用API处理位图

procedure TForm1.Button1Click(Sender: TObject);
var
  dc : hdc;
  MemDc : hdc;
  MemBitmap : hBitmap;
  OldMemBitmap : hBitmap;
begin

{Get the handle to the screen's dc}
  dc := GetDc();

{Create and retrieve a handle to a memory dc based on the screen}
  MemDc := CreateCompatibleDc(dc);

{Create a bitmap that is compatible with the display.}
{Note: if you pass "MemDc" to CreateCompatibleBitmap()}
{instead of "dc", you will get a monochrome bitmap!}
  MemBitmap := CreateCompatibleBitmap(dc, , );

{Release the screen dc}
  ReleaseDc(, dc);

{Select the bitmap surface into the MemDc}
{remembering the default bitmap}
  OldMemBitmap := SelectObject(MemDc, MemBitmap);

{Draw on the MemoryDc}
  PatBlt(MemDc, , , , , WHITENESS);
  Ellipse(MemDc, , , , );

{Copy the MemDc to the Form Canvas}
  BitBlt(Form1.Canvas.Handle,
         , ,
         , ,
         MemDc,
         , ,
         SRCCOPY);

{Select the default bitmap back into the memory dc}
  SelectObject(MemDc, OldMemBitmap);

{Note: You can now use the memory bitmap handle with}
{functions such as GetDiBits()}

{Delete the Memory Bitmap}
  DeleteObject(MemBitmap);

{Delete the MemoryDc}
  DeleteDc(MemDc);

end;