windows程序设计关于BITMAP的一些小疑问

HBITMAP StretchBitmap (HBITMAP hBitmap1)

{

     BITMAP     bm1, bm2 ;

     HBITMAP    hBitmap2 ;

     HDC        hdc, hdcMem1, hdcMem2 ;

     int        cxChar, cyChar ;



          // Get the width and height of a system font character



     cxChar = LOWORD (GetDialogBaseUnits ()) ;

     cyChar = HIWORD (GetDialogBaseUnits ()) ;



          // Create 2 memory DCs compatible with the display

     

     hdc = CreateIC (TEXT ("DISPLAY"), NULL, NULL, NULL) ;

     hdcMem1 = CreateCompatibleDC (hdc) ;

     hdcMem2 = CreateCompatibleDC (hdc) ;

     DeleteDC (hdc) ;



          // Get the dimensions of the bitmap to be stretched

     

     GetObject (hBitmap1, sizeof (BITMAP), (PTSTR) &bm1) ;



          // Scale these dimensions based on the system font size

     

     bm2 = bm1 ;

     bm2.bmWidth      = (cxChar * bm2.bmWidth)  / 4 ;

     bm2.bmHeight     = (cyChar * bm2.bmHeight) / 8 ;

     bm2.bmWidthBytes = ((bm2.bmWidth + 15) / 16) * 2 ;



          // Create a new bitmap of larger size

     

     hBitmap2 = CreateBitmapIndirect (&bm2) ;



          // Select the bitmaps in the memory DCs and do a StretchBlt

     

     SelectObject (hdcMem1, hBitmap1) ;

     SelectObject (hdcMem2, hBitmap2) ;

     

     StretchBlt (hdcMem2, 0, 0, bm2.bmWidth, bm2.bmHeight,

                 hdcMem1, 0, 0, bm1.bmWidth, bm1.bmHeight, SRCCOPY) ;



          // Clean up

     

     DeleteDC (hdcMem1) ;

     DeleteDC (hdcMem2) ;

     DeleteObject (hBitmap1) ;

     

     return hBitmap2 ;

}