怎么得到图像的内容？

	CDC dc;

	CDC tdc;

	CBitmap bm;

	CBitmap *pOld;



	dc.CreateDC("DISPLAY", NULL, NULL, NULL);

	int Width = GetSystemMetrics(SM_CXSCREEN);

	int Height = GetSystemMetrics(SM_CYSCREEN);

	bm.CreateCompatibleBitmap(&dc, Width, Height);

	tdc.CreateCompatibleDC(&dc);

	pOld = tdc.SelectObject(&bm);

	

	//复制图像

	tdc.BitBlt(0, 0, Width, Height, &dc, 0, 0, SRCCOPY);

	tdc.SelectObject(pOld);

	BITMAP btm;

	bm.GetBitmap(&btm);

	

	DWORD size = btm.bmWidthBytes * btm.bmHeight;

	LPSTR lpData = (LPSTR)GlobalAllocPtr(GPTR, size);

	

	CString st;

	st.Format("%d, %d",size, sizeof(lpData));

	MessageBox(st);//输出“5184000, 4”

	BITMAPINFOHEADER bih;

	bih.biBitCount = btm.bmBitsPixel;

	bih.biClrImportant = 0;

	bih.biClrUsed = 0;

	bih.biCompression = 0;

	bih.biHeight = btm.bmHeight;

	bih.biPlanes = 1;

	bih.biSize = sizeof(BITMAPINFOHEADER);

	bih.biSizeImage = size;

	bih.biWidth = btm.bmWidth;

	bih.biXPelsPerMeter = 0;

	bih.biYPelsPerMeter = 0;

	GetDIBits(dc, bm, 0, bih.biHeight, lpData, (BITMAPINFO*)&bih, DIB_RGB_COLORS);

	

	st.Format("%d, %d",size, sizeof(lpData));

	MessageBox(st);//输出“5184000, 4”



	CString stout;

	stout = lpData;

	st.Format("%d",stout.GetLength());

	MessageBox(st);//输出“3”



	BITMAPFILEHEADER bfh;

	bfh.bfReserved1 = bfh.bfReserved2 = 0;

	bfh.bfType = ((WORD)('M'<< 8)|'B');

	bfh.bfSize = 54+size;

	bfh.bfOffBits = 54;



	CFile bf;

	if(bf.Open("pict.bmp", CFile::modeCreate|CFile::modeWrite))

	{

		bf.WriteHuge(&bfh, sizeof(BITMAPFILEHEADER));

		bf.WriteHuge(&bih, sizeof(BITMAPINFOHEADER));

		bf.WriteHuge(lpData, size);

		bf.Close();

		nCount++;

	}

	GlobalFreePtr(lpData);