fix(image): preserve transparency when compressing sent images (#1940)

Author: YeungKC

lib/account/send_message_helper.dart

@@ -1393,7 +1393,7 @@ class SendMessageHelper {
       conversationId,
       messageId,
       null,
-      mimeType: defaultMimeType,
+      mimeType: type.mimeType,
     );
     attachment = await attachment.create(recursive: true);
     await attachment.writeAsBytes(data);

lib/utils/image.dart

@@ -7,12 +7,20 @@ import 'load_balancer_utils.dart';
 const _kMaxDimension = 1920;
 const _kQuality = 85;
 
-enum ImageType { gif, jpeg }
+enum ImageType { gif, jpeg, png }
 
 extension ImageTypeExtension on ImageType {
-  String get mimeType => this == ImageType.gif ? 'image/gif' : 'image/jpeg';
+  String get mimeType => switch (this) {
+    ImageType.gif => 'image/gif',
+    ImageType.jpeg => 'image/jpeg',
+    ImageType.png => 'image/png',
+  };
 
-  String get extension => this == ImageType.gif ? 'gif' : 'jpg';
+  String get extension => switch (this) {
+    ImageType.gif => 'gif',
+    ImageType.jpeg => 'jpg',
+    ImageType.png => 'png',
+  };
 }
 
 (Uint8List, ImageType, int, int)? compressImage(Uint8List data) {
@@ -47,6 +55,18 @@ extension ImageTypeExtension on ImageType {
     image = copyResize(image, width: targetWidth, height: targetHeight);
   }
 
+  final maxChannelValue = image.maxChannelValue;
+  final hasTransparency = image.any((pixel) => pixel.a < maxChannelValue);
+
+  if (hasTransparency) {
+    return (
+      Uint8List.fromList(encodePng(image)),
+      ImageType.png,
+      image.width,
+      image.height,
+    );
+  }
+
   return (
     JpegEncoder(quality: _kQuality).encode(image),
     ImageType.jpeg,

macos/Podfile.lock

@@ -196,7 +196,7 @@ SPEC CHECKSUMS:
   file_picker: 7584aae6fa07a041af2b36a2655122d42f578c1a
   file_selector_macos: 9e9e068e90ebee155097d00e89ae91edb2374db7
   flutter_app_icon_badge: e05568a57ac0d98a0e7bc35c5b9ae9b118109b1e
-  flutter_local_notifications: 4bf37a31afde695b56091b4ae3e4d9c7a7e6cda0
+  flutter_local_notifications: 1fc7ffb10a83d6a2eeeeddb152d43f1944b0aad0
   FlutterMacOS: d0db08ddef1a9af05a5ec4b724367152bb0500b1
   irondash_engine_context: 893c7d96d20ce361d7e996f39d360c4c2f9869ba
   local_auth_darwin: c3ee6cce0a8d56be34c8ccb66ba31f7f180aaebb

pubspec.lock

@@ -174,10 +174,10 @@ packages:
     dependency: "direct dev"
     description:
       name: build_runner
-      sha256: "7981eb922842c77033026eb4341d5af651562008cdb116bdfa31fc46516b6462"
+      sha256: "521daf8d189deb79ba474e43a696b41c49fb3987818dbacf3308f1e03673a75e"
       url: "https://pub.dev"
     source: hosted
-    version: "2.12.2"
+    version: "2.13.1"
   built_collection:
     dependency: transitive
     description:
@@ -2358,4 +2358,4 @@ packages:
     version: "3.1.3"
 sdks:
   dart: ">=3.11.0 <4.0.0"
-  flutter: ">=3.38.0"
+  flutter: ">=3.38.1 <4.0.0"

test/utils/image_test.dart

@@ -0,0 +1,26 @@
+import 'dart:typed_data';
+
+import 'package:flutter_app/utils/image.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:image/image.dart' as img;
+
+void main() {
+  test('compressImage preserves transparency for alpha images', () {
+    final source = img.Image(width: 2, height: 1, numChannels: 4)
+      ..setPixelRgba(0, 0, 0, 0, 0, 0)
+      ..setPixelRgba(1, 0, 255, 255, 255, 255);
+
+    final encoded = Uint8List.fromList(img.encodePng(source));
+    final result = compressImage(encoded);
+
+    expect(result, isNotNull);
+
+    final (bytes, type, _, _) = result!;
+    final decoded = img.decodeImage(bytes);
+
+    expect(type.mimeType, 'image/png');
+    expect(decoded, isNotNull);
+    expect(decoded!.getPixel(0, 0).a.toInt(), 0);
+    expect(decoded.getPixel(1, 0).a.toInt(), 255);
+  });
+}