[mob] Wrote util methods to generate face thumbnails from an image path

Need to decide on which util method to use of the two after performance testing

mobile/lib/utils/face/face_util.dart

@@ -0,0 +1,101 @@
+import "dart:typed_data";
+
+import "package:computer/computer.dart";
+import "package:flutter_image_compress/flutter_image_compress.dart";
+import "package:image/image.dart" as img;
+import "package:logging/logging.dart";
+import "package:photos/face/model/box.dart";
+
+final _logger = Logger("FaceUtil");
+final _computer = Computer.shared();
+
+///Convert img.Image to ui.Image and use RawImage to display.
+Future<List<img.Image>> generateImgFaceThumbnails(
+  String imagePath,
+  List<FaceBox> faceBoxes,
+) async {
+  final faceThumbnails = <img.Image>[];
+
+  img.Image? image =
+      await _computer.compute(_decodeImageFile, param: {"filePath": imagePath});
+
+  if (image == null) {
+    _logger.info(
+      "Failed to decode image. Compressing to jpg and decoding",
+    );
+    final compressedJPGImage =
+        await FlutterImageCompress.compressWithFile(imagePath);
+    image = await _computer.compute(
+      _decodeJpg,
+      param: {"image": compressedJPGImage},
+    );
+
+    if (image == null) {
+      throw Exception("Failed to decode image");
+    }
+  }
+
+  for (FaceBox faceBox in faceBoxes) {
+    final croppedImage = cropFaceBoxFromImage(image, faceBox);
+    faceThumbnails.add(croppedImage);
+  }
+
+  return faceThumbnails;
+}
+
+Future<List<Uint8List>> generateJpgFaceThumbnails(
+  String imagePath,
+  List<FaceBox> faceBoxes,
+) async {
+  img.Image? image =
+      await _computer.compute(_decodeImageFile, param: {"filePath": imagePath});
+
+  if (image == null) {
+    _logger.info(
+      "Failed to decode image. Compressing to jpg and decoding",
+    );
+    final compressedJPGImage =
+        await FlutterImageCompress.compressWithFile(imagePath);
+    image = await _computer.compute(
+      _decodeJpg,
+      param: {"image": compressedJPGImage},
+    );
+
+    if (image == null) {
+      throw Exception("Failed to decode image");
+    }
+  }
+  final croppedImages = <img.Image>[];
+  for (FaceBox faceBox in faceBoxes) {
+    final croppedImage = cropFaceBoxFromImage(image, faceBox);
+    croppedImages.add(croppedImage);
+  }
+
+  return await _computer
+      .compute(_encodeImagesToJpg, param: {"images": croppedImages});
+}
+
+/// Returns an Image from 'package:image/image.dart'
+img.Image cropFaceBoxFromImage(img.Image image, FaceBox faceBox) {
+  return img.copyCrop(
+    image,
+    x: (image.width * faceBox.xMin).round(),
+    y: (image.height * faceBox.yMin).round(),
+    width: (image.width * faceBox.width).round(),
+    height: (image.height * faceBox.height).round(),
+    antialias: false,
+  );
+}
+
+List<Uint8List> _encodeImagesToJpg(Map args) {
+  final images = args["images"] as List<img.Image>;
+  return images.map((img.Image image) => img.encodeJpg(image)).toList();
+}
+
+Future<img.Image?> _decodeImageFile(Map args) async {
+  return await img.decodeImageFile(args["filePath"]);
+}
+
+img.Image? _decodeJpg(Map args) {
+  return img.decodeJpg(args["image"])!;
+}