private int computeColorDepth(int colorcount)
  {
    // color depth = log-base-2 of maximum number of simultaneous colors, i.e.
    // bits per color-index pixel    
    if (colorcount <= 2)
      return 1;      
    if (colorcount <= 4)
      return 2;      
    if (colorcount <= 16)
      return 4;
    return 8;    
  }