根据RGB，计算灰度值

算法来源

http://hi.baidu.com/dplet/blog/item/9d331f6596be28faf63654bf.html

http://www.cnblogs.com/yuanbao/archive/2007/09/25/905322.html

/// <summary>
/// 根据RGB，计算灰度值
/// </summary>
/// <param name="posClr">Color值</param>
/// <returns>灰度值，整型</returns>
private int GetGrayNumColor(System.Drawing.Color posClr)
{
    int i = (int)(0.299 * posClr.R + 0.587 * posClr.G + 0.114 * posClr.B);
    int i2=(posClr.R * 19595 + posClr.G * 38469 + posClr.B * 7472) >> 16;
    return i2;
}