Contributing & lisp/image-mode.el patch

[Benjamin R. White]

Hello,

I've been an Emacs user for a reasonable portion of my life and recently
decided I'd like to start contributing to GNU/Emacs development. With
the aim of starting off by scratching itches, I've thought before that
in image mode there should be a way of navigating around an image within
an *image-mode* buffer using the mouse. To this effect I've made the
following (albeit, trivial) patch that allows you to use the mouse-2
button to drag around an image that is larger than the window displaying
it.

I'm a relative newcomer to Lisp so you may judge my code to not be
worthwhile, however I'm always willing to receive constructive criticism
where it is due, and suggestions on how I can improve my coding style.

I read within etc/CONTRIBUTE that I needed to contact this list to
obtain forms so that the GNU/Emacs project may benefit by my
contributions, so to this effect I've been subscribed to the digest
version of the list for about a week. It's a bit scary for a newbie to
see all the arguments that go on around here.

Cheers,
Benjamin

2010-10-10  Benjamin R. White  <address@hidden>

        * image-mode.el: Function and mapping for image-dragging
        within a window.

# Bazaar merge directive format 2 (Bazaar 0.90)
# revision_id: address@hidden
# target_branch: http://bzr.savannah.gnu.org/r/emacs/trunk
# testament_sha1: be306a8c74d88c7c60c045fb1442688c981f4cdc
# timestamp: 2010-10-10 21:06:34 +0100
# base_revision_id: address@hidden
# 
# Begin patch
=== modified file 'lisp/image-mode.el'
--- lisp/image-mode.el  2010-09-24 03:06:33 +0000
+++ lisp/image-mode.el  2010-10-07 19:31:46 +0000
@@ -290,6 +290,48 @@
     (set-frame-width  (selected-frame) (car size))
     (set-frame-height (selected-frame) (cdr size))))
 
+(defun image-mouse-scroll-position (start-event)
+  "When bound to a down-mouse-X input START-EVENT, will smoothly
+drag the viewpoint on the image buffer that the window displays."
+  (interactive "e")
+  (let* ((echo-keystrokes 0)
+        (start-posn (event-start start-event))
+        (x-offset (window-hscroll (car start-posn)))
+        (y-offset (window-vscroll (car start-posn) t))
+        (imframe-cwidth (frame-char-width 
+                         (window-frame (car start-posn))))
+        (startpos (nth 8 start-posn))
+        event now-posn nowpos x-diff y-diff )
+    (select-window (posn-window start-posn))
+    (track-mouse 
+      (while (progn 
+              ;; as used in mouse.el:mouse-drag-track
+              (setq event (read-event))
+              (or (mouse-movement-p event)
+                  (memq (car-safe event) '(switch-frame select-window))))
+       (setq now-posn (event-start event))
+       ;; check if drag is within image on frame
+       (if (and (eq (nth 7 start-posn) (nth 7 now-posn))
+                (eq (nth 1 start-posn) (nth 1 now-posn)))
+           (progn 
+             (setq nowpos (nth 8 now-posn))
+             (setq x-diff (- (car startpos) (car nowpos)))
+             (setq y-diff (- (cdr startpos) (cdr nowpos)))
+
+             (set-window-vscroll (nth 0 start-posn) 
+                                 (+ y-offset y-diff) t)
+             (if (< (/ imframe-cwidth 2)
+                    (abs x-diff))
+                 (set-window-hscroll 
+                  (car start-posn) 
+                  (+ x-offset 
+                     (/ (if (> x-diff 0) 
+                            (+ x-diff (/ imframe-cwidth 2))
+                          (- x-diff (/ imframe-cwidth 2))) 
+                        imframe-cwidth))))
+             (setq x-offset (window-hscroll (car now-posn)))
+             (setq y-offset (window-vscroll (car now-posn) t))))))))
+
 ;;; Image Mode setup
 
 (defvar image-type nil
@@ -321,6 +363,7 @@
     (define-key map [remap move-end-of-line] 'image-eol)
     (define-key map [remap beginning-of-buffer] 'image-bob)
     (define-key map [remap end-of-buffer] 'image-eob)
+    (define-key map [down-mouse-2] 'image-mouse-scroll-position)
     map)
   "Major mode keymap for viewing images in Image mode.")
 

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