Add commands for navigating multi-frame images

* lisp/image.el (image-nth-frame): New, split from image-animate-timeout.
(image-animate-timeout): Use image-nth-frame.

* lisp/image-mode.el (image-goto-frame, image-next-frame)
(image-previous-frame): New commands.
(image-mode-map): Add new frame commands.

* etc/NEWS: Mention this.

etc/NEWS

@@ -141,6 +141,10 @@ amounts of data into the ERC input.
 visit the next image file and the previous image file in the same
 directory, respectively.
 
+*** New commands to show specific frames of multi-frame images.
+`f' (`image-next-frame') and `b' (`image-previous-frame') visit the
+next or previous frame.  `F' (`image-goto-frame') shows a specific frame.
+
 ---
 *** The command `image-mode-fit-frame' deletes other windows.
 When toggling, it restores the frame's previous window configuration.

lisp/ChangeLog

+2013-02-16  Glenn Morris  <rgm@gnu.org>
+
+	* image.el (image-nth-frame): New, split from image-animate-timeout.
+	(image-animate-timeout): Use image-nth-frame.
+	* image-mode.el (image-goto-frame, image-next-frame)
+	(image-previous-frame): New commands.
+	(image-mode-map): Add new frame commands.
+
 2013-02-16  Jonas Bernoulli  <jonas@bernoul.li>
 
 	* emacs-lisp/tabulated-list.el (tabulated-list-print-col):

lisp/image-mode.el

@@ -340,6 +340,9 @@ call."
     (define-key map (kbd "S-SPC")     'image-scroll-down)
     (define-key map (kbd "DEL")       'image-scroll-down)
     (define-key map (kbd "RET")       'image-toggle-animation)
+    (define-key map "F" 'image-goto-frame)
+    (define-key map "f" 'image-next-frame)
+    (define-key map "b" 'image-previous-frame)
     (define-key map "n" 'image-next-file)
     (define-key map "p" 'image-previous-file)
     (define-key map [remap forward-char] 'image-forward-hscroll)
@@ -627,6 +630,37 @@ Otherwise it plays once, then stops."
 	    (image-animate image index
 			   (if image-animate-loop t)))))))))
 
+(defun image-goto-frame (n &optional relative)
+  "Show frame N of a multi-frame image.
+Optional argument OFFSET non-nil means interpret N as relative to the
+current frame.  Frames are indexed from 1."
+  (interactive
+   (list (or current-prefix-arg
+	     (read-number "Show frame number: "))))
+  (let ((image (image-get-display-property))
+	animation)
+    (cond
+     ((null image)
+      (error "No image is present"))
+     ((null image-current-frame)
+      (message "No image animation."))
+     (t
+      (image-nth-frame image (if relative (+ n image-current-frame) (1- n)))))))
+
+(defun image-next-frame (&optional n)
+  "Switch to the next frame of a multi-frame image.
+With optional argument N, switch to the Nth frame after the current one.
+If N is negative, switch to the Nth frame before the current one."
+  (interactive "p")
+  (image-goto-frame n t))
+
+(defun image-previous-frame (&optional n)
+  "Switch to the previous frame of a multi-frame image.
+With optional argument N, switch to the Nth frame before the current one.
+If N is negative, switch to the Nth frame after the current one."
+  (interactive "p")
+  (image-next-frame (- n)))
+
 
 ;;; Switching to the next/previous image
 

lisp/image.el

@@ -660,6 +660,17 @@ number, play until that number of seconds has elapsed."
 (defvar-local image-current-frame nil
   "The frame index of the current animated image.")
 
+(defun image-nth-frame (image n &optional nocheck)
+  "Show frame N of IMAGE.
+Frames are indexed from 0.  Optional argument NOCHECK non-nil means
+do not check N is within the range of frames present in the image."
+  (unless nocheck
+    (if (< n 0) (setq n 0)
+      (setq n (min n (1- (car (image-animated-p image)))))))
+  (plist-put (cdr image) :index n)
+  (setq image-current-frame n)
+  (force-window-update))
+
 ;; FIXME? The delay may not be the same for different sub-images,
 ;; hence we need to call image-animated-p to return it.
 ;; But it also returns count, so why do we bother passing that as an
@@ -674,9 +685,7 @@ LIMIT determines when to stop.  If t, loop forever.  If nil, stop
  after displaying the last animation frame.  Otherwise, stop
  after LIMIT seconds have elapsed.
 The minimum delay between successive frames is 0.01s."
-  (plist-put (cdr image) :index n)
-  (setq image-current-frame n)
-  (force-window-update)
+  (image-nth-frame image n t)
   (setq n (1+ n))
   (let* ((time (float-time))
 	 (animation (image-animated-p image))