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procedure TForm1.ScreenCapture(Bmp: TBitmap; Oben, Links, Hoehe, Breite: Integer);
var
  DeskWnd: HWnd;
  DeskDC: HDC;
  DeskCv: TCanvas;
  R: TRect;
  W, H: Integer;
begin
  if Bmp = nil then Exit;
  DeskWnd := GetDesktopWindow;
  DeskDC := GetWindowDC(DeskWnd);
  DeskCv := TCanvas.Create;
  DeskCv.Handle := DeskDC;
  W := Breite;
  H := Hoehe;
  R := Bounds(Links, Oben, W, H);
  try
    Bmp.HandleType := bmDIB;
    Bmp.PixelFormat := pf24Bit;
    Bmp.Width := W;
    Bmp.Height := H;
    Bmp.Canvas.CopyMode := cmSrcCopy;
    Bmp.Canvas.CopyRect(R, DeskCv, R);
  finally
    DeskCv.Free;
    ReleaseDC(DeskWnd, DeskDC);
  end;
end;                    

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ScreenCapture(Bmp, StrToInt(edtX.Text), StrToInt(edtY.Text), StrToInt(edtHeight.Text), StrToInt(edtWidth.Text));
  Bmp2Jpg(BMP, 100, sFileName);                    

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procedure MakeScreenShot(x,y,h,b:integer;var result:TBitmap);
var
  DesktopDC: HDC;
begin
  DesktopDC := GetDC(0);
  try
    result.width:=b;
    result.height:=h;
  
    BitBlt(Result.Canvas.Handle, 0, 0, b, h, DesktopDC, x, y, SRCCOPY);
  finally
    DeleteDC(DesktopDC);
  end;
end;                    

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procedure TForm1.Button3Click(Sender: TObject);
var
  DesktopDC: HDC;
  b:TBitmap;
begin
  if not SD.Execute then exit;
  DesktopDC := GetDC(0);
  b:=TBitmap.Create;
  try
    b.width:=100;
    b.height:=100;

    BitBlt(b.Canvas.Handle,0, 0, 100, 100, DesktopDC, 10, 10, SRCCOPY);
  finally
    DeleteDC(DesktopDC);
  end;
  b.SaveToFile(SD.FileName+'.bmp');
  b.Free;
end;