How to take screenshot of obscured window in C++ o

I'm trying to figure out how to take a screenshot of a window that is currently not focused, so there is a good chance that the window will be partially or fully obscured by other windows.

I've found an example here on this link Get a screenshot of a window that is cover or not visible or minimized with Xcomposite extension for X11 but I can't make it work, any time I take a screenshot I get only strange output, mostly black, like I'm accessing the wrong buffer or something.

  XID xid = windowID;  // Checked and confirmed that the window ID is correct
  XGetWindowAttributes( display, windowID, &attrributes );

  XCompositeRedirectWindow (display, xid, CompositeRedirectAutomatic);

  Pixmap pixmap = XCompositeNameWindowPixmap (display, xid);

  // Extract the data
  XRenderPictFormat *format = XRenderFindVisualFormat (display, attrributes.visual);

  XRenderPictureAttributes pa;
  pa.subwindow_mode = IncludeInferiors;
  Picture picture = XRenderCreatePicture (display, xid, format, CPSubwindowMode, &pa);

  QPixmap finalPix (attrributes.width, attrributes.height);
  XRenderComposite (display, PictOpSrc, picture, None, finalPix.x11PictureHandle(), 0,0, 0,0, 0,0, attrributes.width, attrributes.height);

  XFreePixmap (display, pixmap);
  XCompositeUnredirectWindow (display, xid, CompositeRedirectAutomatic);

  return finalPix;

(Edit: This screenshot was taken from a fully visible window, not an obscured window, so I guess currently the issue is not even that X11 doesn't draw it but my implementation seems to be not working and I can't figure out why.)
And this is how a screenshot of my konsole window looks:

First of all Qt has this feature. You can use: QScreen::grabWindow. Problem is that documentation says:

Note on X11 that if the given window doesn't have the same depth as the root window, and another window partially or entirely obscures the one you grab, you will not get pixels from the overlying window. The contents of the obscured areas in the pixmap will be undefined and uninitialized.

So this will simplify your code, but obscured parts of window will still remains as a problem. Looks like functionality of x11 won't let to resolve this issue.

There is a good example how to use this feature.