How to take a screenshot of the Active Window in Delphi?

Question

For full screenshots, I use this code:

form1.Hide;
sleep(500);
bmp := TBitmap.Create;
bmp.Height := Screen.Height;
bmp.Width := Screen.Width;
DCDesk := GetWindowDC(GetDesktopWindow);
BitBlt(bmp.Canvas.Handle, 0, 0, Screen.Width, Screen.Height, DCDesk, 0, 0, SRCCOPY);
form1.Show ;
FileName := 'Screenshot_'+FormatDateTime('mm-dd-yyyy-hhnnss',now());
bmp.SaveToFile(Format('C:\Screenshots\%s.bmp', [FileName]));
ReleaseDC(GetDesktopWindow, DCDesk);
bmp.Free;

How can I convert that to take a screenshot of only the active window.

Answer 1

This combines all the approaches described so far. It also handles multiple-monitor scenarios.

Pass in the kind of screenshot you want, and a TJpegImage, and it will assign your requested screenshot to that image.

///////////
uses
  Jpeg;

type  //define an ENUM to describe the possible screenshot types.
  TScreenShotType = (sstActiveWindow, sstActiveClientArea,
    sstPrimaryMonitor, sstDesktop);
///////////

procedure TfrmMain.GetScreenShot(shotType: TScreenShotType;
  var img: TJpegImage);
var
  w,h: integer;
  DC: HDC;
  hWin: Cardinal;
  r: TRect;
  tmpBmp: TBitmap;
begin
  hWin := 0;
  case shotType of
    sstActiveWindow:
      begin
        //only the active window
        hWin := GetForegroundWindow;
        dc := GetWindowDC(hWin);
        GetWindowRect(hWin,r);
        w := r.Right - r.Left;
        h := r.Bottom - r.Top;
      end;  //sstActiveWindow
    sstActiveClientArea:
      begin
        //only the active client area (active window minus title bars)
        hWin := GetForegroundWindow;
        dc := GetDC(hWin);
        GetWindowRect(hWin,r);
        w := r.Right - r.Left;
        h := r.Bottom - r.Top;
      end;  //sstActiveClientArea
    sstPrimaryMonitor:
      begin
        //only the primary monitor.  If 1 monitor, same as sstDesktop.
        hWin := GetDesktopWindow;
        dc := GetDC(hWin);
        w := GetDeviceCaps(DC,HORZRES);
        h := GetDeviceCaps(DC,VERTRES);
      end;  //sstPrimaryMonitor
    sstDesktop:
      begin
        //ENTIRE desktop (all monitors)
        dc := GetDC(GetDesktopWindow);
        w := Screen.DesktopWidth;
        h := Screen.DesktopHeight;
      end;  //sstDesktop
    else begin
      Exit;
    end;  //case else
  end;  //case

  //convert to jpg
  tmpBmp := TBitmap.Create;
  try
    tmpBmp.Width := w;
    tmpBmp.Height := h;
    BitBlt(tmpBmp.Canvas.Handle,0,0,tmpBmp.Width,
      tmpBmp.Height,DC,0,0,SRCCOPY);
    img.Assign(tmpBmp);
  finally
    ReleaseDC(hWin,DC);
    FreeAndNil(tmpBmp);
  end;  //try-finally
end;

Answer 2

1. First of all you have to get the right window. As sharptooth already noted you should use GetForegroundWindow instead of GetDesktopWindow. You have done it right in your improved version.
2. But then you have to resize your bitmap to the actual size of the DC/Window. You haven't done this yet.
3. And then make sure you don't capture some fullscreen window!

When I executed your code, my Delphi IDE was captured and as it is on fullscreen by default, it created the illusion of a fullscreen screenshot. (Even though your code is mostly correct)

Considering the above steps, I was successfully able to create a single-window screenshot with your code.

Just a hint: You can GetDC instead of GetWindowDC if you are only interested in the client area. (No window borders)

EDIT: Here's what I made with your code:

You should not use this code! Look at the improved version below.

procedure TForm1.Button1Click(Sender: TObject);
const
  FullWindow = True; // Set to false if you only want the client area.
var
  hWin: HWND;
  dc: HDC;
  bmp: TBitmap;
  FileName: string;
  r: TRect;
  w: Integer;
  h: Integer;
begin
  form1.Hide;
  sleep(500);
  hWin := GetForegroundWindow;

  if FullWindow then
  begin
    GetWindowRect(hWin,r);
    dc := GetWindowDC(hWin) ;
  end else
  begin
    Windows.GetClientRect(hWin, r);
    dc := GetDC(hWin) ;
  end;

  w := r.Right - r.Left;
  h := r.Bottom - r.Top;

  bmp := TBitmap.Create;
  bmp.Height := h;
  bmp.Width := w;
  BitBlt(bmp.Canvas.Handle, 0, 0, w, h, DC, 0, 0, SRCCOPY);
  form1.Show ;
  FileName := 'Screenshot_'+FormatDateTime('mm-dd-yyyy-hhnnss',now());
  bmp.SaveToFile(Format('C:\Screenshots\%s.bmp', [FileName]));
  ReleaseDC(hwin, DC);
  bmp.Free;
end;

EDIT 2: As requested I'm adding a better version of the code, but I'm keeping the old one as a reference. You should seriously consider using this instead of your original code. It'll behave much nicer in case of errors. (Resources are cleaned up, your form will be visible again, ...)

procedure TForm1.Button1Click(Sender: TObject);
const
  FullWindow = True; // Set to false if you only want the client area.
var
  Win: HWND;
  DC: HDC;
  Bmp: TBitmap;
  FileName: string;
  WinRect: TRect;
  Width: Integer;
  Height: Integer;
begin
  Form1.Hide;
  try
    Application.ProcessMessages; // Was Sleep(500);
    Win := GetForegroundWindow;

    if FullWindow then
    begin
      GetWindowRect(Win, WinRect);
      DC := GetWindowDC(Win);
    end else
    begin
      Windows.GetClientRect(Win, WinRect);
      DC := GetDC(Win);
    end;
    try
      Width := WinRect.Right - WinRect.Left;
      Height := WinRect.Bottom - WinRect.Top;

      Bmp := TBitmap.Create;
      try
        Bmp.Height := Height;
        Bmp.Width := Width;
        BitBlt(Bmp.Canvas.Handle, 0, 0, Width, Height, DC, 0, 0, SRCCOPY);
        FileName := 'Screenshot_' + 
          FormatDateTime('mm-dd-yyyy-hhnnss', Now());
        Bmp.SaveToFile(Format('C:\Screenshots\%s.bmp', [FileName]));
      finally
        Bmp.Free;
      end;
    finally
      ReleaseDC(Win, DC);
    end;
  finally
    Form1.Show;
  end;
end;