static double[] bwImage_standardDeviationPerRow(BWImage img) {
  int w = img.getWidth(), h = img.getHeight();
  double[] l = new[h];
  double[] row = new[w];
  for y to h: {
    for x to w: row[x] = img.getFloatPixel(x, y);
    l[y] = standardDeviation(row);
  }
  ret l;
}