From f9c2f5ef91322eaf136db96253455a6c31c454ef Mon Sep 17 00:00:00 2001
From: Benjamin Gilbert <bgilbert@cs.cmu.edu>
Date: Thu, 11 Jun 2026 03:00:58 -0500
Subject: [PATCH 1/3] demo: get jQuery from the CDN we're already using

Conserve HTTP connections.

Signed-off-by: Benjamin Gilbert <bgilbert@cs.cmu.edu>
---
 demo/index.html | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/demo/index.html b/demo/index.html
index a993639..b9f492b 100644
--- a/demo/index.html
+++ b/demo/index.html
@@ -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>

From 4a55303f747d7e4fc9162971ba4f3f166d1a3936 Mon Sep 17 00:00:00 2001
From: Benjamin Gilbert <bgilbert@cs.cmu.edu>
Date: Thu, 11 Jun 2026 03:11:18 -0500
Subject: [PATCH 2/3] synctiles: unprettify and gzip JSON

S3 can't negotiate Content-Encoding, but we can precompress objects and
unconditionally set the Content-Encoding header.  This works for browsers,
which always set Accept-Encoding: gzip, but causes problems for curl
(without --compressed) and Python requests etc.  However, our JSON is
really only intended for browsers (and for synctiles itself) and we want
to reduce JSON sizes before adding sparse bitmaps.

Support uncompressed info.json and slide.json until existing metadata has
been converted over.

Signed-off-by: Benjamin Gilbert <bgilbert@cs.cmu.edu>
---
 demo/_synctiles.py | 21 +++++++++++++++++----
 1 file changed, 17 insertions(+), 4 deletions(-)

diff --git a/demo/_synctiles.py b/demo/_synctiles.py
index 14c1037..bc19903 100755
--- a/demo/_synctiles.py
+++ b/demo/_synctiles.py
@@ -25,6 +25,7 @@
 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
@@ -262,10 +263,15 @@ 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',
         )
 
@@ -403,8 +409,11 @@ 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 +638,12 @@ 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

From dd914ec6a34697205a3d98b5f6eecfcf277c17b0 Mon Sep 17 00:00:00 2001
From: Benjamin Gilbert <bgilbert@cs.cmu.edu>
Date: Thu, 11 Jun 2026 10:43:28 -0500
Subject: [PATCH 3/3] demo: make a bitmap of blank tiles instead of uploading
 them

When tiling, don't upload completely white tiles; add them to a bitmap
instead.  If the bitmap is non-empty, Base64-encode it and add it to
info.json.  Then, if a bit is set, tell OpenSeadragon the tile doesn't
exist so OSD won't fetch it, using pixels from the next lower resolution
instead.

We can't simply omit blank tiles because OSD unavoidably logs a console
message for every fetch failure.  Using the bitmap also saves a round
trip.

Retile the demo site.

Signed-off-by: Benjamin Gilbert <bgilbert@cs.cmu.edu>
---
 demo/_synctiles.py | 48 ++++++++++++++++++++++++++++++++++++++++++----
 demo/index.html    | 44 ++++++++++++++++++++++++++++++++++++++++--
 2 files changed, 86 insertions(+), 6 deletions(-)

diff --git a/demo/_synctiles.py b/demo/_synctiles.py
index bc19903..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
@@ -21,6 +21,7 @@
 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
@@ -58,7 +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'
@@ -179,6 +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):
@@ -276,6 +283,30 @@ def upload_metadata(
         )
 
 
+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
@@ -285,9 +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,
@@ -358,13 +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()
@@ -391,6 +430,7 @@ def progress() -> None:
         'name': associated,
         'mpp': mpp,
         'source': source,
+        'sparse': sparse_map.save(),
     }
 
 
diff --git a/demo/index.html b/demo/index.html
index b9f492b..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
@@ -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,