Desktop QR: official camera_windows 0.3.0+1, scan Windows frames in memory

lib/utilities/camera_preview_frame.dart

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+import 'dart:typed_data';
+
+import 'package:image/image.dart' as img;
+
+/// Why a frame captured from a camera preview can't be scanned.
+enum BlankPreviewFrame {
+  /// Nothing was drawn: the preview texture has no camera frame yet, or the
+  /// camera has been closed.
+  transparent,
+
+  /// Every visible pixel is black or close to it, as with a closed privacy
+  /// shutter.
+  black,
+}
+
+// Sensor noise keeps a covered webcam from being exactly 0.
+const _blackLevel = 16;
+
+/// Returns why [rgba] can't be scanned, or null if it has picture content.
+///
+/// [rgba] is tightly packed 8-bit RGBA, as from
+/// `ui.Image.toByteData(format: ui.ImageByteFormat.rawRgba)`.
+BlankPreviewFrame? blankPreviewFrame(Uint8List rgba) {
+  var anyVisible = false;
+  for (var i = 0; i + 3 < rgba.length; i += 4) {
+    if (rgba[i + 3] == 0) continue;
+    anyVisible = true;
+    if (rgba[i] > _blackLevel ||
+        rgba[i + 1] > _blackLevel ||
+        rgba[i + 2] > _blackLevel) {
+      return null;
+    }
+  }
+  return anyVisible ? BlankPreviewFrame.black : BlankPreviewFrame.transparent;
+}
+
+/// Builds an image from an RGBA capture of a horizontally mirrored camera
+/// preview, flipped back so a QR code in it reads the right way round.
+///
+/// [rgba] is tightly packed 8-bit RGBA of [width] x [height] pixels.
+img.Image imageFromMirroredPreviewRgba(
+  Uint8List rgba, {
+  required int width,
+  required int height,
+}) {
+  if (width <= 0 || height <= 0) {
+    throw ArgumentError("Invalid frame size ${width}x$height");
+  }
+  final byteLength = width * height * 4;
+  if (rgba.length < byteLength) {
+    throw ArgumentError.value(
+      rgba.length,
+      "rgba",
+      "Too short for a ${width}x$height RGBA frame",
+    );
+  }
+
+  // sublist copies into a fresh buffer, so the 32-bit view is aligned even
+  // if the caller's bytes start at an odd offset.
+  final src = rgba.sublist(0, byteLength).buffer.asUint32List();
+  final flipped = Uint32List(width * height);
+  for (var y = 0; y < height; y++) {
+    final row = y * width;
+    final last = row + width - 1;
+    for (var x = 0; x < width; x++) {
+      flipped[row + x] = src[last - x];
+    }
+  }
+
+  return img.Image.fromBytes(
+    width: width,
+    height: height,
+    bytes: flipped.buffer,
+    numChannels: 4,
+    order: img.ChannelOrder.rgba,
+  );
+}