"Convert to Black & White" function - it is far to aggressive, smallest shadow is converted to all black

[AhabLives]

I am using the flutter package document_scanner_flutter 0.2.5. I would like to edit the "Convert to Black & White" function - it is far to aggressive, any shadow on document is converted to all black. I can not seem to find this exact function with in the package. I would like to update the "B & W" function with code below (But I can not seem to find it).
Any help you could provide in decreasing the exposure or aggressive shadow conversion in B & W - would be greatly appreciated.

 static Future<Uint8List?> toBlackAndWhite(Uint8List picture) async {
    final appDir = await getTemporaryDirectory();
    File pictureFile = File('${appDir.path}/image_open.jpg');
    await pictureFile.writeAsBytes(picture);

    var res = await Cv2.gaussianBlur(
      pathFrom: CVPathFrom.GALLERY_CAMERA,
      pathString: pictureFile.path,
      kernelSize: [3, 3],
      sigmaX: 0,
    );

    if (res == null) return null;
    await pictureFile.writeAsBytes(res);

    res = await Cv2.adaptiveThreshold(
      pathFrom: CVPathFrom.GALLERY_CAMERA,
      pathString: pictureFile.path,
      maxValue: 255,
      adaptiveMethod: Cv2.ADAPTIVE_THRESH_GAUSSIAN_C,
      thresholdType: Cv2.THRESH_BINARY,
      blockSize: 7,
      constantValue: 2,
    );

    if (res == null) return null;
    await pictureFile.writeAsBytes(res);

    res = await Cv2.medianBlur(
      pathFrom: CVPathFrom.GALLERY_CAMERA,
      pathString: pictureFile.path,
      kernelSize: 3,
    );

    return res;
  }