// threshold = 0 to 1
static L<Float> audio_changePoints1(BWImage img, float threshold) {
  int w = img.getWidth();
  float[] f = bwImageColumnBrightnesses(img);
  
  new L<Float> points;
  float value = f[0];
  for (int i = 1; i < l(f); i++)
    if (abs(f[i]-value) >= threshold) {
      points.add(i-0.5f);
      value = f[i];
    }
  ret points;
}