOWQB62pdZ3ORyrao=" crossorigin="anonymous"></script>
 <script src="https://cdn.jsdelivr.net/npm/openseadragon@6.0.2/build/openseadragon/openseadragon.min.js" integrity="sha256-xFw3UC7oKMnWjRwWFCtFNv5UgUx1xnqzFw8aCVkn7UY=" crossorigin="anonymous"></script>
 <script src="openseadragon-scalebar.js"></script>
 <script>
@@ -70,6 +70,7 @@
     var stamp;
     var groups;
     var viewer;
+    var image;
     var status_skipped;
 
     function check_status() {
@@ -114,6 +115,22 @@
     });
     setInterval(check_status, 300000);
 
+    function decode_sparse_bitmaps(image) {
+        $.each(image.sparse, function(level, sparse) {
+            if (Uint8Array.fromBase64) {
+                sparse.bitmap = Uint8Array.fromBase64(sparse.bitmap, {
+                    lastChunkHandling: "strict"
+                });
+            } else {
+                var decoded = atob(sparse.bitmap);
+                sparse.bitmap = new Uint8Array(decoded.length);
+                for (var i = 0; i < decoded.length; i++) {
+                    sparse.bitmap[i] = decoded.charCodeAt(i);
+                }
+            }
+        });
+    }
+
     function populate_images(data, status, xhr) {
         stamp = data.stamp;
         groups = data.groups;
@@ -140,7 +157,9 @@
                     a.data('slide', j);
                     a.data('image', k);
                     $('<li/>').appendTo(images).append(a);
+                    decode_sparse_bitmaps(image);
                 });
+                decode_sparse_bitmaps(slide.slide);
             });
         });
         var versions = $('<p id="versions">');
@@ -201,7 +220,6 @@
         var slide_id = link.data('slide');
         var associated_id = link.data('image');
         var slide = groups[group_id].slides[slide_id];
-        var image;
         if (typeof associated_id === 'undefined') {
             image = slide.slide;
         } else {
@@ -254,6 +272,28 @@
                 viewer.source.getTileUrl = function() {
                     return getTileUrl.apply(this, arguments) + '?v=' + stamp;
                 };
+                var tileExists = viewer.source.tileExists;
+                viewer.source.tileExists = function(level, x, y) {
+                    if (!tileExists.apply(this, arguments)) {
+                        return false;
+                    }
+                    if (!image.sparse) {
+                        return true;
+                    }
+                    var sparse = image.sparse[level];
+                    if (!sparse) {
+                        return true;
+                    }
+                    if (x < 0 || y < 0 ||
+                        x >= sparse.tiles[0] || y >= sparse.tiles[1]) {
+                        return false;
+                    }
+                    var idx = y * sparse.tiles[0] + x;
+                    if (sparse.bitmap[idx >> 3] & (1 << (idx & 7))) {
+                        return false;
+                    }
+                    return true;
+                };
             });
             viewer.scalebar({
                 xOffset: 10,