Delphi - Capture a Window Image

I wanted to develop my own print screen function - but instead of just capturing the image to the clipboard, I wanted the option to also write the window image directly to a jpeg or png file.

Overview | Displaying the Target Window - SetForegroundWindow Hide and Show Capture the Desktop | Capture the Image | Comments

Overview

Since I want to copy a part of the screen image, the windows api call - BitBlt - was used. For that to work, the code needs

The handle of the source window
The size of the copied image
A bitmap to hold the copy

Since BitBlt actually copies the pixels shown on the screen, the source window must be on top of all the other windows.

Displaying the Target Window

Once a handle is available, the window must be fully displayed on the screen to capture an image of it. You would think that providing the device context - GetWindowDC(hnd) - would be enough, but it is not! Apparently, BitBlt uses the context to get just the screen location and window size, but then gets the pixels directly from the screen.

SetForegroundWindow

One technique is to move the target window to the top of the z-order. Unfortunately, this takes a bit of time and there is no way to get a message indicating that the operation is complete. I "solved" this problem by setting a timer - 100 ms was enough for my test case, but no value is ever going to work all the time.

procedure TForm1.Some_UIButtonMouseUp(Sender: TObject;
  Button: TMouseButton; Shift: TShiftState; X, Y: Integer);
var
 hnd        : HWND;
 DestRect   : TRect;   // and the variables are actually pointers
 bm         : TBitmap;
begin
 DestRect.TopLeft := (Sender as TControl).ClientToScreen( point(x, y) );

 hnd := WindowFromPoint(DestRect.TopLeft);
 SetForegroundWindow(hnd);      // This takes a "long" time

  // other code here

  //  wait until the image has time to be displayed
 Timer1.enabled := true;
 while Timer1.enabled do        // 100 ms is enough for test purposes
   Application.ProcessMessages; // allow other tasks to run

 BitBlt(bm.Canvas.Handle,0,0,bm.Width,bm.Height,
           GetWindowDC(hnd),0,0,SRCCOPY);

 SetForegroundWindow(form1.Handle); // restore the capture window

end;

procedure TForm1.Timer1Timer(Sender: TObject);
begin
  Timer1.enabled := false; // just a flag
end;

To get the capture program back on top, I had to use windows again to change its z-order - I could not find a Delphi function to accomplish that.

Hide and Show

Another way to accomplish this is to hide and show the form capturing the image.

procedure TForm1.Some_UIButtonMouseUp(Sender: TObject;
  Button: TMouseButton; Shift: TShiftState; X, Y: Integer);
var
 hnd        : HWND;
 DestRect   : TRect;   // and the variables are actually pointers
 bm         : TBitmap;
begin
 DestRect.TopLeft := (Sender as TControl).ClientToScreen( point(x, y) );

 hnd := WindowFromPoint(DestRect.TopLeft);
  // SetForegroundWindow(hnd);           // not used when hide and show are used

  // other code here

  //  wait until the image has time to be displayed
 hide;                       // hide the capture form
 Timer1.enabled := true;
 while Timer1.enabled do        // 100 ms is enough for thest purposes
   Application.ProcessMessages; // allow other tasks to run

 BitBlt(bm.Canvas.Handle,0,0,bm.Width,bm.Height,
           GetWindowDC(hnd),0,0,SRCCOPY);

 show;                       // put this form back on top

 // SetForegroundWindow(form1.Handle);  // not used when hide and show are used

end;

procedure TForm1.Timer1Timer(Sender: TObject);
begin
  Timer1.enabled := false; // just a flag
end;

Since this technique does not change the windows z-order, the selected form must already be on top of the other windows. In addition, the timer is still required to allow the target image enough time to be drawn before it is captured.

As a result, I use this technique when a sub-window is used to capture graphs from a parent form, but not when capturing a random window in another application.

By the way, when the code is encapsulated in a component, self will no longer represent the form. In that case, the following modification is necessary.

GetParentForm(self).hide;    // needed only if the desktop is captured

GetParentForm(self).show;    // make the form visible again

Capture the Desktop

When capturing the desktop, merely changing the z-order will not hide the application. To accomplish that, it is necessary to also hide the window. As a result, the "best" approach is to both

Hide the capture program, and
Change the z-order

Perhaps providing checkboxes so the user can control these is should be considered.

Capture the Image

The actual capture code uses BitBlt, a handle, and a bitmap.

procedure xxxx();
var
 hnd        : HWND;
 SourceRect : TRect;   // apparently, TRect is automatically allocated
 DestRect   : TRect;   // and the variables are actually pointers
 bm         : TBitmap;
begin

  // other code to get the handle and display the window here

  GetWindowRect(hnd, SourceRect);
  bm := TBitmap.Create;
  try
    bm.Width  := SourceRect.Right  - SourceRect.Left;
    bm.Height := SourceRect.Bottom - SourceRect.Top ;

    DestRect.Left   := 0;
    DestRect.Top    := 0;
    DestRect.Right  := bm.Width;
    DestRect.Bottom := bm.Height;

    bm.Canvas.FillRect(DestRect); // probably not necessary
    bm.Canvas.Lock;
    try

      BitBlt(bm.Canvas.Handle,0,0,bm.Width,bm.Height,
           GetWindowDC(hnd),0,0,SRCCOPY);
    finally
      bm.Canvas.Unlock;
      Image1.Picture.Bitmap.Assign(bm); // display the image
      bm.Free;
    end;
  except
    bm.Free;
    raise;
  end;

end;

Comments

The code I used actually does not capture a form window - it gets the window (control) that is under the mouse. As a result, it is able to capture a piece of a form. For some applications, that might be considered a bug. However, I like it because it makes it easier to document some of the controls I design. To get the whole form, release the mouse on a blank area .. or on the form window border.

Unfortunately, when a complete application window (form) is captured in Windows XP, whatever is behind the two upper corners is also captured. This is because those corners are rounded. (This is how I know that BitBlt is copying pixels from the screen and not from some internal context buffer.) Therefore, you need to place objects of the appropriate color behind the corners.

With transparent window borders (Vista and beyond), additional noise will be added to the image.

To provide a white background, you can open notepad and center that target window over that.

Author: Robert Clemenzi