Using the built-in backslash fringe bitmap

Question

The built-in backslash fringe bitmap is somewhat mysterious in that it is not available to the user through a common name such as backslash or back-slash. I created my own -- using fringe-helper-convert and define-fringe-bitmap -- but it looks like a miniature staircase instead of a good looking smooth line.

Is there a special name for the built-in backslash fringe bitmap, and if so, what is it called?

The following snippet demonstrates how to display a bitmap in the right fringe (assuming the fringe is visible and wide enough) -- the example uses a question-mark since the built-in backslash appears to be unavailable?

(let ((peol (point-at-eol)))
  (overlay-put (make-overlay peol peol) 'after-string
    (propertize (char-to-string ?\uE000) 'display
      '(right-fringe question-mark font-lock-keyword-face))))

Here are links to the limited documentation regarding fringe bitmaps:

http://www.gnu.org/software/emacs/manual/html_node/elisp/Fringe-Bitmaps.html

http://www.gnu.org/software/emacs/manual/html_node/elisp/Customizing-Bitmaps.html

---

The built-in fringe backslash is visible in the right fringe in a GUI version of Emacs when the fringes are turned OFF -- i.e., (set-window-fringes (selected-window) 0 0)

---

Here is my staircase-looking version of the backslash, which is not a smooth line.

;; AUTHOR:  Nikolaj Schumacher -- https://github.com/nschum/fringe-helper.el
;;
(defun fringe-helper-convert (&rest strings)
"Convert STRINGS into a vector usable for `define-fringe-bitmap'.
Each string in STRINGS represents a line of the fringe bitmap.
Periods (.) are background-colored pixel; Xs are foreground-colored. The
fringe bitmap always is aligned to the right. If the fringe has half
width, only the left 4 pixels of an 8 pixel bitmap will be shown.
For example, the following code defines a diagonal line.
\(fringe-helper-convert
\"XX......\"
\"..XX....\"
\"....XX..\"
\"......XX\"\)"
  (unless (cdr strings)
  ;; only one string, probably with newlines
    (setq strings (split-string (car strings) "\n")))
  (apply 'vector
    (mapcar
      (lambda (str)
        (let ((num 0))
          (dolist (c (string-to-list str))
            (setq num (+ (* num 2) (if (eq c ?.) 0 1))))
          num))
      strings)))

(define-fringe-bitmap 'backslash (fringe-helper-convert
  "XX......"
  "XX......"
  " XX....."
  ".XX....."
  "..XX...."
  "..XX...."
  "...XX..."
  "...XX..."
  "....XX.."
  "....XX.."
  ".....XX."
  ".....XX.") nil nil 'center)

Answer 1

When fringes are turned OFF in a GUI version of Emacs, the continuation image that is displayed in what appeared to be the right fringe is actually a glyph, not a bitmap. Therefore, users must create their own backslash if they want to display that image in the fringe -- i.e., the staircase-looking bitmap in the question is about as good as it gets.

See: http://www.gnu.org/software/emacs/manual/html_node/elisp/Display-Tables.html

See also -- the following is the relevant excerpt from xdisp.c:

/* Get information about special display element WHAT in an
   environment described by IT.  WHAT is one of IT_TRUNCATION or
   IT_CONTINUATION.  Maybe produce glyphs for WHAT if IT has a
   non-null glyph_row member.  This function ensures that fields like
   face_id, c, len of IT are left untouched.  */

static void
produce_special_glyphs (struct it *it, enum display_element_type what)
{
  struct it temp_it;
  Lisp_Object gc;
  GLYPH glyph;

  temp_it = *it;
  temp_it.object = make_number (0);
  memset (&temp_it.current, 0, sizeof temp_it.current);

  if (what == IT_CONTINUATION)
    {
      /* Continuation glyph.  For R2L lines, we mirror it by hand.  */
      if (it->bidi_it.paragraph_dir == R2L)
    SET_GLYPH_FROM_CHAR (glyph, '/');
      else
    SET_GLYPH_FROM_CHAR (glyph, '\\');
      if (it->dp
      && (gc = DISP_CONTINUE_GLYPH (it->dp), GLYPH_CODE_P (gc)))
    {
      /* FIXME: Should we mirror GC for R2L lines?  */
      SET_GLYPH_FROM_GLYPH_CODE (glyph, gc);
      spec_glyph_lookup_face (XWINDOW (it->window), &glyph);
    }
    }
  else if (what == IT_TRUNCATION)
    {
      /* Truncation glyph.  */
      SET_GLYPH_FROM_CHAR (glyph, '$');
      if (it->dp
      && (gc = DISP_TRUNC_GLYPH (it->dp), GLYPH_CODE_P (gc)))
    {
      /* FIXME: Should we mirror GC for R2L lines?  */
      SET_GLYPH_FROM_GLYPH_CODE (glyph, gc);
      spec_glyph_lookup_face (XWINDOW (it->window), &glyph);
    }
    }
  else
    emacs_abort ();