Take a Screen Shot of an Inactive Window

Using PrintWindow API from Delphi

Taking a screen shot of a window using Delphi code is rather easy.
A screen shot (screen capture) is a copy of the screen's contents that can be saved as a graphics file or displayed in a graphics "aware" control, for example TImage.

In most cases you will want to take a screen shot of the active window or the Windows Desktop.

What if you need to do a screen capture of all the running applications - most of them will be inactive and not visible to the user?

WindowSnap - Inactive Window Screen Capture

Windows XP also introduces the new printing API, PrintWindow. This API enables the caller to snapshot a visual copy of a window into a device context.

Drop a TImage (named "Image1") on a form and use the following code:

WindowSnap(Self.Handle, Image1.Picture.Bitmap) ;
Image1.Refresh;

The actual WindowSnap function is defined as:

function WindowSnap(windowHandle: HWND; bmp: TBitmap): boolean;
var
  r: TRect;
  user32DLLHandle: THandle;
  printWindowAPI: function(sourceHandle: HWND; destinationHandle: HDC; nFlags: UINT): BOOL; stdcall;
begin
  result := False;
  user32DLLHandle := GetModuleHandle(user32) ;
  if user32DLLHandle <> 0 then
  begin
    @printWindowAPI := GetProcAddress(huser32, 'PrintWindow') ;
    if @printWindowAPI <> nil then
    begin
      GetWindowRect(windowHandle, r) ;
      bmp.Width := r.Right - r.Left;
      bmp.Height := r.Bottom - r.Top;
      bmp.Canvas.Lock;
      try
        result := printWindowAPI(windowHandle, bmp.Canvas.Handle, 0) ;
      finally
        bmp.Canvas.Unlock;
      end;
    end;
  end;
end; (*WindowSnap*)

Note that the first parameter to the WindowSnap procedure is a HWND value (THandle) - the handle of the window you want to capture.

WinDowse is an extremely convenient and easy to use tool for obtaining necessary technical information about any window (handle, child windows, etc.)

Here's an idea: enumerate top-level windows (to grab their handles) and create your own task-switcher :)

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