turn greyscale Int16 data into a red-green heatmap

greyscale_heatmap.dart

import 'dart:typed_data';

import 'package:image/image.dart' as img;

const dim = 512;
const maxInt16Value = 32767;
const maxInt8Value = 255;

final Int16List testData = () {
  final buffer = Int16List(dim * dim);
  for (var y = 0; y < dim; y++) {
    for (var x = 0; x < dim; x++) {
      final value = (x + y) * (maxInt16Value / ((dim - 1) * 2));
      buffer[y * dim + x] = value.toInt();
    }
  }
  return buffer;
}();

Future<void> makeHeatmap({int numFilesToUse = 0}) async {
  final normalizedData = Uint8List(testData.length);

  for (var i = 0; i < testData.length; i++) {
    final int16Value = testData[i];
    final normalizedDouble = (int16Value / maxInt16Value) * maxInt8Value;
    normalizedData[i] = normalizedDouble.round().clamp(0, maxInt8Value);
  }

  final imageWithPalette = img.Image(
    width: dim,
    height: dim,
    withPalette: true,
  );

  final palette = imageWithPalette.palette!;

  for (var i = 0; i <= maxInt8Value; i++) {
    final r = i;
    final g = maxInt8Value - i;
    const b = 0;

    palette
      ..set(i, 0, r.clamp(0, maxInt8Value))
      ..set(i, 1, g.clamp(0, maxInt8Value))
      ..set(i, 2, b);
  }

  for (var y = 0; y < dim; y++) {
    for (var x = 0; x < dim; x++) {
      imageWithPalette.setPixelIndex(
        x,
        y,
        normalizedData[y * dim + x],
      );
    }
  }

  final cmd = img.Command()
    ..image(imageWithPalette)
    ..encodePngFile(
      'heatmap.png',
    );
  await cmd.executeThread();
}

Future<void> main(List<String> arguments) async {
  await makeHeatmap();
}