誤差

やられた（苦笑

using System;

public enum SupportedColors
{
    Mono        = 1,
    Bpp16       = 4,
    Bpp256      = 8,
    Bpp4096     = 12,
    Bpp32768    = 15,
    Bpp65536    = 16,
}

public class Program
{
    static void Main()
    {
        foreach (int colors in 
            new int[] { 2, 16, 256, 4096, 32768, 65536 })
        {
            SupportedColors col = ToSupportedColors(colors);
            
            Console.WriteLine("{0} is {1} (depth {2})",
                    colors, col, (int) col);
        }
    }
    
    static SupportedColors ToSupportedColors(int colors)
    {
        return (SupportedColors) Math.Log(colors, 2);
        // return (SupportedColors) (int) Math.Log(colors, 2);
        // return (SupportedColors) Convert.ToInt32(Math.Log(colors, 2));
    }
}

/* Output:
  2 is Mono (depth 1)
  16 is Bpp16 (depth 4)
  256 is Bpp256 (depth 8)
  4096 is 11 (depth 11)
  32768 is Bpp32768 (depth 15)
  65536 is Bpp65536 (depth 16)
 */