static float[] anal_singlePixelDents(float[] d) {
  int n = l(d);
  float[] x = new float[n];
  for (int i = 1; i < n-1; i++) {
    float a = d[i-1], b = d[i], c = d[i+1];
    if (a > b && c > b)
      x[i] = a+c-2*b;
  }
  ret x;
}