截屏問題！急死了。

截屏問題！急死了。 Delphi / Windows SDK/API
http://www.delphi2007.net/DelphiMultimedia/html/delphi_20061106092726174.html
我想截屏時只截取指定地區，而不總是全屏，不知道該怎麼做。  
  高手幫幫忙。。

var  
  ghdc:   hdc;  
  gtop,gleft,gwidth,gheight:   integer;//截屏的範圍  
  begin  
    ghdc:=GetDC(0);  
                      //設定截屏的範圍  
    gtop:=10;  
    gleft:=10;  
    gwidth:=   100;  
    gheight:=100;  
    BitBlt(Image1.Canvas.Handle,0,0,100,100,ghdc,10,10,SRCCOPY);  
  end;

gtop,gleft,gwidth,gheight這四個參數怎麼在BitBlt裡沒有用到？  
  BitBlt這個函數怎麼使用？  
 

he   BitBlt   function   performs   a   bit-block   transfer   of   the   color   data   corresponding   to   a   rectangle   of   pixels   from   the   specified   source   device   context   into   a   destination   device   context.    
   
  BOOL   BitBlt(  
   
          HDC   hdcDest,//   handle   to   destination   device   context    
          int   nXDest,//   x-coordinate   of   destination   rectangle's   upper-left   corner  
          int   nYDest,//   y-coordinate   of   destination   rectangle's   upper-left   corner  
          int   nWidth,//   width   of   destination   rectangle    
          int   nHeight,//   height   of   destination   rectangle    
          HDC   hdcSrc,//   handle   to   source   device   context    
          int   nXSrc,//   x-coordinate   of   source   rectangle's   upper-left   corner      
          int   nYSrc,//   y-coordinate   of   source   rectangle's   upper-left   corner  
          DWORD   dwRop   //   raster   operation   code    
        );  
     
   
  Parameters  
   
  hdcDest  
   
  Identifies   the   destination   device   context.    
   
  nXDest  
   
  Specifies   the   logical   x-coordinate   of   the   upper-left   corner   of   the   destination   rectangle.  
   
  nYDest  
   
  Specifies   the   logical   y-coordinate   of   the   upper-left   corner   of   the   destination   rectangle.    
   
  nWidth  
   
  Specifies   the   logical   width   of   the   source   and   destination   rectangles.    
   
  nHeight  
   
  Specifies   the   logical   height   of   the   source   and   the   destination   rectangles.    
   
  hdcSrc  
   
  Identifies   the   source   device   context.    
   
  nXSrc  
   
  Specifies   the   logical   x-coordinate   of   the   upper-left   corner   of   the   source   rectangle.    
   
  nYSrc  
   
  Specifies   the   logical   y-coordinate   of   the   upper-left   corner   of   the   source   rectangle.    
   
  dwRop  
   
  Specifies   a   raster-operation   code.   These   codes   define   how   the   color   data   for   the   source   rectangle   is   to   be   combined   with   the   color   data   for   the   destination   rectangle   to   achieve   the   final   color.    
  The   following   list   shows   some   common   raster   operation   codes:    
   
  ValueDescription  
  BLACKNESSFills   the   destination   rectangle   using   the   color   associated   with   index   0   in   the   physical   palette.   (This   color   is   black   for   the   default   physical   palette.)  
  DSTINVERTInverts   the   destination   rectangle.  
  MERGECOPYMerges   the   colors   of   the   source   rectangle   with   the   specified   pattern   by   using   the   Boolean   AND   operator.  
  MERGEPAINTMerges   the   colors   of   the   inverted   source   rectangle   with   the   colors   of   the   destination   rectangle   by   using   the   Boolean   OR   operator.  
  NOTSRCCOPYCopies   the   inverted   source   rectangle   to   the   destination.  
  NOTSRCERASECombines   the   colors   of   the   source   and   destination   rectangles   by   using   the   Boolean   OR   operator   and   then   inverts   the   resultant   color.  
  PATCOPYCopies   the   specified   pattern   into   the   destination   bitmap.  
  PATINVERTCombines   the   colors   of   the   specified   pattern   with   the   colors   of   the   destination   rectangle   by   using   the   Boolean   XOR   operator.  
  PATPAINTCombines   the   colors   of   the   pattern   with   the   colors   of   the   inverted   source   rectangle   by   using   the   Boolean   OR   operator.   The   result   of   this   operation   is   combined   with   the   colors   of   the   destination   rectangle   by   using   the   Boolean   OR   operator.  
  SRCANDCombines   the   colors   of   the   source   and   destination   rectangles   by   using   the   Boolean   AND   operator.  
  SRCCOPYCopies   the   source   rectangle   directly   to   the   destination   rectangle.  
  SRCERASECombines   the   inverted   colors   of   the   destination   rectangle   with   the   colors   of   the   source   rectangle   by   using   the   Boolean   AND   operator.  
  SRCINVERTCombines   the   colors   of   the   source   and   destination   rectangles   by   using   the   Boolean   XOR   operator.  
  SRCPAINTCombines   the   colors   of   the   source   and   destination   rectangles   by   using   the   Boolean   OR   operator.  
  WHITENESSFills   the   destination   rectangle   using   the   color   associated   with   index   1   in   the   physical   palette.   (This   color   is   white   for   the   default   physical   palette.)  
     
   
  Return   Values  
   
  If   the   function   succeeds,   the   return   value   is   nonzero.  
  If   the   function   fails,   the   return   value   is   zero.   To   get   extended   error   information,   call   GetLastError.    
   
  Remarks  
   
  If   a   rotation   or   shear   transformation   is   in   effect   in   the   source   device   context,   BitBlt   returns   an   error.   If   other   transformations   exist   in   the   source   device   context   (and   a   matching   transformation   is   not   in   effect   in   the   destination   device   context),   the   rectangle   in   the   destination   device   context   is   stretched,   compressed,   or   rotated   as   necessary.    
  If   the   color   formats   of   the   source   and   destination   device   contexts   do   not   match,   the   BitBlt   function   converts   the   source   color   format   to   match   the   destination   format.    
   
  When   an   enhanced   metafile   is   being   recorded,   an   error   occurs   if   the   source   device   context   identifies   an   enhanced-metafile   device   context.    
  Not   all   devices   support   the   BitBlt   function.   For   more   information,   see   the   RC_BITBLT   raster   capability   entry   in   GetDeviceCaps  
  .    
  BitBlt   returns   an   error   if   the   source   and   destination   device   contexts   represent   different   devices.

哈哈，忘用了，就是bitblt裡面的100，10那幾個數，我直接寫上了