(defun window-recenter-region-start-position (&optional from to window buffer)
  "Return window start position for recentered region.
WINDOW specifies the window whose start position to return and defaults
to the selected window.  BUFFER specifies the buffer that contains the
region and defaults to WINDOW's buffer. FROM and TO specify the
beginning and end of the region in BUFFER and default to the active
region or the entire text of BUFFER."
  (let* ((window (or window (selected-window)))
	 (buffer (or buffer (window-buffer)))
	 (from (or from
		   (and (eq buffer (current-buffer))
			(region-active-p)
			(region-beginning))
		   (with-current-buffer buffer
		     (point-min))))
	 (to (or to
		 (and (eq buffer (current-buffer))
		      (region-active-p)
		      (region-end))
		 (with-current-buffer buffer
		   (point-max))))
	 (body-width (window-body-width window))
	 (body-height (window-body-height window))
	 old-buffer old-start old-point height start)
    (unless (eq (window-buffer window) buffer)
      (setq old-buffer (window-buffer window))
      (setq old-start (window-start window))
      (setq old-point (window-point window))
      (set-window-buffer window buffer))
    (setq height
	  (/ (cdr (window-text-pixel-size
		   window from to body-width body-height))
	     (frame-char-height (window-frame window))))
    (save-excursion
      (goto-char from)
      (when (<= height body-height)
	(forward-line (- (max (/ (- body-height height) 2) 0))))
      (setq start (pos-bol)))
    (when old-buffer
      (set-window-buffer window old-buffer)
      (set-window-point window old-point)
      (set-window-start window old-point))
    start))

(defun recenter-region ()
  (interactive)
  (mark-paragraph)
  (set-window-start
   (selected-window) (window-recenter-region-start-position)))