/// <summary>
/// 反色处理
/// </summary>
private Bitmap Inverse(Bitmap bmp)
{
    BitmapData srcdat = bmp.LockBits(new Rectangle(Point.Empty, bmp.Size), ImageLockMode.ReadWrite, PixelFormat.Format24bppRgb); // 锁定位图
    unsafe // 不安全代码
    {
        byte* pix = (byte*)srcdat.Scan0; // 像素首地址
        for (int i = 0; i < srcdat.Stride * srcdat.Height; i++)
        {
            pix[i] = (byte)(255 - pix[i]);
        }
        bmp.UnlockBits(srcdat); // 解锁
        return bmp;
    }
}