static double[] bwImage_averageBrightnessPerRow(BWImage img) {
  int w = img.getWidth(), h = img.getHeight();
  double[] l = new[h];
  for y to h: {
    double sum = 0;
    for x to w: sum += img.getFloatPixel(x, y);
    l[y] = sum/w;
  }
  ret l;
}