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  private Bitmap ConvertBmp8(Bitmap bmp)

  {

  var bmp8 = new Bitmap(bmp.Width, bmp.Height, System.Drawing.Imaging.PixelFormat.Format8bppIndexed);

 

  //GetDIBitsで8bitビットマップ取得

  bi.bmiHeader.biBitCount = 8;

  bi.bmiHeader.biCompression = BitmapCompressionMode.BI_RGB;

  var hdc = CreateCompatibleDC(IntPtr.Zero);

  var hBmpOld = SelectObject(hdc, hBmp);

  var bmData = bmp8.LockBits(new Rectangle(0, 0, bmp.Width, bmp.Height), System.Drawing.Imaging.ImageLockMode.ReadOnly, System.Drawing.Imaging.PixelFormat.Format8bppIndexed);

  GetDIBits(hdc, hBmp, 0, bmp.Height, bmData.Scan0, ref bi, DibColorsUsage.RGB);