Added crude support for KHR_texture_basisu

lib/src/data_access/image_decoder.dart

@@ -100,6 +100,9 @@ class ImageInfo {
           case ImageCodec.WebP:
             decoder = WebPInfoDecoder(subscription, completer);
             break;
+          case ImageCodec.KTX2:
+            decoder = KTX2InfoDecoder(subscription, completer);
+            break;
           default:
             subscription.cancel();
             completer.completeError(const UnsupportedImageFormatException());
@@ -743,6 +746,59 @@ class WebPInfoDecoder extends ImageInfoDecoder {
   }
 }
 
+class KTX2InfoDecoder extends ImageInfoDecoder {
+  @override
+  String get mimeType => 'image/ktx2';
+
+  // Accumulate 48 bytes of KTX2 header
+  final Uint8List _buffer = Uint8List(48);
+  int _bufferIndex = 0;
+
+  KTX2InfoDecoder(StreamSubscription<List<int>> subscription,
+      Completer<ImageInfo> completer)
+      : super(subscription, completer);
+
+  @override
+  void add(List<int> bytes) {
+    final availableDataLength =
+        min(bytes.length, _buffer.length - _bufferIndex);
+    _buffer.setRange(_bufferIndex, _bufferIndex += availableDataLength, bytes);
+
+    if (_bufferIndex < 48) {
+      return;
+    }
+
+    subscription.cancel();
+
+    final byteData = _buffer.buffer.asByteData();
+
+    // Verify file identifier
+    if (byteData.getUint32(0, Endian.big) != 0xAB4B5458 ||
+        byteData.getUint32(4, Endian.big) != 0x203230BB ||
+        byteData.getUint32(8, Endian.big) != 0x0D0A1A0A) {
+      _abort(const InvalidDataFormatException('Invalid KTX2 file identifier.'));
+      return;
+    }
+
+    final vkFormat = byteData.getUint32(12, Endian.little);
+    final typeSize = byteData.getUint32(16, Endian.little);
+    final width = byteData.getUint32(20, Endian.little);
+    final height = byteData.getUint32(24, Endian.little);
+    final depth = byteData.getUint32(28, Endian.little);
+    final layer = byteData.getUint32(32, Endian.little);
+    final face = byteData.getUint32(36, Endian.little);
+    final level = byteData.getUint32(40, Endian.little);
+    final compression = byteData.getUint32(44, Endian.little);
+
+    // TODO: Proper conversion from vkFormat to format
+    var format = Format.Unknown;
+
+    completer.complete(ImageInfo._(mimeType, 8, format, width, height,
+        colorTransfer: _ColorTransfer.sRGB,
+        colorPrimaries: _ColorPrimaries.sRGB));
+  }
+}
+
 class UnsupportedImageFormatException implements Exception {
   const UnsupportedImageFormatException();
 }

lib/src/ext/KHR_texture_basisu/khr_texture_basisu.dart

@@ -0,0 +1,85 @@
+/*
+ * # Copyright (c) 2016-2019 The Khronos Group Inc.
+ * #
+ * # Licensed under the Apache License, Version 2.0 (the "License");
+ * # you may not use this file except in compliance with the License.
+ * # You may obtain a copy of the License at
+ * #
+ * #     http://www.apache.org/licenses/LICENSE-2.0
+ * #
+ * # Unless required by applicable law or agreed to in writing, software
+ * # distributed under the License is distributed on an "AS IS" BASIS,
+ * # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * # See the License for the specific language governing permissions and
+ * # limitations under the License.
+ */
+
+library gltf.extensions.khr_texture_basisu;
+
+import 'package:gltf/src/base/gltf_property.dart';
+import 'package:gltf/src/ext/extensions.dart';
+
+const String IMAGE_KTX2 = 'image/ktx2';
+
+const Extension khrTextureBasisUExtension = Extension(
+    'KHR_texture_basisu',
+    <Type, ExtensionDescriptor>{
+      Texture: ExtensionDescriptor(KhrTextureBasisUTexture.fromMap)
+    },
+    init: _init);
+
+void _init(Context context) {
+  context.imageMimeTypes.add(IMAGE_KTX2);
+}
+
+const List<String> KHR_TEXTURE_BASISU_TEXTURE_MEMBERS = <String>[SOURCE];
+
+class KhrTextureBasisUTexture extends GltfProperty
+    implements ResourceValidatable {
+  final int _sourceIndex;
+
+  Image _source;
+
+  Image get source => _source;
+
+  KhrTextureBasisUTexture._(
+      this._sourceIndex, Map<String, Object> extensions, Object extras)
+      : super(extensions, extras);
+
+  static KhrTextureBasisUTexture fromMap(
+      Map<String, Object> map, Context context) {
+    if (context.validate) {
+      checkMembers(map, KHR_TEXTURE_BASISU_TEXTURE_MEMBERS, context);
+    }
+
+    return KhrTextureBasisUTexture._(
+        getIndex(map, SOURCE, context, req: false),
+        getExtensions(map, KhrTextureBasisUTexture, context),
+        getExtras(map, context));
+  }
+
+  @override
+  void link(Gltf gltf, Context context) {
+    _source = gltf.images[_sourceIndex];
+    if (context.validate && _sourceIndex != -1) {
+      if (_source == null) {
+        context.addIssue(LinkError.unresolvedReference,
+            name: SOURCE, args: [_sourceIndex]);
+      } else {
+        _source.markAsUsed();
+      }
+    }
+  }
+
+  @override
+  void validateResources(Gltf gltf, Context context) {
+    final mimeType = _source?.mimeType ?? _source?.info?.mimeType;
+    if (mimeType != null && mimeType != IMAGE_KTX2) {
+      context
+          .addIssue(LinkError.textureInvalidImageMimeType, name: SOURCE, args: [
+        mimeType,
+        const [IMAGE_KTX2]
+      ]);
+    }
+  }
+}

lib/src/ext/extensions.dart

@@ -34,6 +34,7 @@ import 'package:gltf/src/ext/KHR_materials_unlit/khr_materials_unlit.dart';
 import 'package:gltf/src/ext/KHR_materials_variants/KHR_materials_variants.dart';
 import 'package:gltf/src/ext/KHR_materials_volume/khr_materials_volume.dart';
 import 'package:gltf/src/ext/KHR_mesh_quantization/khr_mesh_quantization.dart';
+import 'package:gltf/src/ext/KHR_texture_basisu/khr_texture_basisu.dart';
 import 'package:gltf/src/ext/KHR_texture_transform/khr_texture_transform.dart';
 import 'package:gltf/src/ext/VRMC_materials_mtoon/vrmc_materials_mtoon.dart';
 import 'package:gltf/src/ext/VRMC_node_constraint/vrmc_node_constraint.dart';
@@ -129,6 +130,7 @@ const List<Extension> kDefaultExtensions = <Extension>[
   khrMaterialsVariantsExtension,
   khrMaterialsVolumeExtension,
   khrMeshQuantizationExtension,
+  khrTextureBasisUExtension,
   khrTextureTransformExtension,
   vrmcMaterialMtoon,
   vrmcNodeConstraint,