center-current-window.el

(defun glyph-pixel-width (pos)
  (let* ((font (font-at pos))
         (glyphs (when font (font-get-glyphs font pos (1+ pos))))
         (glyph (when glyphs (aref glyphs 0))))
    (if glyph
        (aref glyph 4)
      0)))

(defun line-pixel-width ()
  (save-excursion
    (goto-char (line-beginning-position))
    (let ((sum 0)
          (end (line-end-position)))
      (while (< (point) end)
        (let ((x (glyph-pixel-width (point))))
          (when x
            (setq sum (+ sum x))))
        (forward-char))
      sum)))

(defun buffer-max-line-pixel-width ()
  (save-excursion
    (goto-char (point-min))
    (let ((max (line-pixel-width)))
      (while (eq (forward-line 1) 0)
        (let ((x (line-pixel-width)))
          (when (and x (> x max))
            (setq max x))))
      max)))

(defun center-current-window (&optional pixel-padding)
  "Center the current window, allowing for at least the longest
  lines in the buffer."
  (interactive)
  (let* ((window (get-buffer-window (current-buffer)))
         (max-line-pixel-width (buffer-max-line-pixel-width))
         (window-pixel-width (window-width window 'pixelwise))
         (pixel-padding 30)
         (pixel-margin (max 0 (/ (- window-pixel-width
                                    (+ pixel-padding
                                       max-line-pixel-width))
                                 2)))
         (window-width (window-width window))
         (pixel-ratio (/ (float window-width) (float window-pixel-width)))
         (margin (floor (* pixel-margin pixel-ratio))))
    (set-window-margins window margin margin)))