static float[] bwImageColumnBrightnesses_generic(virtual BWImage img) {
  int w = rcall_int getWidth(img), h = rcall_int getHeight(img);
  float[] f = new float[w];
  for x to w:
    f[x] = toFloat(rcall averageBrightness(rcall clip(img, x, 0, 1, h)));
  ret f;
}