code: static double luminance( double color ) {
          long argb = Double.doubleToRawLongBits( color );
          double r = ((argb >> 32) & 0xFFFF) / (double)0xFF00;
          double g = ((argb >> 16) & 0xFFFF) / (double)0xFF00;
          double b = ((argb) & 0xFFFF) / (double)0xFF00;
          return (0.2126 * r) + (0.7152 * g) + (0.0722 * b);
      }
docs: The less function "luminance".
  
  @param color
  a color value as long
  @return a value in the range from 0.0 to 1.0