RangeError when converting Uint8List to ByteBuffer for image processing in Flutter

Question

I'm working on a Flutter app for image segmentation, where users can upload or capture photos, and the app analyzes these photos using a TensorFlow Lite model. However, I've encountered a RangeError when trying to convert pixel data (after model analysis) into an image using the image library.

Here's a brief overview of my workflow:

1. Users upload/capture a photo.
2. The photo is resized to 512x512 pixels.
3. Pixel data from the resized photo is fed into a TensorFlow Lite model, which outputs a tensor indicating class predictions for each pixel.
4. Based on the model's output, I generate a list of colors (List pixelData) where each pixel is colored red or green, depending on the class prediction.
5. I then convert pixelData into a Uint8List and subsequently into a ByteBuffer, aiming to create a segmented image visualizing the model's predictions.

Here's the problematic part of the code:

List<int> pixelData = [];
// Fill pixelData based on model's output
// ...

// Convert List<int> to Uint8List
Uint8List byteData = Uint8List.fromList(pixelData);

// Attempt to create ByteBuffer from byteData
ByteBuffer buffer = byteData.buffer;

// Try to create an image from the ByteBuffer
// Don't be confused about the imgimg -- i imported the library like that
// import 'package:image/image.dart' as imgimg;
imgimg.Image? segmentedImage = imgimg.Image.fromBytes(
  width: imgWidth,
  height: imgHeight,
  bytes: buffer,
);

The error occurs when trying to create segmentedImage:

RangeError (end): Invalid value: Not in inclusive range 261120..262144: 262656

The length of pixelData is exactly 262144, which is correct for a 512x512 image where each pixel is represented by a single integer color value. The error seems to indicate an issue with the conversion process or the way the ByteBuffer is being handled.

Could anyone provide insight into what might be causing this RangeError and how to correctly convert my pixelData into a ByteBuffer for image creation? Is there a better approach to visualizing the TensorFlow model's output as an image in Flutter?

Any help or suggestions would be greatly appreciated!

If needed, here is a brief overview of the remaining function.

Future<void> _onUseImagePressed() async {
  if (_image == null) return;
  
  // Load and resize the image
  imgimg.Image? image = imgimg.decodeImage(File(_image!.path).readAsBytesSync());
  imgimg.Image resizedImage = imgimg.copyResize(image!, width: 512, height: 512);

  // Prepare the model input
  var input = List.generate(1, (_) => List.generate(512, (_) => List.generate(512, (_) => List.generate(3, (_) => 0.0))));
  
  // Populate input with normalized pixel values
  for (int y = 0; y < 512; y++) {
    for (int x = 0; x < 512; x++) {
      var pixel = resizedImage.getPixel(x, y);
      input[0][y][x][0] = pixel.r / 255.0;
      input[0][y][x][1] = pixel.g / 255.0;
      input[0][y][x][2] = pixel.b / 255.0;
    }
  }

  // Run the TensorFlow Lite model
  final interpreter = await Interpreter.fromAsset('assets/models/production_model_resnet50.tflite');
  var output = List.filled(1 * 512 * 512 * 2, 0).reshape([1, 512, 512, 2]);
  interpreter.run(input, output);

  // Generate pixel data based on model output
  List<int> pixelData = [];
  for (int y = 0; y < 512; y++) {
    for (int x = 0; x < 512; x++) {
      int classIndex = output[0][y][x][0] > output[0][y][x][1] ? 0 : 1;
      int color = classIndex == 0 ? 0xFFFF0000 : 0xFF00FF00;
      pixelData.add(color);
    }
  }

  // Convert pixelData to Uint8List and ByteBuffer
  Uint8List byteData = Uint8List.fromList(pixelData);
  ByteBuffer buffer = byteData.buffer;

  // Attempt to create an image from bytes
  try {
    imgimg.Image? segmentedImage = imgimg.Image.fromBytes(512, 512, buffer);
  } catch (e) {
    print("Error processing model output: $e");
  }
}

Answer 1

Thanks to @jamesdlin for pointing out that Image.fromBytes by default assumes 3 color channels. Without specifying the image format and the number of channels, the Image expected a different structure.

I made the following adjustments:

1. Adjusting pixelData Construction to assign either 0 or 255 based on the model output

List<int> pixelData = [];
for (int y = 0; y < imgHeight; y++) {
  for (int x = 0; x < imgWidth; x++) {
    int classIndex = output[0][y][x][0] > output[0][y][x][1] ? 0 : 1;
    int colorValue = classIndex == 0 ? 0 : 255;
    pixelData.add(colorValue);
  }
}

2. Specify format when calling fromBytes -- imgimg.Format.uint8 and numChannels: 1 to align with the updated pixelData structure

segmentedImage = imgimg.Image.fromBytes(
    width: imgWidth,
    height: imgHeight,
    bytes: buffer,
    format: imgimg.Format.uint8,
    numChannels: 1
);