Bitmap的加载（从屏幕DC到内存DC)

HDC hScrDC,hMemDC;//screen and mem dc
 HBITMAP hBitmap, hOldBitmap;//handle of bitmap
 int xScrn, yScrn;//the resolution of screen
 int nX, nY, nX2, nY2;              
   int nWidth, nHeight;
 nX = rect.left;  
 nY = rect.top;  
 nX2 = rect.right;  
 nY2 = rect.bottom;
 hScrDC = CreateDC("DISPLAY", NULL, NULL, NULL);//create dc for screen
 hMemDC = CreateCompatibleDC(hScrDC);//create compatible mem dc for screen dc
 xScrn = GetDeviceCaps(hScrDC, HORZRES);//get resolution of screen
 yScrn = GetDeviceCaps(hScrDC, VERTRES);
 if (nX < 0)  
  nX = 0;  
 if (nY < 0)  
  nY = 0;  
 if (nX2 > xScrn)  
  nX2 = xScrn;  
 if (nY2 > yScrn)  
  nY2 = yScrn;
 nWidth = nX2 - nX;  
 nHeight = nY2 - nY;
 //hBitmap = CreateCompatibleBitmap(hMemDC, nWidth, nHeight);
 hBitmap = CreateCompatibleBitmap(hScrDC, nWidth, nHeight);//create compatible bitmap for screen dc
 hOldBitmap = (HBITMAP)SelectObject(hMemDC, hBitmap);//select bitmap into the mem dc
 BitBlt(hMemDC, 0, 0, nWidth, nHeight,hScrDC,nX,nY, SRCCOPY);//copy screen dc to mem
 hBitmap =(HBITMAP)SelectObject(hMemDC, hOldBitmap);//get the handle of screen bitmap
 DeleteDC(hScrDC);//clear
 DeleteDC(hMemDC);
 DeleteObject(hOldBitmap);
 return hBitmap;//return the handle of bitmap